CBSE Class 10 · Chemistry
CBSE Class 10 Chemistry Chemical Reactions and Equations Previous Year Questions
Help your child master CBSE Class 10 Chemistry Chemical Reactions and Equations previous year questions with this curated collection sourced from real board papers. Every question — from balancing equations and identifying reaction types to redox and displacement reactions — comes with a detailed step-by-step solution, giving your child the confidence and clarity needed to score well in board exams.
CBSE Class 10 Chemistry Chemical Reactions and Equations — Questions with Solutions
Electrolysis of water is a decomposition reaction. The mass ratio ($M_H : M_O$) of hydrogen and oxygen gases liberated at the electrodes during electrolysis of water is:
Solution
Answer: Option (D) is correct.
Explanation: The balanced equation for electrolysis of water is: $$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$ 2 moles of $H_2O$ produce 2 moles of $H_2$ and 1 mole of $O_2$.
• Mass of hydrogen: $2 \times 2 = 4$ g
• Mass of oxygen: $1 \times 32 = 32$ g
Mass ratio $(H : O) = 4 : 32 = \mathbf{1 : 8}$
Explanation: The balanced equation for electrolysis of water is: $$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$ 2 moles of $H_2O$ produce 2 moles of $H_2$ and 1 mole of $O_2$.
• Mass of hydrogen: $2 \times 2 = 4$ g
• Mass of oxygen: $1 \times 32 = 32$ g
Mass ratio $(H : O) = 4 : 32 = \mathbf{1 : 8}$
Assertion (A): Decomposition reactions are generally endothermic reactions.
Reason (R): Decomposition of organic matter into compost is an exothermic process.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Reason (R): Decomposition of organic matter into compost is an exothermic process.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Solution
Answer: Option (B) is correct.
Assertion (A): True. Most decomposition reactions require energy input (heat, light, or electricity) to break bonds, making them endothermic.
Reason (R): Also true. The microbial breakdown of organic matter (composting) releases heat, making it an exothermic process.
However, Reason (R) is not the correct explanation of Assertion (A) because it describes a biological exception, not the general chemical rule. Hence Option (B) is correct.
Assertion (A): True. Most decomposition reactions require energy input (heat, light, or electricity) to break bonds, making them endothermic.
Reason (R): Also true. The microbial breakdown of organic matter (composting) releases heat, making it an exothermic process.
However, Reason (R) is not the correct explanation of Assertion (A) because it describes a biological exception, not the general chemical rule. Hence Option (B) is correct.
The correct balanced chemical equation showing an exothermic reaction in which natural gas burns in air is:
Solution
Answer: Option (D) is correct.
Explanation: The combustion of methane (natural gas) in oxygen is an exothermic reaction. The correct balanced equation is: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$$ Option (A) is unbalanced. Option (B) has incorrect products ($2CO_2$ is wrong). Option (C) is balanced but does not mention the release of energy. Only Option (D) is both correctly balanced and shows the exothermic nature.
Explanation: The combustion of methane (natural gas) in oxygen is an exothermic reaction. The correct balanced equation is: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$$ Option (A) is unbalanced. Option (B) has incorrect products ($2CO_2$ is wrong). Option (C) is balanced but does not mention the release of energy. Only Option (D) is both correctly balanced and shows the exothermic nature.
Consider the following chemical equation:
$$p \, \text{Al} + q \, \text{H}_2\text{O} \rightarrow s\,\text{Al}_2\text{O}_3 + r \, \text{H}_2$$
To balance this chemical equation, the values of $p, q, r$ and $s$ must be respectively:
Solution
Answer: Option (B) is correct.
Step 1 — Balance Al: $\text{Al}_2\text{O}_3$ has 2 Al, so $p = 2$, $s = 1$.
Step 2 — Balance O: $\text{Al}_2\text{O}_3$ has 3 O, so $q = 3$.
Step 3 — Balance H: 3 $\text{H}_2\text{O}$ provides 6 H atoms, giving $r = 3$.
Balanced equation: $2\,\text{Al} + 3\,\text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 3\,\text{H}_2$
Therefore $p=2,\ q=3,\ r=3,\ s=1$ — Option (B).
Step 1 — Balance Al: $\text{Al}_2\text{O}_3$ has 2 Al, so $p = 2$, $s = 1$.
Step 2 — Balance O: $\text{Al}_2\text{O}_3$ has 3 O, so $q = 3$.
Step 3 — Balance H: 3 $\text{H}_2\text{O}$ provides 6 H atoms, giving $r = 3$.
Balanced equation: $2\,\text{Al} + 3\,\text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 3\,\text{H}_2$
Therefore $p=2,\ q=3,\ r=3,\ s=1$ — Option (B).
Study the following cases:
(i) $\text{CuSO}_4 + \text{Mg} \rightarrow$
(ii) $\text{FeSO}_4 + \text{Pb} \rightarrow$
(iii) $\text{CaSO}_4 + \text{Al} \rightarrow$
(iv) $\text{ZnSO}_4 + \text{Ca} \rightarrow$
The case(s) in which new product(s) will form is/are:
(i) $\text{CuSO}_4 + \text{Mg} \rightarrow$
(ii) $\text{FeSO}_4 + \text{Pb} \rightarrow$
(iii) $\text{CaSO}_4 + \text{Al} \rightarrow$
(iv) $\text{ZnSO}_4 + \text{Ca} \rightarrow$
The case(s) in which new product(s) will form is/are:
Solution
Answer: Option (C) is correct.
(i) $\text{CuSO}_4 + \text{Mg}$: Mg is more reactive than Cu. Reaction occurs: $\text{CuSO}_4 + \text{Mg} \rightarrow \text{MgSO}_4 + \text{Cu}$ ✓
(ii) $\text{FeSO}_4 + \text{Pb}$: Pb is less reactive than Fe. No reaction. ✗
(iii) $\text{CaSO}_4 + \text{Al}$: Although Al is more reactive than Ca, $\text{CaSO}_4$ is relatively stable and Al cannot displace Ca from it. No reaction. ✗
(iv) $\text{ZnSO}_4 + \text{Ca}$: Ca is more reactive than Zn. Reaction occurs: $\text{ZnSO}_4 + \text{Ca} \rightarrow \text{CaSO}_4 + \text{Zn}$ ✓
New products form in (i) and (iv) — Option (C).
(i) $\text{CuSO}_4 + \text{Mg}$: Mg is more reactive than Cu. Reaction occurs: $\text{CuSO}_4 + \text{Mg} \rightarrow \text{MgSO}_4 + \text{Cu}$ ✓
(ii) $\text{FeSO}_4 + \text{Pb}$: Pb is less reactive than Fe. No reaction. ✗
(iii) $\text{CaSO}_4 + \text{Al}$: Although Al is more reactive than Ca, $\text{CaSO}_4$ is relatively stable and Al cannot displace Ca from it. No reaction. ✗
(iv) $\text{ZnSO}_4 + \text{Ca}$: Ca is more reactive than Zn. Reaction occurs: $\text{ZnSO}_4 + \text{Ca} \rightarrow \text{CaSO}_4 + \text{Zn}$ ✓
New products form in (i) and (iv) — Option (C).
In which one of the following situations does a chemical reaction not occur?
Solution
Answer: Option (D) is correct.
Explanation: Melting of glaciers is a physical change — ice converts to water without any change in chemical composition. No new substance is formed, so no chemical reaction occurs.
Options (A), (B), and (C) all involve chemical reactions: souring of milk (bacterial action), fermentation (conversion of sugars to alcohol), and rusting of iron (oxidation) respectively.
Explanation: Melting of glaciers is a physical change — ice converts to water without any change in chemical composition. No new substance is formed, so no chemical reaction occurs.
Options (A), (B), and (C) all involve chemical reactions: souring of milk (bacterial action), fermentation (conversion of sugars to alcohol), and rusting of iron (oxidation) respectively.
Assertion (A): All exothermic reactions are accompanied with evolution of heat and light.
Reason (R): Combination reactions may or may not be exothermic.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true, but R is not the correct explanation of A.
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Reason (R): Combination reactions may or may not be exothermic.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true, but R is not the correct explanation of A.
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Solution
Answer: Option (D) is correct.
Assertion (A): False. Exothermic reactions always release heat, but they do not necessarily produce light. For example, dissolving NaOH in water releases heat but no light.
Reason (R): True. Combination reactions can be exothermic (e.g., formation of water from $H_2$ and $O_2$) or endothermic. The category is broad.
Therefore A is false, R is true — Option (D).
Assertion (A): False. Exothermic reactions always release heat, but they do not necessarily produce light. For example, dissolving NaOH in water releases heat but no light.
Reason (R): True. Combination reactions can be exothermic (e.g., formation of water from $H_2$ and $O_2$) or endothermic. The category is broad.
Therefore A is false, R is true — Option (D).
The main observations while performing the experiment of burning magnesium ribbon in air are:
(i) Magnesium ribbon burns with a dazzling white flame.
