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Chemical Reactions and Equations: Class 10 Science Practice Questions

30 original exam-pattern questions with full answers, matched to the current CBSE Class 10 paper design, including case-based questions. Attempt each question before opening the answer — or start a free 14-day trial ↓ for the full bank.

Q1Case-based4 marks

Priya is helping her mother prepare lemonade. She squeezes lemon juice into a glass of water and adds a pinch of baking soda (sodium hydrogen carbonate). She immediately notices a fizzing action in the glass. Her mother tells her this is because a chemical reaction is taking place.

Priya is helping her mother prepare lemonade. She squeezes lemon juice into a glass of water and adds a pinch of baking soda (sodium hydrogen carbonate). She immediately notices a fizzing action in the glass. Her mother tells her this is because a chemical reaction is taking place.

(a) Identify the type of chemical reaction occurring when lemon juice (citric acid) reacts with baking soda.
(b) Name the gas responsible for the fizzing action observed by Priya.
(c) (i) Write the balanced chemical equation for the reaction between sodium hydrogen carbonate (NaHCO₃) and citric acid, represented here as a general acid HCl for simplicity:
NaHCO₃ + HCl → ?
(ii) Priya also notices that the mixture feels slightly cool to the touch after the reaction. What does this tell us about the nature of this reaction — exothermic or endothermic? Give one reason to support your answer.

Show answer
(a) The type of chemical reaction occurring is a double displacement reaction (also accepted: neutralisation reaction). Lemon juice contains citric acid and baking soda is a base; the acid and base react by exchanging ions to form a salt, water, and carbon dioxide gas. [1 mark]

(b) The gas responsible for the fizzing action is carbon dioxide (CO₂). [1 mark]

(c) (i) The balanced chemical equation is:

NaHCO₃ + HCl → NaCl + H₂O + CO₂↑

Sodium hydrogen carbonate reacts with hydrochloric acid to form sodium chloride (common salt), water, and carbon dioxide gas. The equation is balanced as written — one Na, one H (from HCO₃) + one H (from HCl) on left giving H₂O, one C, three O (from HCO₃) giving CO₂ + H₂O, and one Cl on each side. [1 mark]

(ii) The reaction is endothermic in nature. Priya notices the mixture feels cool to the touch, which means the reaction absorbs heat energy from the surroundings. When a reaction absorbs heat from the surroundings, the temperature of the surroundings (the glass and its contents) drops, making it feel cool. This is the characteristic of an endothermic reaction. [1 mark]
Q2Case-based4 marks

A science teacher sets up four test tubes for a class demonstration and records the following observations:

Test Tube P: A piece of zinc metal is dropped into dilute hydrochloric acid. Brisk effervescence is observed and the metal slowly dissolves.

Test Tube Q: Silver chloride powder is spread on a watch glass and placed in bright sunlight. The white powder gradually turns grey.

Test Tube R: Barium chloride solution is mixed with sodium sulphate solution. A dense white precipitate immediately forms.

Test Tube S: Powdered calcium oxide is carefully added to water in a beaker. The beaker becomes very hot to touch.

A science teacher sets up four test tubes for a class demonstration and records the following observations:

Test Tube P: A piece of zinc metal is dropped into dilute hydrochloric acid. Brisk effervescence is observed and the metal slowly dissolves.

Test Tube Q: Silver chloride powder is spread on a watch glass and placed in bright sunlight. The white powder gradually turns grey.

Test Tube R: Barium chloride solution is mixed with sodium sulphate solution. A dense white precipitate immediately forms.

Test Tube S: Powdered calcium oxide is carefully added to water in a beaker. The beaker becomes very hot to touch.

(a) Identify the type of chemical reaction occurring in Test Tube Q. Name the energy source that drives this reaction. [1 mark]

(b) Test Tube R shows a double displacement reaction. Write the balanced chemical equation for this reaction, including state symbols. [1 mark]

(c) (i) The teacher says that Test Tubes P and S represent two different types of reactions, yet both share a common characteristic. Name both reaction types and state the ONE common characteristic they share. [1 mark]
(ii) Explain why the reaction in Test Tube S is considered useful in everyday life. Give ONE specific application. [1 mark]

OR

(c) (i) In Test Tube P, hydrogen gas is evolved. A student claims that if dilute nitric acid were used instead of dilute hydrochloric acid, the same gas would be produced. Is the student correct? Justify your answer with a chemical reason. [1 mark]
(ii) Write the balanced chemical equation for the decomposition of silver chloride (Test Tube Q) and identify what is oxidised and what is reduced in this reaction. [1 mark]

Show answer
(a) Test Tube Q shows a photochemical decomposition reaction. The energy source that drives this reaction is sunlight (solar energy / light energy). [1 mark]

(b) The reaction in Test Tube R:

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)

[Award ½ mark for correct products with correct formula; ½ mark for balanced equation with correct state symbols including ↓ for BaSO₄] [1 mark]

(c)(i) Test Tube P shows a displacement reaction (zinc displaces hydrogen from HCl). Test Tube S shows a combination reaction (CaO + H₂O → Ca(OH)₂).

The ONE common characteristic they share is that both reactions are exothermic — they release heat energy to the surroundings. [1 mark]

(c)(ii) The reaction of calcium oxide with water (slaking of lime) produces calcium hydroxide [Ca(OH)₂], commonly called slaked lime. It is used in whitewashing walls of buildings, which gives them a clean white finish and acts as a disinfectant. [1 mark]

[Award 1 mark for any correct, specific application: e.g. whitewashing / making mortar / treating acidic soil / manufacturing cement — with a brief reason]

---

OR

(c)(i) The student is INCORRECT. When dilute nitric acid reacts with zinc (or any metal), hydrogen gas is NOT produced. This is because nitric acid is a strong oxidising agent and it immediately oxidises any hydrogen gas produced back to water. Instead, gases such as nitric oxide (NO) with dilute HNO₃ are produced, not H₂. [1 mark]

(c)(ii) Balanced equation for decomposition of silver chloride:

2AgCl(s) →[Sunlight]→ 2Ag(s) + Cl₂(g)↑

In this reaction:
— Silver (Ag⁺ in AgCl) GAINS electrons (is reduced to Ag metal).
— Chloride (Cl⁻ in AgCl) LOSES electrons (is oxidised to Cl₂ gas).
This is therefore a redox (oxidation-reduction) reaction. [1 mark]

[Award ½ mark for correct balanced equation; ½ mark for correctly identifying what is oxidised AND what is reduced — both required]
Q3Case-based4 marks

Riya is helping her mother cook dinner. She notices that when her mother lights the gas stove, the blue flame burns steadily and the food gets cooked. Later, her mother asks her to squeeze a lemon (which contains citric acid) over the cooked dal. Riya also sees a packet of baking powder in the kitchen shelf. She reads that baking powder contains sodium hydrogen carbonate (NaHCO₃), which releases CO₂ gas when heated, making cakes soft and fluffy.

Riya is helping her mother cook dinner. She notices that when her mother lights the gas stove, the blue flame burns steadily and the food gets cooked. Later, her mother asks her to squeeze a lemon (which contains citric acid) over the cooked dal. Riya also sees a packet of baking powder in the kitchen shelf. She reads that baking powder contains sodium hydrogen carbonate (NaHCO₃), which releases CO₂ gas when heated, making cakes soft and fluffy.

Based on this everyday context, answer the following questions:

(a) The burning of LPG (a hydrocarbon fuel) on the gas stove is an example of which type of chemical reaction? Name it. [1]

(b) Is the burning of LPG an exothermic or endothermic reaction? Give one reason. [1]

(c) Write the balanced chemical equation for the decomposition of sodium hydrogen carbonate (NaHCO₃) on heating. Also name the type of decomposition reaction it represents and give one reason for your answer. [2]

Show answer
(a) The burning of LPG is an example of a combustion reaction.
[1 mark]

(b) The burning of LPG is an exothermic reaction.
Reason: During combustion, heat and light energy are released into the surroundings.
[1 mark]

(c) Balanced chemical equation for thermal decomposition of sodium hydrogen carbonate:

2NaHCO₃ →(Heat)→ Na₂CO₃ + H₂O + CO₂↑

Type: It is a thermal decomposition reaction.
Reason: Because a single compound (sodium hydrogen carbonate) breaks down into simpler substances on the application of heat.
[1 mark for correct balanced equation + 1 mark for correct type with reason]

[Examiner note: Award ½ mark for correct but unbalanced equation. Accept 'decomposition reaction' without the qualifier 'thermal' for ½ mark; full mark only if 'thermal' or 'heat-induced' decomposition is specified with reason.]
Q4Case-based4 marks

A science teacher demonstrated the following experiment in class:

She took four separate test tubes and performed these reactions:

• Test Tube P: Added a few drops of silver nitrate solution to sodium chloride solution. A white solid settled at the bottom.
• Test Tube Q: Dissolved a small amount of calcium oxide in water and touched the outside of the test tube. It felt very hot.
• Test Tube R: Heated copper(II) sulphate pentahydrate crystals (blue). The solid turned white and water drops appeared on the cooler upper part of the tube.
• Test Tube S: Passed electricity through acidified water using carbon electrodes. Bubbles appeared at both electrodes.