(ii) A white powder is formed.
(iii) Magnesium ribbon vaporizes.
(iv) Aqueous solution of the white powder turns blue litmus to red.
Which of the following options is correct?
(i) Magnesium ribbon burns with a dazzling white flame.
(ii) A white powder is formed.
(iii) Magnesium ribbon vaporizes.
(iv) Aqueous solution of the white powder turns blue litmus to red.
Which of the following options is correct?
Solution
Answer: Option (C) is correct.
Explanation:
• (i) True — Mg burns with a characteristic dazzling white flame.
• (ii) True — MgO (magnesium oxide), a white powder, is formed.
• (iii) False — Magnesium does not vaporize during burning; it reacts with oxygen.
• (iv) False — MgO is a basic oxide; its aqueous solution (Mg(OH)₂) turns red litmus blue, not the other way around.
Therefore, correct observations are (i) and (ii) — Option (C).
Explanation:
• (i) True — Mg burns with a characteristic dazzling white flame.
• (ii) True — MgO (magnesium oxide), a white powder, is formed.
• (iii) False — Magnesium does not vaporize during burning; it reacts with oxygen.
• (iv) False — MgO is a basic oxide; its aqueous solution (Mg(OH)₂) turns red litmus blue, not the other way around.
Therefore, correct observations are (i) and (ii) — Option (C).
A metal, M, displaces iron from aqueous solution of ferrous sulphate but fails to do so in the case of aqueous solution of aluminium sulphate. The metal M is:
Solution
Answer: Option (D) is correct.
Explanation: In the reactivity series: Aluminium > Zinc > Iron > Copper.
• M displaces iron from $\text{FeSO}_4$ → M is more reactive than Fe.
• M cannot displace aluminium from $\text{Al}_2(\text{SO}_4)_3$ → M is less reactive than Al.
Zinc (Zn) fits perfectly between Al and Fe. It displaces Fe ($Zn + FeSO_4 \rightarrow ZnSO_4 + Fe$) but cannot displace Al. Hence M = Zinc — Option (D).
Explanation: In the reactivity series: Aluminium > Zinc > Iron > Copper.
• M displaces iron from $\text{FeSO}_4$ → M is more reactive than Fe.
• M cannot displace aluminium from $\text{Al}_2(\text{SO}_4)_3$ → M is less reactive than Al.
Zinc (Zn) fits perfectly between Al and Fe. It displaces Fe ($Zn + FeSO_4 \rightarrow ZnSO_4 + Fe$) but cannot displace Al. Hence M = Zinc — Option (D).
The values of a, b, c and d in the following balanced chemical equation are respectively:
$$a \, \text{Pb(NO}_3)_2 \, \xrightarrow{\text{heat}} \, b \, \text{PbO} + c \, \text{NO}_2 + d \, \text{O}_2$$
Solution
Answer: Option (D) is correct.
Step 1 — Balance Pb: Set $b=2$, so $a=2$.
Step 2 — Balance N: 2 molecules of $\text{Pb(NO}_3)_2$ have $2 \times 2 = 4$ N atoms, so $c = 4$.
Step 3 — Balance O: LHS: $2 \times 6 = 12$ O. RHS: $2(\text{PbO}) + 4(\text{NO}_2) = 2 + 8 = 10$. Remaining $= 2$ O, so $d = 1$ (one $O_2$). Total RHS oxygen $= 2+8+2 = 12$ ✓
Balanced: $2\,\text{Pb(NO}_3)_2 \xrightarrow{\text{heat}} 2\,\text{PbO} + 4\,\text{NO}_2 + \text{O}_2$
Therefore $a=2,\ b=2,\ c=4,\ d=1$ — Option (D).
Step 1 — Balance Pb: Set $b=2$, so $a=2$.
Step 2 — Balance N: 2 molecules of $\text{Pb(NO}_3)_2$ have $2 \times 2 = 4$ N atoms, so $c = 4$.
Step 3 — Balance O: LHS: $2 \times 6 = 12$ O. RHS: $2(\text{PbO}) + 4(\text{NO}_2) = 2 + 8 = 10$. Remaining $= 2$ O, so $d = 1$ (one $O_2$). Total RHS oxygen $= 2+8+2 = 12$ ✓
Balanced: $2\,\text{Pb(NO}_3)_2 \xrightarrow{\text{heat}} 2\,\text{PbO} + 4\,\text{NO}_2 + \text{O}_2$
Therefore $a=2,\ b=2,\ c=4,\ d=1$ — Option (D).
Assertion (A): Silver chloride turns grey in sunlight.
Reason (R): Decomposition of silver chloride into silver and chlorine takes place by sunlight.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true, but R is not the correct explanation of A.
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Reason (R): Decomposition of silver chloride into silver and chlorine takes place by sunlight.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true, but R is not the correct explanation of A.
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Solution
Answer: Option (A) is correct.
Explanation: Silver chloride (white) turns grey in sunlight because sunlight causes it to decompose into silver (grey metal) and chlorine gas: $$2\text{AgCl} \xrightarrow{\text{sunlight}} 2\text{Ag} + \text{Cl}_2$$ Reason (R) correctly and completely explains Assertion (A). Both are true and R is the correct explanation of A.
Explanation: Silver chloride (white) turns grey in sunlight because sunlight causes it to decompose into silver (grey metal) and chlorine gas: $$2\text{AgCl} \xrightarrow{\text{sunlight}} 2\text{Ag} + \text{Cl}_2$$ Reason (R) correctly and completely explains Assertion (A). Both are true and R is the correct explanation of A.
In the following experimental setup of electrolysis of water, if P and Q are the gases collected in the test tubes enclosing the electrodes R and S, then select the option/options in which the matching is correct:
(i) P – Oxygen gas, R – Anode
(ii) P – Hydrogen gas, R – Cathode
(iii) Q – Hydrogen gas, S – Cathode
(iv) Q – Oxygen gas, S – Anode
(i) P – Oxygen gas, R – Anode
(ii) P – Hydrogen gas, R – Cathode
(iii) Q – Hydrogen gas, S – Cathode
(iv) Q – Oxygen gas, S – Anode
Solution
Answer: Option (C) is correct.
Explanation: During electrolysis of water:
• Hydrogen ($H_2$) is produced at the cathode (negative electrode) — double the volume of oxygen.
• Oxygen ($O_2$) is produced at the anode (positive electrode).
Gas P has smaller volume → oxygen ($O_2$) → R is the anode. (i) ✓
Gas Q has larger volume → hydrogen ($H_2$) → S is the cathode. (iii) ✓
Correct matchings: (i) and (iii) — Option (C).
Explanation: During electrolysis of water:
• Hydrogen ($H_2$) is produced at the cathode (negative electrode) — double the volume of oxygen.
• Oxygen ($O_2$) is produced at the anode (positive electrode).
Gas P has smaller volume → oxygen ($O_2$) → R is the anode. (i) ✓
Gas Q has larger volume → hydrogen ($H_2$) → S is the cathode. (iii) ✓
Correct matchings: (i) and (iii) — Option (C).
Select from the following a statement which is not true about burning of magnesium ribbon in air:
Solution
Answer: Option (C) is correct (as the statement that is NOT true).
Explanation: The burning of magnesium in air is an exothermic reaction, releasing a significant amount of heat and light — not endothermic. Options (A), (B), and (D) are all true observations about this reaction.
Explanation: The burning of magnesium in air is an exothermic reaction, releasing a significant amount of heat and light — not endothermic. Options (A), (B), and (D) are all true observations about this reaction.
The colour of the solution observed after about 1 hour of placing iron nails in copper sulfate solution is:
Solution
Answer: Option (B) is correct.
Explanation: Iron displaces copper from copper sulfate solution because Fe is more reactive than Cu: $$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$ • The blue colour of $\text{CuSO}_4$ fades as copper is removed.
• The solution turns pale green due to the formation of $\text{FeSO}_4$ (iron(II) sulfate).
• Reddish-brown copper deposits on the iron nails.
Explanation: Iron displaces copper from copper sulfate solution because Fe is more reactive than Cu: $$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$ • The blue colour of $\text{CuSO}_4$ fades as copper is removed.
• The solution turns pale green due to the formation of $\text{FeSO}_4$ (iron(II) sulfate).
• Reddish-brown copper deposits on the iron nails.
Examples of thermal decomposition reactions are:
(i) $2 \, \text{AgCl} \rightarrow 2 \, \text{Ag} + \text{Cl}_2$
(ii) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$
(iii) $2 \, \text{H}_2\text{O} \rightarrow 2 \, \text{H}_2 + \text{O}_2$
(iv) $2 \, \text{KClO}_3 \rightarrow 2 \, \text{KCl} + 3 \, \text{O}_2$
(i) $2 \, \text{AgCl} \rightarrow 2 \, \text{Ag} + \text{Cl}_2$
(ii) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$
(iii) $2 \, \text{H}_2\text{O} \rightarrow 2 \, \text{H}_2 + \text{O}_2$
(iv) $2 \, \text{KClO}_3 \rightarrow 2 \, \text{KCl} + 3 \, \text{O}_2$
Solution
Answer: Option (D) is correct.
• (i) AgCl decomposition — requires light (photodecomposition), not heat. ✗
• (ii) CaCO₃ decomposition — classic thermal decomposition: $\text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2$ ✓
• (iii) H₂O decomposition — occurs via electrolysis (electrical energy), not thermal. ✗
• (iv) KClO₃ decomposition — decomposes on heating to give KCl and O₂ ✓
Thermal decomposition examples: (ii) and (iv) — Option (D).
• (i) AgCl decomposition — requires light (photodecomposition), not heat. ✗
• (ii) CaCO₃ decomposition — classic thermal decomposition: $\text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2$ ✓
• (iii) H₂O decomposition — occurs via electrolysis (electrical energy), not thermal. ✗
• (iv) KClO₃ decomposition — decomposes on heating to give KCl and O₂ ✓
Thermal decomposition examples: (ii) and (iv) — Option (D).
A student performs the following experiment in his school laboratory:
List two observations to justify that in this experiment a chemical change has taken place.
List two observations to justify that in this experiment a chemical change has taken place.
Answer
Observations:
1. Effervescence (bubbling) is observed due to the evolution of hydrogen gas.
2. Zinc granules slowly dissolve and the solution may get warm (rise in temperature).
Zinc granules react with dilute sulphuric acid, producing hydrogen gas and zinc sulphate in a single displacement reaction: $$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$$ The formation of a new substance ($ZnSO_4$) and the evolution of a gas confirm that a chemical change has occurred.
1. Effervescence (bubbling) is observed due to the evolution of hydrogen gas.
2. Zinc granules slowly dissolve and the solution may get warm (rise in temperature).
Zinc granules react with dilute sulphuric acid, producing hydrogen gas and zinc sulphate in a single displacement reaction: $$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$$ The formation of a new substance ($ZnSO_4$) and the evolution of a gas confirm that a chemical change has occurred.
What happens when (write balanced chemical equation):
(a) Lead nitrate is thermally decomposed?
(b) Natural gas burns in oxygen (or air)?
(a) Lead nitrate is thermally decomposed?
(b) Natural gas burns in oxygen (or air)?
Answer
(a) Thermal decomposition of lead nitrate:
$$2Pb(NO_3)_2 \xrightarrow{\text{heat}} 2PbO + 4NO_2 + O_2$$ Brown fumes of $NO_2$ are observed. This is a thermal decomposition reaction.
(b) Natural gas (methane) burns in oxygen:
$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$ This is a combination/combustion reaction and is exothermic in nature.
$$2Pb(NO_3)_2 \xrightarrow{\text{heat}} 2PbO + 4NO_2 + O_2$$ Brown fumes of $NO_2$ are observed. This is a thermal decomposition reaction.
(b) Natural gas (methane) burns in oxygen:
$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$ This is a combination/combustion reaction and is exothermic in nature.
Translate the following statements into chemical equations and then balance them:
(a) Nitric acid reacts with calcium hydroxide to form calcium nitrate and water.
(b) Sodium chloride reacts with silver nitrate to form silver chloride and sodium nitrate.
(a) Nitric acid reacts with calcium hydroxide to form calcium nitrate and water.
(b) Sodium chloride reacts with silver nitrate to form silver chloride and sodium nitrate.
Answer
(a) Nitric acid + Calcium hydroxide:
Unbalanced: $HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + H_2O(l)$
Balanced: $$2HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l)$$
(b) Sodium chloride + Silver nitrate:
Balanced: $$NaCl(aq) + AgNO_3(aq) \rightarrow AgCl(s) + NaNO_3(aq)$$ (The equation is already balanced as written. AgCl precipitates as a white solid.)
Unbalanced: $HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + H_2O(l)$
Balanced: $$2HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l)$$
(b) Sodium chloride + Silver nitrate:
Balanced: $$NaCl(aq) + AgNO_3(aq) \rightarrow AgCl(s) + NaNO_3(aq)$$ (The equation is already balanced as written. AgCl precipitates as a white solid.)
(a) (i) Define the term decomposition reaction. Write one chemical equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
(ii) Decomposition of vegetable matter into compost is considered an exothermic reaction. Why?
OR
(b) Why are decomposition reactions called the opposite of combination reactions? Write one chemical equation each for these two types of reactions, mentioning the name of the reactant(s) and the product(s) involved.
(ii) Decomposition of vegetable matter into compost is considered an exothermic reaction. Why?
OR
(b) Why are decomposition reactions called the opposite of combination reactions? Write one chemical equation each for these two types of reactions, mentioning the name of the reactant(s) and the product(s) involved.
Answer
(a)(i) Decomposition Reaction: A reaction in which a single reactant breaks down into two or more simpler products, typically requiring energy input.
• Heat (thermal decomposition): $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ • Light (photodecomposition): $$2AgCl \xrightarrow{\text{light}} 2Ag + Cl_2$$ • Electricity (electrolytic decomposition): $$2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$$
(a)(ii) The decomposition of vegetable matter into compost is exothermic because microorganisms break down organic bonds and form new, more stable compounds. This process releases heat into the surroundings, making it exothermic — the compost heap actually warms up during decomposition.
(b) OR: In a decomposition reaction, one compound breaks into simpler substances, while in a combination reaction, two or more substances join to form one compound. They are reverse processes of each other.
Decomposition: Calcium carbonate (reactant) → Calcium oxide + Carbon dioxide (products): $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ Combination: Nitrogen + Hydrogen (reactants) → Ammonia (product): $$N_2 + 3H_2 \rightarrow 2NH_3$$
• Heat (thermal decomposition): $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ • Light (photodecomposition): $$2AgCl \xrightarrow{\text{light}} 2Ag + Cl_2$$ • Electricity (electrolytic decomposition): $$2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$$
(a)(ii) The decomposition of vegetable matter into compost is exothermic because microorganisms break down organic bonds and form new, more stable compounds. This process releases heat into the surroundings, making it exothermic — the compost heap actually warms up during decomposition.
(b) OR: In a decomposition reaction, one compound breaks into simpler substances, while in a combination reaction, two or more substances join to form one compound. They are reverse processes of each other.
Decomposition: Calcium carbonate (reactant) → Calcium oxide + Carbon dioxide (products): $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ Combination: Nitrogen + Hydrogen (reactants) → Ammonia (product): $$N_2 + 3H_2 \rightarrow 2NH_3$$
Balance the following chemical equations:
(a) $\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{sunlight, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2 + \text{H}_2\text{O}$
(b) $\text{Pb(NO}_3)_2 \xrightarrow{\text{Heat}} \text{PbO} + \text{NO}_2 + \text{O}_2$
(a) $\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{sunlight, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2 + \text{H}_2\text{O}$
(b) $\text{Pb(NO}_3)_2 \xrightarrow{\text{Heat}} \text{PbO} + \text{NO}_2 + \text{O}_2$
Answer
(a) Photosynthesis (balanced): $$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} \xrightarrow{\text{sunlight, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 + \text{H}_2\text{O}$$
(Balance C: 6 on each side; H: 12 from $6H_2O$; O: 18 on each side.)
(b) Thermal decomposition of lead nitrate (balanced): $$2\,\text{Pb(NO}_3)_2 \xrightarrow{\text{heat}} 2\,\text{PbO} + 4\,\text{NO}_2 + \text{O}_2$$
(b) Thermal decomposition of lead nitrate (balanced): $$2\,\text{Pb(NO}_3)_2 \xrightarrow{\text{heat}} 2\,\text{PbO} + 4\,\text{NO}_2 + \text{O}_2$$
List the possible sources of energy required in decomposition reactions. Illustrate any one with a suitable example.
Answer
Sources of energy in decomposition reactions:
1. Heat (Thermal energy)
2. Light (Photonic / Radiant energy)
3. Electricity (Electrical energy)
Example — Thermal decomposition of Calcium Carbonate:
When calcium carbonate ($CaCO_3$) is heated strongly, it decomposes into calcium oxide ($CaO$) and carbon dioxide ($CO_2$): $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ Heat energy breaks the chemical bonds in $CaCO_3$, causing it to decompose. This reaction is used industrially in the manufacture of cement and lime.
1. Heat (Thermal energy)
2. Light (Photonic / Radiant energy)
3. Electricity (Electrical energy)
Example — Thermal decomposition of Calcium Carbonate:
When calcium carbonate ($CaCO_3$) is heated strongly, it decomposes into calcium oxide ($CaO$) and carbon dioxide ($CO_2$): $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ Heat energy breaks the chemical bonds in $CaCO_3$, causing it to decompose. This reaction is used industrially in the manufacture of cement and lime.