A science teacher demonstrated the following experiment in class:

She took four separate test tubes and performed these reactions:

• Test Tube P: Added a few drops of silver nitrate solution to sodium chloride solution. A white solid settled at the bottom.
• Test Tube Q: Dissolved a small amount of calcium oxide in water and touched the outside of the test tube. It felt very hot.
• Test Tube R: Heated copper(II) sulphate pentahydrate crystals (blue). The solid turned white and water drops appeared on the cooler upper part of the tube.
• Test Tube S: Passed electricity through acidified water using carbon electrodes. Bubbles appeared at both electrodes.

Based on your understanding of chemical reactions, answer the following:

(a) Identify the type of chemical reaction occurring in Test Tube P and name the white solid formed. [1]

(b) Is the reaction in Test Tube Q exothermic or endothermic? Give one reason. [1]

(c) (i) Write the balanced chemical equation, with conditions, for the reaction in Test Tube R. Name the type of reaction. [1]
(ii) Write the balanced chemical equation, with conditions, for the reaction in Test Tube S. Name the type of reaction. [1]

OR

(c) The teacher told the class that the reactions in Test Tubes R and S are both examples of decomposition reactions, but they are triggered by different means.
(i) Name the specific type of decomposition occurring in each of Test Tube R and Test Tube S. [1]
(ii) Identify one more everyday example of the type of decomposition occurring in Test Tube S, and write its balanced chemical equation. [1]

Show answer
(a) The reaction in Test Tube P is a double displacement (precipitation) reaction.

When silver nitrate solution is added to sodium chloride solution, the silver ions (Ag⁺) and chloride ions (Cl⁻) combine to form an insoluble compound that settles as a white precipitate.

The white solid formed is silver chloride (AgCl).

[1 mark: ½ for correct reaction type + ½ for name of white solid]

---

(b) The reaction in Test Tube Q is exothermic.

When calcium oxide (CaO) dissolves in water to form calcium hydroxide [Ca(OH)₂], heat is released to the surroundings. This is why the outside of the test tube felt hot — energy is given out during the reaction, causing a rise in temperature of the surroundings.

[1 mark: ½ for correct identification + ½ for valid reason referencing heat release / rise in temperature]

---

(c) Main option:

(i) Test Tube R:

Observation: Blue copper(II) sulphate pentahydrate crystals turn white on heating, and water is driven off.

Balanced equation:

CuSO₄·5H₂O →(Heat)→ CuSO₄ + 5H₂O

(Copper sulphate pentahydrate → Copper sulphate (anhydrous) + Water)

Type of reaction: Thermal decomposition reaction — because a single compound breaks down into simpler substances on the application of heat.

[½ mark for correct balanced equation with condition; ½ mark for correct reaction type]

(ii) Test Tube S:

Observation: Acidified water (H₂O) is split into hydrogen gas at the cathode and oxygen gas at the anode on passing electricity.

Balanced equation:

2H₂O →(Electrolysis)→ 2H₂↑ + O₂↑

Type of reaction: Electrolytic decomposition (electrolysis) reaction — because a single compound (water) is decomposed by the application of electrical energy.

[½ mark for correct balanced equation with condition; ½ mark for correct reaction type]

---

(c) OR option:

(i)
- Test Tube R: Thermal decomposition — the compound (CuSO₄·5H₂O) is decomposed by the application of heat energy.
- Test Tube S: Electrolytic decomposition (electrolysis) — the compound (water) is decomposed by the application of electrical energy.

[½ + ½ mark for correctly naming both types with the energy source]

(ii) An everyday example of electrolytic decomposition is the decomposition of lead(II) bromide during electrolysis:

PbBr₂ →(Electrolysis)→ Pb + Br₂

Alternatively, the decomposition of sodium chloride solution (brine) is also acceptable:

2NaCl(aq) + 2H₂O(l) →(Electrolysis)→ 2NaOH(aq) + Cl₂↑ + H₂↑

[½ mark for a correct named everyday example; ½ mark for its correctly balanced equation with condition 'Electrolysis' written above arrow]

---

Examiner note: Award marks for any chemically correct and balanced equation. Deduct ½ mark for each unbalanced equation. Accept state symbols where provided. Conditions must be written above/below the arrow, not in prose.
Q5Case-based4 marks

Mia is a Class 10 student doing a home experiment. She dissolves a small piece of zinc metal in dilute sulphuric acid taken in a conical flask and observes brisk effervescence. She then takes a separate test tube containing copper turnings and adds the same dilute sulphuric acid — no reaction occurs. Curious, she adds a few drops of dilute nitric acid to another piece of copper and this time observes that the copper slowly dissolves, a colourless gas is produced which turns reddish-brown in air, and no effervescence of the kind seen with zinc is noticed.

Mia is a Class 10 student doing a home experiment. She dissolves a small piece of zinc metal in dilute sulphuric acid taken in a conical flask and observes brisk effervescence. She then takes a separate test tube containing copper turnings and adds the same dilute sulphuric acid — no reaction occurs. Curious, she adds a few drops of dilute nitric acid to another piece of copper and this time observes that the copper slowly dissolves, a colourless gas is produced which turns reddish-brown in air, and no effervescence of the kind seen with zinc is noticed.

Read the above scenario carefully and answer the following questions:
(a) Identify the type of reaction taking place when zinc reacts with dilute sulphuric acid. Give one reason for your answer.
(b) Why does copper not react with dilute sulphuric acid? What does this tell us about the position of copper in the reactivity series?
(c) (i) Name the colourless gas produced when copper reacts with dilute nitric acid and explain why it turns reddish-brown in air.
(ii) Why is hydrogen gas NOT produced when a metal reacts with dilute nitric acid, even though hydrogen gas is produced when the same metal reacts with dilute sulphuric acid?

Show answer
(a) The reaction between zinc and dilute sulphuric acid is a displacement reaction (also accepted: single displacement reaction).
Reason: Zinc is higher in the reactivity series than hydrogen, so it displaces hydrogen from the dilute sulphuric acid to form zinc sulphate and hydrogen gas.
Balanced equation:
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂↑
[1 mark: reaction type + reason + equation; accept equation without state symbols for 1 mark]

(b) Copper does not react with dilute sulphuric acid because copper is lower than hydrogen in the reactivity series. Only metals placed above hydrogen in the reactivity series can displace hydrogen from dilute acids. Since copper is below hydrogen, it cannot displace hydrogen from H₂SO₄ and hence no reaction occurs.
This tells us that copper is less reactive than hydrogen, i.e., copper is placed below hydrogen in the electrochemical / reactivity series.
[1 mark: correct reason with reactivity series reference]

(c)
(i) The colourless gas produced when copper reacts with dilute nitric acid is nitric oxide (NO).
Balanced equation:
3Cu(s) + 8HNO₃(dilute)(aq) → 3Cu(NO₃)₂(aq) + 4H₂O(l) + 2NO↑(g)
Nitric oxide (NO) is a colourless gas. When it comes in contact with oxygen (O₂) present in air, it is immediately oxidised to nitrogen dioxide (NO₂), which is a reddish-brown gas. This is why the colourless gas turns reddish-brown in air.
Reaction: 2NO(g) + O₂(g) → 2NO₂(g) [reddish-brown]
[1 mark: name of gas + reason for colour change + equation for Cu + dil. HNO₃]

(ii) Dilute nitric acid (HNO₃) is a strong oxidising agent. Even though the metal displaces hydrogen from the acid, the HNO₃ immediately oxidises any hydrogen gas (H₂) produced back to water (H₂O). Therefore, hydrogen gas is not liberated; instead, the reduction of HNO₃ itself produces nitrogen oxides (NO or NO₂) as the gaseous product.
(Verbatim MS phrase: 'Nitric acid is a strong oxidising agent and oxidises H₂ gas produced to water.')
[1 mark: correct explanation — HNO₃ is a strong oxidising agent that oxidises H₂ to water]

[Total: 4 marks — (a) 1 mark, (b) 1 mark, (c)(i) 1 mark, (c)(ii) 1 mark]
Q6Case-based4 marks

A student is investigating how iron behaves in different chemical environments. She sets up three test tubes:

• Test tube P: Iron nails placed in dilute sulphuric acid (H₂SO₄)
• Test tube Q: Iron nails placed in copper sulphate solution (CuSO₄)
• Test tube R: Iron nails placed in silver nitrate solution (AgNO₃)

In Test tube P, she observes brisk effervescence and the iron slowly dissolves. In Test tube Q, she notices the iron surface turns reddish-brown and the blue colour of the solution fades. In Test tube R, she observes a shiny grey deposit forming on the iron nail and the solution becomes pale green.

Read the following passage and answer the questions that follow.

A student is investigating how iron behaves in different chemical environments. She sets up three test tubes:

• Test tube P: Iron nails placed in dilute sulphuric acid (H₂SO₄)
• Test tube Q: Iron nails placed in copper sulphate solution (CuSO₄)
• Test tube R: Iron nails placed in silver nitrate solution (AgNO₃)

In Test tube P, she observes brisk effervescence and the iron slowly dissolves. In Test tube Q, she notices the iron surface turns reddish-brown and the blue colour of the solution fades. In Test tube R, she observes a shiny grey deposit forming on the iron nail and the solution becomes pale green.

(a) Identify the type of chemical reaction occurring in Test tube P. Give one reason.
(b) In Test tube Q, which metal is deposited on the iron nail? Write the balanced chemical equation for this reaction.
(c) The student claims that both reactions in Test tubes Q and R are of the same type, but the reaction in Test tube R releases more energy than in Test tube Q. Justify whether her claim about the reaction type is correct. Also identify which substance is oxidised and which is reduced in the reaction occurring in Test tube R.