(a) What is a reactivity series of elements? How is it developed? Arrange the following elements as they appear in the reactivity series: Aluminum, Calcium, Copper, Lead.
(b) Write a balanced chemical equation to show the reaction of iron (III) oxide ($Fe_2O_3$) with aluminum.
(b) Write a balanced chemical equation to show the reaction of iron (III) oxide ($Fe_2O_3$) with aluminum.
Answer
(a) Reactivity Series: A list of elements (mostly metals) arranged in decreasing order of their reactivity — their tendency to lose electrons and react with water, acids, or other substances.
It is developed by observing:
• How easily metals react with water and acids.
• Their ability to displace other metals from solutions.
• Their tendency to form oxides and compounds.
Arrangement (most to least reactive):
Calcium > Aluminum > Lead > Copper
(b) Thermite Reaction (balanced):
$$\text{Fe}_2\text{O}_3 + 2\,\text{Al} \rightarrow 2\,\text{Fe} + \text{Al}_2\text{O}_3$$ Aluminum (more reactive) displaces iron from its oxide, releasing a large amount of heat. This highly exothermic reaction is used in thermite welding of railway tracks.
It is developed by observing:
• How easily metals react with water and acids.
• Their ability to displace other metals from solutions.
• Their tendency to form oxides and compounds.
Arrangement (most to least reactive):
Calcium > Aluminum > Lead > Copper
(b) Thermite Reaction (balanced):
$$\text{Fe}_2\text{O}_3 + 2\,\text{Al} \rightarrow 2\,\text{Fe} + \text{Al}_2\text{O}_3$$ Aluminum (more reactive) displaces iron from its oxide, releasing a large amount of heat. This highly exothermic reaction is used in thermite welding of railway tracks.
How is a double displacement reaction different from a displacement reaction? Explain giving examples in the form of balanced chemical equations.
Answer
Displacement Reaction: One element displaces another from its compound. Only one ion exchange occurs (one element replaces one other).
General form: $AB + C \rightarrow AC + B$
Example: Zinc displaces copper from copper sulphate: $$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$$
Double Displacement Reaction: Two compounds mutually exchange their ions to form two new compounds. One product is typically a precipitate, gas, or water.
General form: $AB + CD \rightarrow AD + CB$
Example: Silver nitrate and sodium chloride exchange ions: $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s)\downarrow + NaNO_3(aq)$$ Key difference: In displacement, one element replaces another; in double displacement, two compounds exchange their ions with each other.
General form: $AB + C \rightarrow AC + B$
Example: Zinc displaces copper from copper sulphate: $$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$$
Double Displacement Reaction: Two compounds mutually exchange their ions to form two new compounds. One product is typically a precipitate, gas, or water.
General form: $AB + CD \rightarrow AD + CB$
Example: Silver nitrate and sodium chloride exchange ions: $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s)\downarrow + NaNO_3(aq)$$ Key difference: In displacement, one element replaces another; in double displacement, two compounds exchange their ions with each other.
State the change that is observed when a China dish containing copper powder is heated over the flame of a burner.
1) Name the phenomenon responsible for the change and write the balanced equation for the chemical reaction that occurs.
2) How is this reaction different from the reaction that occurs when copper wares kept in open air slowly lose their shiny brown surface and gain a coat?
3) Write chemical name of the coating and state its colour.
1) Name the phenomenon responsible for the change and write the balanced equation for the chemical reaction that occurs.
2) How is this reaction different from the reaction that occurs when copper wares kept in open air slowly lose their shiny brown surface and gain a coat?
3) Write chemical name of the coating and state its colour.
Answer
When a China dish containing copper powder is heated, the shiny brown copper turns black due to the formation of copper(II) oxide.
1) Phenomenon — Oxidation:
Copper gains oxygen from the air, forming a black oxide coating. Balanced equation: $$2Cu(s) + O_2(g) \rightarrow 2CuO(s)$$
2) Difference:
When copper wares are kept in open air, they react with moisture and carbon dioxide to form copper carbonate — a greenish coating. This is a slow reaction in humid air:
$$2Cu(s) + CO_2(g) + H_2O(l) \rightarrow CuCO_3(s) + H_2(g)$$ Heating in air forms black CuO, while exposure to humid air forms a green carbonate coating — different products under different conditions.
3) Chemical name: Copper carbonate ($CuCO_3$)
Colour: Green
1) Phenomenon — Oxidation:
Copper gains oxygen from the air, forming a black oxide coating. Balanced equation: $$2Cu(s) + O_2(g) \rightarrow 2CuO(s)$$
2) Difference:
When copper wares are kept in open air, they react with moisture and carbon dioxide to form copper carbonate — a greenish coating. This is a slow reaction in humid air:
$$2Cu(s) + CO_2(g) + H_2O(l) \rightarrow CuCO_3(s) + H_2(g)$$ Heating in air forms black CuO, while exposure to humid air forms a green carbonate coating — different products under different conditions.
3) Chemical name: Copper carbonate ($CuCO_3$)
Colour: Green
Define oxidation. Identify and name the substance oxidised in the following reaction:
$$\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$$
Answer
Oxidation: Oxidation is the process in which a substance gains oxygen or loses hydrogen (in broader terms, loses electrons). The substance that undergoes oxidation is called the reducing agent.
In the given reaction: $$CuO + H_2 \rightarrow Cu + H_2O$$ • Hydrogen ($H_2$) gains oxygen to form water ($H_2O$) → Hydrogen is oxidised.
• Copper oxide ($CuO$) loses oxygen to form copper ($Cu$) → CuO is reduced.
Substance oxidised: Hydrogen ($H_2$)
In the given reaction: $$CuO + H_2 \rightarrow Cu + H_2O$$ • Hydrogen ($H_2$) gains oxygen to form water ($H_2O$) → Hydrogen is oxidised.
• Copper oxide ($CuO$) loses oxygen to form copper ($Cu$) → CuO is reduced.
Substance oxidised: Hydrogen ($H_2$)
(A) Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation. Balance the following chemical equation:
$$\text{Zn} + \text{H}_3\text{PO}_4 \rightarrow \text{Zn}_3\text{(PO}_4)_2 + \text{H}_2$$ OR
(B) Define a precipitation reaction. Give its example and also express the reaction in the form of a balanced chemical equation.
(B) Define a precipitation reaction. Give its example and also express the reaction in the form of a balanced chemical equation.
Answer
(A) Why balance a chemical equation?
We balance chemical equations to satisfy the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. The number of atoms of each element must be equal on both sides.
Balancing the equation:
Step 1 — Balance Zn: 3 Zn needed on left: $3Zn + H_3PO_4 \rightarrow Zn_3(PO_4)_2 + H_2$
Step 2 — Balance $PO_4$: 2 groups on right, so $2H_3PO_4$ on left: $3Zn + 2H_3PO_4 \rightarrow Zn_3(PO_4)_2 + H_2$
Step 3 — Balance H: 6 H on left, so $3H_2$ on right:
$$3Zn + 2H_3PO_4 \rightarrow Zn_3(PO_4)_2 + 3H_2$$
(B) OR — Precipitation Reaction:
A reaction in which two solutions combine to form an insoluble solid (precipitate) is called a precipitation reaction.
Example: Silver nitrate reacts with sodium chloride to form a white precipitate of silver chloride: $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s)\downarrow + NaNO_3(aq)$$ AgCl is the precipitate — insoluble in water and appears as a white solid.
We balance chemical equations to satisfy the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. The number of atoms of each element must be equal on both sides.
Balancing the equation:
Step 1 — Balance Zn: 3 Zn needed on left: $3Zn + H_3PO_4 \rightarrow Zn_3(PO_4)_2 + H_2$
Step 2 — Balance $PO_4$: 2 groups on right, so $2H_3PO_4$ on left: $3Zn + 2H_3PO_4 \rightarrow Zn_3(PO_4)_2 + H_2$
Step 3 — Balance H: 6 H on left, so $3H_2$ on right:
$$3Zn + 2H_3PO_4 \rightarrow Zn_3(PO_4)_2 + 3H_2$$
(B) OR — Precipitation Reaction:
A reaction in which two solutions combine to form an insoluble solid (precipitate) is called a precipitation reaction.
Example: Silver nitrate reacts with sodium chloride to form a white precipitate of silver chloride: $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s)\downarrow + NaNO_3(aq)$$ AgCl is the precipitate — insoluble in water and appears as a white solid.
Name the compound used in black and white photography. State whether the reaction that occurs is exothermic or endothermic. Give justification for your answer.
Answer
Compound used: Silver bromide ($AgBr$) is used in black and white photography.
Nature of reaction: The reaction is endothermic.
Justification: When silver bromide is exposed to sunlight during photography, it absorbs light energy to decompose into silver metal and bromine. Energy is absorbed from surroundings, making it endothermic: $$AgBr \xrightarrow{\text{light}} Ag + Br^-$$ The subsequent development reaction also requires energy input. Since energy is absorbed rather than released, the process is classified as endothermic.