Show answer
(a) The reaction in Test tube P is a displacement reaction (also a redox reaction). Iron displaces hydrogen from dilute sulphuric acid because iron is more reactive than hydrogen (iron is above hydrogen in the reactivity series). The balanced equation is:

Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂↑

[1 mark: correct type named + reason based on reactivity series]

(b) Copper (Cu) is deposited on the iron nail.

Iron displaces copper from copper sulphate solution because iron is higher in the reactivity series than copper.

Balanced equation:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

[1 mark: correct metal named + balanced equation with state symbols]

(c) The student's claim about the reaction TYPE is CORRECT. Both reactions in Test tubes Q and R are displacement reactions — in each case, a more reactive metal (iron) displaces a less reactive metal (copper or silver) from its salt solution. Iron is above both copper and silver in the reactivity series.

However, her claim about energy cannot be confirmed from the information given — the passage does not provide any energy data, and CBSE syllabus does not require comparison of energy released in displacement reactions at this level.

For the reaction in Test tube R:

Fe(s) + 2AgNO₃(aq) → Fe(NO₃)₂(aq) + 2Ag(s)

• Substance oxidised: Iron (Fe)
Fe loses 2 electrons: Fe → Fe²⁺ + 2e⁻ (its oxidation state increases from 0 to +2)

• Substance reduced: Silver ions (Ag⁺)
Ag⁺ gains 1 electron: Ag⁺ + e⁻ → Ag (its oxidation state decreases from +1 to 0)

It is a displacement reaction as well as a redox reaction because iron is oxidised and silver ions are reduced simultaneously.

[2 marks: reaction type correctly justified with reactivity series reasoning (1 mark) + both oxidised and reduced substances correctly identified with electron-transfer reasoning (1 mark)]
Q7Case-based4 marks

A science teacher performed the following demonstration in class. She took three test tubes labelled P, Q, and R. In test tube P, she added a piece of magnesium ribbon and poured dilute hydrochloric acid over it — brisk effervescence was observed and the test tube became warm. In test tube Q, she placed silver chloride (AgCl) near a window in sunlight for 20 minutes — the white solid gradually turned grey. In test tube R, she mixed solutions of barium chloride (BaCl₂) and sodium sulphate (Na₂SO₄) — a dense white precipitate was immediately formed.

A science teacher performed the following demonstration in class:

She took three test tubes labelled P, Q, and R.

• In test tube P, she added a piece of magnesium ribbon and poured dilute hydrochloric acid over it. Brisk effervescence was observed and the test tube became warm.

• In test tube Q, she dissolved silver chloride (AgCl) in a beaker, placed the beaker near a window in sunlight for 20 minutes. The white solid gradually turned grey.

• In test tube R, she mixed solutions of barium chloride (BaCl₂) and sodium sulphate (Na₂SO₄). A dense white precipitate was immediately formed.

Based on this demonstration, answer the following:

(a) Identify the type of chemical reaction taking place in test tube P. Give one reason for your answer. [1]

(b) The reaction in test tube Q is used in black-and-white photography. Identify the type of reaction and write the balanced chemical equation for it. [1]

(c) (i) Write the balanced chemical equation for the reaction in test tube R, using correct state symbols.
(ii) Give one reason why this type of reaction is called a double displacement reaction. [2]

OR

(c) (i) The teacher noted that the test tube P felt warm after the reaction. What does this indicate about the energy change in this reaction? Name this type of reaction based on energy change.
(ii) Write the balanced chemical equation for the reaction in test tube P with correct state symbols. [2]

Show answer
(a) The reaction in test tube P is a displacement reaction (also accepted: single displacement reaction). [½ mark]
Reason: Magnesium is more reactive than hydrogen and therefore displaces hydrogen from dilute hydrochloric acid, producing magnesium chloride and hydrogen gas. [½ mark]

(b) The reaction in test tube Q is a photochemical decomposition reaction — it is triggered by light (sunlight). [½ mark]
Balanced chemical equation:

2AgCl(s) →[Sunlight]→ 2Ag(s) + Cl₂(g) [½ mark]

(The grey colour is due to the formation of fine silver metal particles.)

(c)
(i) Balanced chemical equation for the reaction in test tube R:

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq) [1 mark]

(State symbols mandatory: (aq) for dissolved reactants and NaCl; (s)↓ for the insoluble white precipitate of barium sulphate.)

(ii) This reaction is called a double displacement reaction because the two reactant compounds (BaCl₂ and Na₂SO₄) exchange their ions with each other — Ba²⁺ combines with SO₄²⁻ and Na⁺ combines with Cl⁻ — forming two new compounds. [1 mark]

OR

(c)
(i) The test tube P felt warm because heat energy is released during the reaction. This indicates that the reaction is exothermic — the chemical energy of the reactants is higher than that of the products, and the difference is released as heat to the surroundings. [1 mark]

(ii) Balanced chemical equation for the reaction in test tube P:

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)↑ [1 mark]

(State symbols: Mg is solid, HCl is in aqueous solution, MgCl₂ dissolves in water, and H₂ is a gas evolved upward.)
Q8Case-based4 marks

A Class 10 student, Riya, is studying decomposition reactions in her chemistry lab. Her teacher shows her three different experiments. Experiment 1: A white powder (Lead nitrate) is heated in a test tube. A reddish-brown gas is produced and a yellow residue remains. Experiment 2: A green powder (Ferrous sulphate crystals) is heated. The green colour changes and a gas with the smell of burning sulphur is produced. Experiment 3: Silver chloride (a white solid) is kept in a transparent container near a window on a sunny day. It slowly turns grey.

A Class 10 student, Riya, is studying decomposition reactions in her chemistry lab. Her teacher shows her three different experiments:

Experiment 1: A white powder (Lead nitrate) is heated in a test tube. A reddish-brown gas is produced and a yellow residue remains.

Experiment 2: A green powder (Ferrous sulphate crystals) is heated. The green colour changes and a gas with the smell of burning sulphur is produced.

Experiment 3: Silver chloride (a white solid) is kept in a transparent container near a window on a sunny day. It slowly turns grey.

Based on the above information, answer the following questions:

(a) Identify the type of decomposition reaction occurring in Experiment 3. Give one reason for your answer. [1 mark]

(b) In Experiment 1, the reddish-brown gas produced is NO₂. Name the type of decomposition reaction in Experiment 1. [1 mark]

(c) Write the balanced chemical equations for Experiment 1 and Experiment 2, with appropriate conditions. [2 marks]

OR

(c) For Experiment 2, the residue left after heating ferrous sulphate is reddish-brown. (i) Name this residue. (ii) Write the balanced chemical equation for Experiment 2, with appropriate conditions. (iii) Is this reaction exothermic or endothermic? Give one reason. [2 marks]

Show answer
(a) Experiment 3 is a photochemical decomposition reaction.
Reason: Silver chloride decomposes in the presence of sunlight (light energy), not heat or electricity. [1 mark]

(b) Experiment 1 is a thermal decomposition reaction, because lead nitrate decomposes on heating (heat energy is used). [1 mark]

(c) Balanced chemical equations:

Experiment 1 — Decomposition of Lead Nitrate:

Observation: White solid turns yellow (PbO formed); reddish-brown gas (NO₂) evolved.

2Pb(NO₃)₂ →(Heat)→ 2PbO + 4NO₂↑ + O₂↑

(Lead nitrate → Lead oxide + Nitrogen dioxide + Oxygen)

Experiment 2 — Decomposition of Ferrous Sulphate:

Observation: Green colour of ferrous sulphate disappears; reddish-brown residue (Fe₂O₃) and gases with smell of burning sulphur (SO₂, SO₃) are produced.

2FeSO₄ →(Heat)→ Fe₂O₃ + SO₂↑ + SO₃↑

(Ferrous sulphate → Ferric oxide + Sulphur dioxide + Sulphur trioxide) [1 + 1 = 2 marks]

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OR (c)

(i) The reddish-brown residue formed is Ferric oxide (Fe₂O₃). [½ mark]

(ii) Balanced chemical equation for Experiment 2:

Observation: Green ferrous sulphate loses its green colour on heating; gases with burning sulphur odour are evolved and reddish-brown ferric oxide remains.

2FeSO₄ →(Heat)→ Fe₂O₃ + SO₂↑ + SO₃↑

(Ferrous sulphate → Ferric oxide + Sulphur dioxide + Sulphur trioxide) [1 mark]

(iii) This reaction is endothermic.
Reason: The reaction requires heat energy to be supplied continuously to decompose ferrous sulphate. Energy is absorbed from the surroundings, not released. [½ mark]
Q9MCQ1 mark

When calcium carbonate is heated strongly, it undergoes thermal decomposition. Identify the solid product 'P' formed in the following reaction:

CaCO₃ →(Δ) 'P' + CO₂

Show answer
Option (B) is correct.