Nature of reaction: The reaction is endothermic.
Justification: When silver bromide is exposed to sunlight during photography, it absorbs light energy to decompose into silver metal and bromine. Energy is absorbed from surroundings, making it endothermic: $$AgBr \xrightarrow{\text{light}} Ag + Br^-$$ The subsequent development reaction also requires energy input. Since energy is absorbed rather than released, the process is classified as endothermic.
While burning a magnesium ribbon in air, list two safety measures which should be followed. Also state two observations of this activity.
Answer
Safety Measures:
1. Wear Protective Goggles — Burning Mg produces intense white light that can cause eye damage. Goggles protect against bright light and heat.
2. Keep a Fire Extinguisher or Bucket of Sand Nearby — Mg burns at very high temperatures. Water should never be used to extinguish it. Sand or a CO₂ extinguisher can control the fire safely.
Observations:
1. Magnesium burns with a dazzling white light, producing intense brightness.
2. A white powdery ash (magnesium oxide, MgO) is formed as a product of the combustion.
1. Wear Protective Goggles — Burning Mg produces intense white light that can cause eye damage. Goggles protect against bright light and heat.
2. Keep a Fire Extinguisher or Bucket of Sand Nearby — Mg burns at very high temperatures. Water should never be used to extinguish it. Sand or a CO₂ extinguisher can control the fire safely.
Observations:
1. Magnesium burns with a dazzling white light, producing intense brightness.
2. A white powdery ash (magnesium oxide, MgO) is formed as a product of the combustion.
Samples of four metals A, B, C, and D were added one by one to the following solutions. The results obtained were tabulated as follows:
(i) Which is the least reactive metal?
(ii) What would be observed if C is added to a solution of copper sulphate?
(iii) Arrange the metals A, B, C, and D in the order of their decreasing reactivity.
(i) Which is the least reactive metal?
(ii) What would be observed if C is added to a solution of copper sulphate?
(iii) Arrange the metals A, B, C, and D in the order of their decreasing reactivity.
Answer
(i) Least reactive metal: Metal D — it shows no reaction with any of the solutions, meaning it cannot displace any other metal from solution.
(ii) C added to copper sulphate: There would be no reaction. Metal C is less reactive than copper and cannot displace copper ions from $CuSO_4$ solution. The solution remains blue and no deposit forms on the metal.
(iii) Decreasing order of reactivity:
• B displaces all three metals → most reactive.
• A displaces copper → more reactive than Cu, less than B.
• C displaces iron → more reactive than Fe and Cu, less than A.
• D displaces no metal → least reactive.
B > A > C > D
(ii) C added to copper sulphate: There would be no reaction. Metal C is less reactive than copper and cannot displace copper ions from $CuSO_4$ solution. The solution remains blue and no deposit forms on the metal.
(iii) Decreasing order of reactivity:
• B displaces all three metals → most reactive.
• A displaces copper → more reactive than Cu, less than B.
• C displaces iron → more reactive than Fe and Cu, less than A.
• D displaces no metal → least reactive.
B > A > C > D
Write chemical equation for the chemical reaction between the aqueous solutions of sodium sulphate and barium chloride. State two types of chemical reactions in which this reaction can be placed.
Answer
Balanced Chemical Equation: $$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s)\downarrow + 2\text{NaCl}(aq)$$
A white precipitate of barium sulphate ($BaSO_4$) is formed.
Two types of reactions this can be classified as:
1. Precipitation Reaction — because an insoluble solid precipitate (white $BaSO_4$) is formed from two soluble salts.
2. Double Displacement Reaction — because $Na^+$ exchanges with $Ba^{2+}$ and $SO_4^{2-}$ exchanges with $Cl^-$ to form two new compounds.
Two types of reactions this can be classified as:
1. Precipitation Reaction — because an insoluble solid precipitate (white $BaSO_4$) is formed from two soluble salts.
2. Double Displacement Reaction — because $Na^+$ exchanges with $Ba^{2+}$ and $SO_4^{2-}$ exchanges with $Cl^-$ to form two new compounds.
A copper wire on burning in flame gets coated with a black substance. Write the chemical equation of the reaction that takes place. How can this chemical change be reversed?
Answer
When copper burns in air, it reacts with oxygen to form copper(II) oxide — the black coating:
$$2Cu(s) + O_2(g) \rightarrow 2CuO(s)$$
Reversing the change: The black copper(II) oxide can be reduced back to copper by heating it in the presence of hydrogen gas. Hydrogen removes the oxygen from CuO, restoring the copper: $$CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(g)$$ The black coating disappears and the shiny brown copper is recovered.
$$2Cu(s) + O_2(g) \rightarrow 2CuO(s)$$
Reversing the change: The black copper(II) oxide can be reduced back to copper by heating it in the presence of hydrogen gas. Hydrogen removes the oxygen from CuO, restoring the copper: $$CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(g)$$ The black coating disappears and the shiny brown copper is recovered.
A light green coloured solution of sulphate salt of metal ‘P’ is taken in a beaker, a rod of another metal ‘Q’ is put in this solution. After about 1 hour, the solution becomes colourless.
• Identify the metals ‘P’ and ‘Q’ and write the chemical equation for the reaction that occurs.
• State the conclusion of this reaction in terms of the reactivity series of metals.
• Identify the metals ‘P’ and ‘Q’ and write the chemical equation for the reaction that occurs.
• State the conclusion of this reaction in terms of the reactivity series of metals.
Answer
Metal P = Iron (Fe) — FeSO₄ is light green in colour.
Metal Q = Zinc (Zn) — Zinc is more reactive than iron and displaces it.
Chemical Equation: $$Zn(s) + FeSO_4(aq) \rightarrow ZnSO_4(aq) + Fe(s)$$ As Zn displaces Fe, $FeSO_4$ (light green) is replaced by $ZnSO_4$ (colourless), making the solution colourless. Iron metal deposits on the zinc rod.
Conclusion: This reaction confirms that Zinc is more reactive than Iron in the reactivity series. A more reactive metal displaces a less reactive metal from its salt solution — a fundamental principle of the reactivity series.
Metal Q = Zinc (Zn) — Zinc is more reactive than iron and displaces it.
Chemical Equation: $$Zn(s) + FeSO_4(aq) \rightarrow ZnSO_4(aq) + Fe(s)$$ As Zn displaces Fe, $FeSO_4$ (light green) is replaced by $ZnSO_4$ (colourless), making the solution colourless. Iron metal deposits on the zinc rod.
Conclusion: This reaction confirms that Zinc is more reactive than Iron in the reactivity series. A more reactive metal displaces a less reactive metal from its salt solution — a fundamental principle of the reactivity series.
Write balanced chemical equations for the reactions that occur when:
(a) Steam is passed over red hot iron.
(b) Natural gas is burnt in air.
(c) Glucose reacts with oxygen in the cells of our body and provides energy.
(a) Steam is passed over red hot iron.
(b) Natural gas is burnt in air.
(c) Glucose reacts with oxygen in the cells of our body and provides energy.
Answer
(a) Steam over red hot iron: $$Fe(s) + H_2O(g) \rightarrow FeO(s) + H_2(g)$$
(Iron reacts with steam to form iron oxide and hydrogen gas.)
(b) Natural gas (methane) burns in air: $$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$$ (Combustion — an exothermic reaction producing $CO_2$ and water.)
(c) Cellular respiration (glucose + oxygen): $$C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l) + \text{Energy}$$ (Aerobic respiration — an exothermic process providing energy to body cells.)
(b) Natural gas (methane) burns in air: $$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$$ (Combustion — an exothermic reaction producing $CO_2$ and water.)
(c) Cellular respiration (glucose + oxygen): $$C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l) + \text{Energy}$$ (Aerobic respiration — an exothermic process providing energy to body cells.)
Explain why respiration is considered as an exothermic reaction. Give the chemical equation for this reaction.
Answer
Respiration as an Exothermic Reaction:
Respiration is considered exothermic because it releases energy in the form of heat during the oxidation of glucose in body cells. The energy released is greater than the energy absorbed, which is the defining feature of an exothermic reaction.
During respiration, glucose reacts with oxygen to produce carbon dioxide, water, and energy. This released energy powers all physiological activities — movement, growth, maintaining body temperature, and more.
Chemical equation for aerobic respiration: $$C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l) + \text{Energy}$$ Since energy is released as a product, respiration is classified as an exothermic process.
Respiration is considered exothermic because it releases energy in the form of heat during the oxidation of glucose in body cells. The energy released is greater than the energy absorbed, which is the defining feature of an exothermic reaction.
During respiration, glucose reacts with oxygen to produce carbon dioxide, water, and energy. This released energy powers all physiological activities — movement, growth, maintaining body temperature, and more.
Chemical equation for aerobic respiration: $$C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l) + \text{Energy}$$ Since energy is released as a product, respiration is classified as an exothermic process.
Consider the unbalanced chemical equations of the chemical reactions listed below:
(1) $CaO + H_2O \rightarrow Ca(OH)_2$
(2) $Pb(NO_3)_2 \rightarrow PbO + NO_2 + O_2$
(3) $CuSO_4 + Fe \rightarrow FeSO_4 + Cu$
(4) $Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + NaCl$
(i) Select combination reaction from the reactions listed above. Is this reaction endothermic or exothermic?
(ii) Which one of these reactions represents a displacement reaction? State the colour change observed in this reaction.
(iii) (a) Identify from the above list a reaction in which thermal decomposition occurs. Write the balanced chemical equation to represent thermal decomposition of ferrous sulphate.
OR
(iii) (b) Write the word equation for the double displacement reaction given in the above list. Giving reason, state whether this reaction is a redox reaction or not.
(1) $CaO + H_2O \rightarrow Ca(OH)_2$
(2) $Pb(NO_3)_2 \rightarrow PbO + NO_2 + O_2$
(3) $CuSO_4 + Fe \rightarrow FeSO_4 + Cu$
(4) $Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + NaCl$
(i) Select combination reaction from the reactions listed above. Is this reaction endothermic or exothermic?
(ii) Which one of these reactions represents a displacement reaction? State the colour change observed in this reaction.
(iii) (a) Identify from the above list a reaction in which thermal decomposition occurs. Write the balanced chemical equation to represent thermal decomposition of ferrous sulphate.
OR
(iii) (b) Write the word equation for the double displacement reaction given in the above list. Giving reason, state whether this reaction is a redox reaction or not.
Answer
(i) Combination Reaction: Reaction (1) — $CaO + H_2O \rightarrow Ca(OH)_2$
Two reactants combine to form a single product (calcium hydroxide).
This reaction is exothermic — it releases a large amount of heat when quicklime reacts with water.
(ii) Displacement Reaction: Reaction (3) — $CuSO_4 + Fe \rightarrow FeSO_4 + Cu$
Iron (more reactive) displaces copper from copper sulphate.
Colour change: The solution changes from blue (copper sulphate) to green (iron(II) sulphate). A reddish-brown deposit of copper forms on the iron.
(iii-a) Thermal Decomposition: Reaction (2) — $Pb(NO_3)_2 \xrightarrow{\text{heat}} PbO + NO_2 + O_2$
Lead nitrate decomposes on heating, releasing brown fumes of $NO_2$.
Balanced equation for thermal decomposition of ferrous sulphate: $$2FeSO_4 \xrightarrow{\text{heat}} Fe_2O_3 + SO_2 + SO_3$$
OR (iii-b) Double Displacement Reaction: Reaction (4)
Word equation: Sodium sulphate + Barium chloride → Barium sulphate (white precipitate) + Sodium chloride
Is this a redox reaction? No. In this reaction, only ion exchange occurs between the two compounds. There is no change in the oxidation states of any element — sodium, barium, sulphate, and chloride all retain the same oxidation states before and after the reaction. Since no electron transfer takes place, it is not a redox reaction. It is a precipitation (double displacement) reaction.
Two reactants combine to form a single product (calcium hydroxide).
This reaction is exothermic — it releases a large amount of heat when quicklime reacts with water.
(ii) Displacement Reaction: Reaction (3) — $CuSO_4 + Fe \rightarrow FeSO_4 + Cu$
Iron (more reactive) displaces copper from copper sulphate.
Colour change: The solution changes from blue (copper sulphate) to green (iron(II) sulphate). A reddish-brown deposit of copper forms on the iron.
(iii-a) Thermal Decomposition: Reaction (2) — $Pb(NO_3)_2 \xrightarrow{\text{heat}} PbO + NO_2 + O_2$
Lead nitrate decomposes on heating, releasing brown fumes of $NO_2$.
Balanced equation for thermal decomposition of ferrous sulphate: $$2FeSO_4 \xrightarrow{\text{heat}} Fe_2O_3 + SO_2 + SO_3$$
OR (iii-b) Double Displacement Reaction: Reaction (4)
Word equation: Sodium sulphate + Barium chloride → Barium sulphate (white precipitate) + Sodium chloride
Is this a redox reaction? No. In this reaction, only ion exchange occurs between the two compounds. There is no change in the oxidation states of any element — sodium, barium, sulphate, and chloride all retain the same oxidation states before and after the reaction. Since no electron transfer takes place, it is not a redox reaction. It is a precipitation (double displacement) reaction.
(a) What is a chemical reaction? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place.
OR
(b) (i) Define a decomposition reaction. How can we say that (I) electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions? Mention the type of energy involved in each case.
(ii) The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other. Justify this statement with the help of chemical equations.
OR
(b) (i) Define a decomposition reaction. How can we say that (I) electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions? Mention the type of energy involved in each case.
(ii) The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other. Justify this statement with the help of chemical equations.
Answer
(a) Chemical Reaction: A process in which one or more substances (reactants) are converted into new substances (products) with different chemical properties, involving breaking and formation of chemical bonds.
(i) Change of Colour — Activity:
When an iron nail is dipped in copper sulphate solution, the blue colour fades and a brown deposit of copper forms. Iron displaces copper: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$ (Blue → colourless/pale green; brown copper deposits on nail.)
(ii) Change in Temperature — Activity:
When quicklime (CaO) is mixed with water, heat is released and the vessel becomes hot: $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ Rise in temperature shows an exothermic chemical change has occurred.
(b)(i) Decomposition Reaction: A reaction in which a single compound breaks down into two or more simpler substances, requiring energy input (heat, light, or electricity).
(I) Electrolysis of water — water (single compound) breaks into hydrogen and oxygen: $$2H_2O \xrightarrow{\text{electric current}} 2H_2 + O_2$$ Energy used: Electrical energy
(II) Blackening of silver bromide in sunlight — AgBr decomposes into silver and bromine: $$2AgBr \xrightarrow{\text{sunlight}} 2Ag + Br_2$$ Energy used: Light (photonic) energy
Both involve a single compound splitting into simpler products → confirmed decomposition reactions.
(b)(ii) Opposite Reactions:
Formation of CaO — Decomposition reaction: $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ Formation of Ca(OH)₂ — Combination reaction: $$CaO + H_2O \rightarrow Ca(OH)_2$$ In the first, one compound breaks into two simpler ones. In the second, two compounds join to form one — they are reverse processes (decomposition vs. combination), justifying the statement.
(i) Change of Colour — Activity:
When an iron nail is dipped in copper sulphate solution, the blue colour fades and a brown deposit of copper forms. Iron displaces copper: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$ (Blue → colourless/pale green; brown copper deposits on nail.)
(ii) Change in Temperature — Activity:
When quicklime (CaO) is mixed with water, heat is released and the vessel becomes hot: $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ Rise in temperature shows an exothermic chemical change has occurred.
(b)(i) Decomposition Reaction: A reaction in which a single compound breaks down into two or more simpler substances, requiring energy input (heat, light, or electricity).
(I) Electrolysis of water — water (single compound) breaks into hydrogen and oxygen: $$2H_2O \xrightarrow{\text{electric current}} 2H_2 + O_2$$ Energy used: Electrical energy
(II) Blackening of silver bromide in sunlight — AgBr decomposes into silver and bromine: $$2AgBr \xrightarrow{\text{sunlight}} 2Ag + Br_2$$ Energy used: Light (photonic) energy
Both involve a single compound splitting into simpler products → confirmed decomposition reactions.
(b)(ii) Opposite Reactions:
Formation of CaO — Decomposition reaction: $$CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$$ Formation of Ca(OH)₂ — Combination reaction: $$CaO + H_2O \rightarrow Ca(OH)_2$$ In the first, one compound breaks into two simpler ones. In the second, two compounds join to form one — they are reverse processes (decomposition vs. combination), justifying the statement.
Balance the following chemical equations:
1. $Fe + H_2O \rightarrow Fe_3O_4 + H_2$
2. $C_3H_8 + O_2 \rightarrow CO_2 + H_2O$
3. $Al + HCl \rightarrow AlCl_3 + H_2$
4. $Na + H_2O \rightarrow NaOH + H_2$
5. $NH_3 + O_2 \rightarrow NO + H_2O$
1. $Fe + H_2O \rightarrow Fe_3O_4 + H_2$
2. $C_3H_8 + O_2 \rightarrow CO_2 + H_2O$
3. $Al + HCl \rightarrow AlCl_3 + H_2$
4. $Na + H_2O \rightarrow NaOH + H_2$
5. $NH_3 + O_2 \rightarrow NO + H_2O$
Answer
1. $$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$$
(Balance Fe: 3; Balance O: 4 $H_2O$; Balance H: $4H_2$)
2. $$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$$ (Balance C: 3; Balance H: 4$H_2O$; Balance O: 5$O_2$)
3. $$2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2$$ (Balance Al: 2; Balance Cl: 6; Balance H: $3H_2$)
4. $$2Na + 2H_2O \rightarrow 2NaOH + H_2$$ (Balance Na: 2; Balance O: 2$H_2O$; Balance H: $H_2$)
5. $$4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O$$ (Balance N: 4; Balance H: $6H_2O$; Balance O: $5O_2$)
2. $$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$$ (Balance C: 3; Balance H: 4$H_2O$; Balance O: 5$O_2$)
3. $$2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2$$ (Balance Al: 2; Balance Cl: 6; Balance H: $3H_2$)
4. $$2Na + 2H_2O \rightarrow 2NaOH + H_2$$ (Balance Na: 2; Balance O: 2$H_2O$; Balance H: $H_2$)
5. $$4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O$$ (Balance N: 4; Balance H: $6H_2O$; Balance O: $5O_2$)
(a) Mention with reason the colour changes observed when:
(i) Silver chloride is exposed to sunlight.