Explanation: When calcium carbonate is heated strongly, it undergoes thermal decomposition — a reaction in which a single compound breaks down into two or more simpler substances on application of heat. CaCO₃ →(Δ) CaO + CO₂. The solid product 'P' is calcium oxide (CaO), commonly known as quicklime. Calcium hydroxide (Ca(OH)₂) is formed only when CaO subsequently reacts with water, not directly from the decomposition of CaCO₃; calcium sulphate and calcium chloride contain sulphate and chloride ions respectively, which are absent from the reactant, so options (A), (C), and (D) are incorrect.
Q10MCQ1 mark

When lead nitrate [Pb(NO₃)₂] is heated strongly, it decomposes to produce lead oxide (PbO), a reddish-brown gas, and oxygen. The reddish-brown gas released during this reaction is:

Show answer
Option (A) is correct.

Explanation: Lead nitrate undergoes thermal decomposition when heated strongly, according to the equation: 2Pb(NO₃)₂ →(Heat)→ 2PbO + 4NO₂↑ + O₂↑. Nitrogen dioxide (NO₂) is a reddish-brown, pungent gas — this characteristic colour is the basis of identification. SO₂ and CO₂ contain no nitrogen and cannot be produced from a nitrate compound, and nitrogen monoxide (NO) is a colourless gas, not reddish-brown.
Q11Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper is obtained and the black colour of copper oxide disappears.
Reason (R): In this reaction, hydrogen gas gains oxygen to form water, so hydrogen is oxidised, while copper oxide loses oxygen, so copper oxide is reduced.

Show answer
Option (a) is correct.

The Assertion is true: when copper oxide (CuO, black solid) reacts with hydrogen gas, it is reduced to copper (reddish-brown solid) and water is formed, so the black colour disappears. The Reason is also true: hydrogen gains oxygen to form water and is therefore oxidised, while copper oxide loses oxygen to form copper and is therefore reduced — this is a redox (oxidation-reduction) reaction. The Reason correctly and completely explains the observation stated in the Assertion, because it is precisely this simultaneous oxidation of hydrogen and reduction of copper oxide that accounts for the disappearance of the black colour and the formation of copper.
Q12MCQ1 mark

When phosphorus burns in excess oxygen, a white solid product is formed. The observations recorded during this reaction are listed below:
(i) White fumes are produced during the reaction.
(ii) A white powdery solid (phosphorus pentoxide, P₂O₅) is formed.
(iii) The reaction releases a large amount of heat and light.
(iv) The aqueous solution of the white solid turns red litmus to blue.

Which of the following observations is INCORRECT?

Show answer
Option (D) is correct.

Explanation: When phosphorus burns in excess oxygen, phosphorus pentoxide (P₂O₅) is formed. P₂O₅ dissolves in water to form phosphoric acid: P₂O₅ + 3H₂O → 2H₃PO₄. Since phosphoric acid is an acid, its aqueous solution turns blue litmus to red — not red litmus to blue. Turning red litmus to blue is the property of a base, not an acid. Therefore, observation (iv) is incorrect. Observations (i), (ii), and (iii) — white fumes, white powdery solid, and release of heat and light — are all correct for this exothermic combustion reaction.
Q13MCQ1 mark

Which of the following reactions is an example of electrolytic decomposition?

Show answer
Option (A) is correct.

Explanation: Electrolytic decomposition is a type of decomposition reaction in which a compound breaks down into simpler substances when electric current is passed through it. In Option (A), water decomposes into hydrogen and oxygen gas on passing electric current — 2H₂O(l) → 2H₂(g) + O₂(g) — making it a classic example of electrolytic decomposition. Option (B) is thermal decomposition (lead nitrate breaks down on heating), Option (C) is photochemical decomposition (silver chloride decomposes in sunlight), and Option (D) is also thermal decomposition (calcium carbonate decomposes on heating). Only Option (A) uses electric current as the energy source for decomposition.
Q14Short Answer1 mark

Assertion (A): When iron nails are added to blue copper sulphate solution, the blue colour of the solution fades gradually.
Reason (R): Iron is more reactive than copper and displaces copper from copper sulphate solution.

Show answer
Option (a) is correct. The Assertion is true: when iron nails are placed in copper sulphate solution, the blue colour fades as copper is displaced and a reddish-brown deposit of copper forms on the iron nails. The Reason is also true: iron is higher in the reactivity series than copper, so it displaces copper from its salt solution according to the reaction Fe + CuSO₄ → FeSO₄ + Cu. The Reason correctly and directly explains the Assertion — the fading of blue colour occurs precisely because iron displaces copper from the solution, forming pale green FeSO₄ in place of blue CuSO₄. This is a single displacement reaction.
Q15Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper is obtained and water is formed.
Reason (R): In this reaction, hydrogen gains oxygen and gets oxidised, while copper oxide loses oxygen and gets reduced.

Show answer
Option (a) is correct.