(ii) A piece of zinc is dropped in copper sulphate solution. Justify your answer by giving reactions involved.
(b) Name the colour of precipitate formed when lead nitrate solution is added to potassium iodide solution.
(i) Silver chloride is exposed to sunlight.
(ii) A piece of zinc is dropped in copper sulphate solution. Justify your answer by giving reactions involved.
(b) Name the colour of precipitate formed when lead nitrate solution is added to potassium iodide solution.
Answer
(a)(i) Silver chloride exposed to sunlight:
Colour change: White → Grey/Dark
Reason: Sunlight causes photodecomposition of AgCl into silver (grey) and chlorine gas: $$2AgCl \xrightarrow{\text{sunlight}} 2Ag + Cl_2$$ The grey silver metal deposited causes the white AgCl to turn grey.
(a)(ii) Zinc dropped in copper sulphate solution:
Colour change: Blue solution → colourless; reddish-brown deposit forms on zinc.
Reason: Zinc is more reactive than copper and displaces it from its solution: $$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$$ Blue $CuSO_4$ is replaced by colourless $ZnSO_4$. Copper metal (reddish-brown) deposits on zinc.
(b) Lead nitrate + Potassium iodide: $$Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$$ The precipitate of lead iodide ($PbI_2$) is yellow in colour.
Colour change: White → Grey/Dark
Reason: Sunlight causes photodecomposition of AgCl into silver (grey) and chlorine gas: $$2AgCl \xrightarrow{\text{sunlight}} 2Ag + Cl_2$$ The grey silver metal deposited causes the white AgCl to turn grey.
(a)(ii) Zinc dropped in copper sulphate solution:
Colour change: Blue solution → colourless; reddish-brown deposit forms on zinc.
Reason: Zinc is more reactive than copper and displaces it from its solution: $$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$$ Blue $CuSO_4$ is replaced by colourless $ZnSO_4$. Copper metal (reddish-brown) deposits on zinc.
(b) Lead nitrate + Potassium iodide: $$Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$$ The precipitate of lead iodide ($PbI_2$) is yellow in colour.
Define a chemical reaction. State four observations to help us determine that a chemical reaction has taken place. Write one example of each observation with a balanced chemical equation.
Answer
Chemical Reaction: A process in which the original substances (reactants) lose their nature and identity and form new substances (products) with different properties, involving the breaking and forming of chemical bonds.
1. Change in Colour:
When iron is placed in copper sulphate solution, the blue colour fades and a brown deposit of copper forms: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$ 2. Evolution of Gas (Effervescence):
Sodium carbonate reacts with dilute hydrochloric acid, releasing $CO_2$ gas: $$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow$$ 3. Change in Temperature:
Quicklime (CaO) reacts with water, releasing heat (exothermic): $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ 4. Formation of Precipitate:
Lead nitrate reacts with potassium iodide, forming a yellow precipitate: $$Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$$
1. Change in Colour:
When iron is placed in copper sulphate solution, the blue colour fades and a brown deposit of copper forms: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$ 2. Evolution of Gas (Effervescence):
Sodium carbonate reacts with dilute hydrochloric acid, releasing $CO_2$ gas: $$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow$$ 3. Change in Temperature:
Quicklime (CaO) reacts with water, releasing heat (exothermic): $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ 4. Formation of Precipitate:
Lead nitrate reacts with potassium iodide, forming a yellow precipitate: $$Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$$
Identify the type of chemical reaction in the following statements and define each of them:
(a) Digestion of food in our body.
(b) Burning of methane gas.
(c) Heating of manganese dioxide with aluminium powder.
(d) The disappearance of the blue colour of the copper sulphate solution when iron filings are added to it.
(e) Adding dilute hydrochloric acid to sodium hydroxide solution to form sodium chloride and water.
(a) Digestion of food in our body.
(b) Burning of methane gas.
(c) Heating of manganese dioxide with aluminium powder.
(d) The disappearance of the blue colour of the copper sulphate solution when iron filings are added to it.
(e) Adding dilute hydrochloric acid to sodium hydroxide solution to form sodium chloride and water.
Answer
(a) Digestion of food — Decomposition Reaction:
Food (complex molecules) is broken down into simpler substances by enzymes. A single compound splits into two or more simpler products. $$AB \rightarrow A + B$$
(b) Burning of methane gas — Combination/Combustion Reaction:
Methane combines with oxygen, releasing heat and light. Two or more reactants form a single type of product set: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}$$
(c) Heating MnO₂ with aluminium — Redox (Displacement) Reaction:
Aluminium reduces manganese dioxide; oxidation states change. Electrons are transferred between species: $$3Al + 2MnO_2 \rightarrow 2Mn + Al_2O_3$$
(d) Blue colour of CuSO₄ disappears — Displacement Reaction:
A more reactive element (iron) displaces a less reactive element (copper) from its compound: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$
(e) HCl + NaOH — Double Displacement (Neutralisation) Reaction:
An acid reacts with a base to form a salt and water. Ions of two compounds exchange places: $$HCl + NaOH \rightarrow NaCl + H_2O$$
Food (complex molecules) is broken down into simpler substances by enzymes. A single compound splits into two or more simpler products. $$AB \rightarrow A + B$$
(b) Burning of methane gas — Combination/Combustion Reaction:
Methane combines with oxygen, releasing heat and light. Two or more reactants form a single type of product set: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}$$
(c) Heating MnO₂ with aluminium — Redox (Displacement) Reaction:
Aluminium reduces manganese dioxide; oxidation states change. Electrons are transferred between species: $$3Al + 2MnO_2 \rightarrow 2Mn + Al_2O_3$$
(d) Blue colour of CuSO₄ disappears — Displacement Reaction:
A more reactive element (iron) displaces a less reactive element (copper) from its compound: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$
(e) HCl + NaOH — Double Displacement (Neutralisation) Reaction:
An acid reacts with a base to form a salt and water. Ions of two compounds exchange places: $$HCl + NaOH \rightarrow NaCl + H_2O$$
The chemical reaction of solid calcium oxide with water is:
Solution
Answer: Option (D) is correct.
Explanation: When calcium oxide (CaO) reacts with water, it forms calcium hydroxide and releases heat: $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ • The reaction is exothermic — heat is released.
• Calcium hydroxide is a base, so its solution has a pH greater than 7 (alkaline/basic).
Explanation: When calcium oxide (CaO) reacts with water, it forms calcium hydroxide and releases heat: $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ • The reaction is exothermic — heat is released.
• Calcium hydroxide is a base, so its solution has a pH greater than 7 (alkaline/basic).
Give one chemical equation each for the chemical reaction in which the following changes occur (Mention the change in temperature/colour and/or the compound precipitated with the equation):
(a) Change in temperature
(b) Change in colour
(c) Formation of precipitate
(a) Change in temperature
(b) Change in colour
(c) Formation of precipitate
Answer
(a) Change in Temperature — Exothermic reaction:
Calcium oxide (quicklime) reacts with water, releasing heat (temperature increases): $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ Change: The mixture becomes hot — rise in temperature observed.
(b) Change in Colour:
Hydrated copper sulphate (blue) loses water on heating to form anhydrous copper sulphate (white): $$CuSO_4 \cdot 5H_2O \xrightarrow{\text{heat}} CuSO_4 + 5H_2O$$ Change: Blue colour → White.
(c) Formation of Precipitate:
Barium chloride reacts with sodium sulphate, forming a white precipitate of barium sulphate: $$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$$ Precipitate: $BaSO_4$ — white, insoluble solid.
Calcium oxide (quicklime) reacts with water, releasing heat (temperature increases): $$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat}$$ Change: The mixture becomes hot — rise in temperature observed.
(b) Change in Colour:
Hydrated copper sulphate (blue) loses water on heating to form anhydrous copper sulphate (white): $$CuSO_4 \cdot 5H_2O \xrightarrow{\text{heat}} CuSO_4 + 5H_2O$$ Change: Blue colour → White.
(c) Formation of Precipitate:
Barium chloride reacts with sodium sulphate, forming a white precipitate of barium sulphate: $$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$$ Precipitate: $BaSO_4$ — white, insoluble solid.
In which one of the following decomposition reactions is energy released?
Solution
Answer: Option (C) is correct.
Explanation: The decomposition of vegetable matter into compost is an exothermic process — microorganisms break down organic matter, releasing energy as heat. The compost heap warms up during this process.
Options (A), (B), and (D) all require external energy input (electrical, light, and thermal respectively) and are therefore endothermic.
Explanation: The decomposition of vegetable matter into compost is an exothermic process — microorganisms break down organic matter, releasing energy as heat. The compost heap warms up during this process.
Options (A), (B), and (D) all require external energy input (electrical, light, and thermal respectively) and are therefore endothermic.
Translate the following statements into balanced chemical equations:
(a) Aluminium reacts with copper chloride to form aluminium chloride and copper.
(b) Zinc reacts with sodium hydroxide to give sodium zincate and hydrogen gas.
(a) Aluminium reacts with copper chloride to form aluminium chloride and copper.
(b) Zinc reacts with sodium hydroxide to give sodium zincate and hydrogen gas.
Answer
(a) Aluminium + Copper chloride: $$2Al + 3CuCl_2 \rightarrow 2AlCl_3 + 3Cu$$
(Al is more reactive than Cu and displaces it — a displacement reaction.)
(b) Zinc + Sodium hydroxide: $$Zn + 2NaOH \rightarrow Na_2[Zn(OH)_4] + H_2$$ (Zinc reacts with sodium hydroxide to form sodium zincate and hydrogen gas.)
(b) Zinc + Sodium hydroxide: $$Zn + 2NaOH \rightarrow Na_2[Zn(OH)_4] + H_2$$ (Zinc reacts with sodium hydroxide to form sodium zincate and hydrogen gas.)
Define a displacement reaction. Name a displacement reaction which is highly exothermic and has its use in joining railway tracks. Explain the process with a balanced chemical equation of the reaction that occurs.
Answer
Displacement Reaction: A chemical reaction in which one element displaces another from a compound because the displacing element is more reactive. A more reactive element takes the place of a less reactive one:
$$A + BC \rightarrow AC + B$$
Highly exothermic displacement reaction — Thermite Reaction:
The reaction of aluminium powder with iron(III) oxide is used in thermite welding to join railway tracks.
Process: A mixture of aluminium powder ($Al$) and iron(III) oxide ($Fe_2O_3$) is ignited. Aluminium, being more reactive, displaces iron from its oxide. The reaction releases enormous heat (up to ~2500°C), producing molten iron that flows into the gap between railway tracks and welds them together.
Balanced Chemical Equation: $$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3 + \text{Heat}$$
Highly exothermic displacement reaction — Thermite Reaction:
The reaction of aluminium powder with iron(III) oxide is used in thermite welding to join railway tracks.
Process: A mixture of aluminium powder ($Al$) and iron(III) oxide ($Fe_2O_3$) is ignited. Aluminium, being more reactive, displaces iron from its oxide. The reaction releases enormous heat (up to ~2500°C), producing molten iron that flows into the gap between railway tracks and welds them together.
Balanced Chemical Equation: $$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3 + \text{Heat}$$
Select exothermic processes from the following:
(i) Dilution of acid
(ii) Burning of natural gas
(iii) Evaporation of water
(iv) Electrolysis of water
(i) Dilution of acid
(ii) Burning of natural gas
(iii) Evaporation of water
(iv) Electrolysis of water
Solution
Answer: Option (A) is correct.
• (i) Dilution of acid — Exothermic: Mixing concentrated acid with water releases heat. ✓
• (ii) Burning of natural gas — Exothermic: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$. Combustion always releases heat. ✓
• (iii) Evaporation of water — Endothermic: Water absorbs heat from surroundings to evaporate. ✗
• (iv) Electrolysis of water — Endothermic: Requires electrical energy input to decompose water. ✗
Exothermic processes: (i) and (ii) — Option (A).
• (i) Dilution of acid — Exothermic: Mixing concentrated acid with water releases heat. ✓
• (ii) Burning of natural gas — Exothermic: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$. Combustion always releases heat. ✓
• (iii) Evaporation of water — Endothermic: Water absorbs heat from surroundings to evaporate. ✗
• (iv) Electrolysis of water — Endothermic: Requires electrical energy input to decompose water. ✗
Exothermic processes: (i) and (ii) — Option (A).
From the following, select a process in which a combination reaction occurs:
Solution
Answer: Option (B) is correct.
Explanation: A combination reaction involves two or more reactants combining to form a single product.
• (B) Burning of carbon (coal): Carbon + oxygen → single product (carbon dioxide): $C + O_2 \rightarrow CO_2$ ✓
• (A) Black and white photography involves photolytic decomposition of AgBr. ✗
• (C) Burning of methane produces two products ($CO_2$ and $H_2O$) — not a combination reaction. ✗
• (D) Refining of copper involves displacement and electrolysis. ✗
Explanation: A combination reaction involves two or more reactants combining to form a single product.
• (B) Burning of carbon (coal): Carbon + oxygen → single product (carbon dioxide): $C + O_2 \rightarrow CO_2$ ✓
• (A) Black and white photography involves photolytic decomposition of AgBr. ✗
• (C) Burning of methane produces two products ($CO_2$ and $H_2O$) — not a combination reaction. ✗
• (D) Refining of copper involves displacement and electrolysis. ✗
Select from the following decomposition reactions in which the source of energy for decomposition is heat:
(i) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$
(ii) $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$
(iii) $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$
(iv) $2\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$
(i) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$
(ii) $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$
(iii) $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$
(iv) $2\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$
Solution
Answer: Option (D) is correct.
• (i) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$: Classic thermal decomposition — heat is required. ✓
• (ii) $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$: Electrolysis — requires electrical energy, not heat. ✗
• (iii) $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$: Photodecomposition — requires light energy. ✗
• (iv) $2\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$: Thermal decomposition of ferrous sulphate on strong heating. ✓
Thermal decomposition reactions: (i) and (iv) — Option (D).
• (i) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$: Classic thermal decomposition — heat is required. ✓
• (ii) $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$: Electrolysis — requires electrical energy, not heat. ✗
• (iii) $2\text{AgBr} \rightarrow 2\text{Ag} + \text{Br}_2$: Photodecomposition — requires light energy. ✗
• (iv) $2\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$: Thermal decomposition of ferrous sulphate on strong heating. ✓
Thermal decomposition reactions: (i) and (iv) — Option (D).
Explore More CBSE Class 10 Chemistry Chapters
Frequently Asked Questions
What does the Chemical Reactions and Equations chapter cover in CBSE Class 10 Chemistry? ⌄
The Chemical Reactions and Equations chapter covers how to write and balance chemical equations, the different types of chemical reactions (combination, decomposition, displacement, double displacement, and oxidation-reduction), and important concepts like the Law of Conservation of Mass, exothermic and endothermic reactions, and the reactivity series of metals. These concepts form the foundation of all chemistry studied in higher classes, making this chapter crucial to master early.
How many marks does Chemical Reactions and Equations carry in the CBSE Class 10 board exam? ⌄
Chemical Reactions and Equations is part of the Science paper’s Chemistry section, which carries significant weightage. Questions from this chapter typically appear as 1-mark MCQs, 2-mark short answer questions, 3-mark descriptive questions, and 5-mark long answer or case study questions. Balancing chemical equations and identifying reaction types are among the most frequently tested skills across all question types.
What are the most important topics students should focus on in Chemical Reactions and Equations? ⌄
Students must focus on balancing chemical equations (a skill tested almost every year), identifying types of reactions from given equations, writing balanced equations for common reactions (like electrolysis of water, thermal decomposition of lead nitrate, and thermite reaction), and understanding the reactivity series for displacement reactions. Assertion-Reason questions on exothermic vs. endothermic reactions and the differences between reaction types are also very frequently asked.
What are the most common mistakes students make in Chemical Reactions and Equations? ⌄
A very common error is leaving chemical equations unbalanced or changing formulae while balancing (students should only change coefficients, never subscripts). Students often confuse endothermic and exothermic reactions, or incorrectly classify reactions — for example, calling combustion of methane a “combination reaction” when it produces two products. Another frequent mistake is not knowing the colour changes in key reactions (iron nails in copper sulphate turning pale green, silver chloride turning grey in sunlight). Practising previous year questions helps identify and correct these gaps.
How does Angle Belearn help students score well in Chemical Reactions and Equations? ⌄
Angle Belearn’s CBSE specialists curate chapter-wise question banks drawn directly from real board papers, paired with clear, step-by-step solutions that show your child exactly how to structure answers for full marks. Regular practice with these verified questions builds the habit of writing balanced equations correctly and identifying reaction types confidently — skills that directly translate to marks in board exams. Your child gains both speed and accuracy by working through questions that mirror what actually appears in the exam.