The Assertion is true: when copper oxide (CuO) reacts with hydrogen gas (H₂), copper metal (Cu) is obtained and water (H₂O) is formed, as represented by the equation CuO + H₂ → Cu + H₂O. The Reason is also true: in this reaction, hydrogen gains oxygen to form water and is therefore oxidised, while copper oxide loses oxygen to form copper and is therefore reduced — making this a redox (oxidation-reduction) reaction. Since the Reason correctly identifies the oxidation and reduction occurring in the reaction and directly explains why the products stated in the Assertion are formed, R is the correct explanation of A.
Q16Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper oxide is reduced to copper.
Reason (R): Reduction is the process of gain of oxygen or loss of hydrogen by a substance during a chemical reaction.

Show answer
Option (c) is correct.

The Assertion is true: when copper oxide reacts with hydrogen gas, the reaction CuO + H₂ → Cu + H₂O shows that copper oxide loses oxygen and is thereby reduced to copper. The Reason, however, is false — it states that reduction is the gain of oxygen or loss of hydrogen, which is precisely the opposite of the correct definition. Reduction is actually the loss of oxygen or the gain of hydrogen by a substance during a chemical reaction. Since the Reason contains an incorrect definition, it cannot explain the Assertion.
Q17MCQ1 mark

Which of the following is an example of a thermal decomposition reaction?

Show answer
Option (A) is correct.

Explanation: Thermal decomposition is a type of decomposition reaction in which a single compound breaks down into two or more simpler substances upon the application of heat energy. Heating calcium carbonate gives calcium oxide and carbon dioxide — CaCO₃ →(Heat)→ CaO + CO₂ — and the sole driving energy is thermal, making it a classic thermal decomposition reaction. Option (B) is incorrect because electrolysis of water uses electrical energy, not heat, and is therefore electrolytic decomposition. Option (C) is incorrect because decomposition of silver bromide is driven by sunlight and is a photochemical (photo-decomposition) reaction. Option (D) is incorrect because combination of hydrogen and oxygen to form water is a combination reaction, not a decomposition reaction.
Q18MCQ1 mark

Identify the gaseous product 'Y' obtained when ferrous sulphate crystals are heated strongly:

FeSO₄ → Fe₂O₃ + 'Y' + SO₃

Show answer
Option (A) is correct.

Explanation: The thermal decomposition of ferrous sulphate follows the balanced equation: 2FeSO₄ →(Heat)→ Fe₂O₃ + SO₂↑ + SO₃↑. Since the question already identifies SO₃ as one product, the unknown gaseous product 'Y' is SO₂ (sulphur dioxide). CO₂, H₂S, and NO₂ contain carbon, hydrogen, and nitrogen respectively — none of which are present in FeSO₄ — so options (B), (C), and (D) are eliminated.
Q19Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper is obtained and water is formed.
Reason (R): In this reaction, hydrogen gains oxygen and is oxidised, while copper oxide loses oxygen and is reduced, making it a redox reaction.

Show answer
Option (a) is correct.

The Assertion is true — when copper oxide reacts with hydrogen gas, the products are copper and water: CuO + H₂ → Cu + H₂O. The Reason is also true and correctly explains the Assertion. In this reaction, hydrogen (H₂) gains oxygen to form water (H₂O), so hydrogen is oxidised; copper oxide (CuO) loses oxygen to form copper (Cu), so copper oxide is reduced. Since oxidation and reduction occur simultaneously, this is indeed a redox reaction, and the redox nature of the reaction is precisely what explains why both copper and water are formed. Thus R is the correct explanation of A.
Q20Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper is obtained and water is formed.
Reason (R): In this reaction, hydrogen gains oxygen to form water, so hydrogen is oxidised, while copper oxide loses oxygen to form copper, so copper oxide is reduced.

Show answer
Option (a) is correct.

The Assertion is true: when copper oxide reacts with hydrogen gas, copper and water are formed — CuO + H₂ → Cu + H₂O. The Reason is also true: hydrogen gains oxygen to form water and is therefore oxidised, while copper oxide loses oxygen to form copper and is therefore reduced, making this a redox reaction. The Reason correctly explains the Assertion because it identifies the mechanism — simultaneous oxidation of hydrogen and reduction of copper oxide — that accounts for the products stated in the Assertion.
Q21Short Answer1 mark

Assertion (A): When copper is deposited on an iron nail dipped in copper sulphate solution, the iron is oxidised and copper is reduced.
Reason (R): Oxidation is the gain of oxygen or loss of hydrogen, whereas reduction is the loss of oxygen or gain of hydrogen; in terms of electrons, oxidation is loss of electrons and reduction is gain of electrons.

Show answer
Option (a) is correct.

Explanation: Assertion A is true. When an iron nail is dipped in copper sulphate solution, the reaction Fe + CuSO₄ → FeSO₄ + Cu occurs; iron loses electrons (Fe → Fe²⁺ + 2e⁻) and is therefore oxidised, while copper ions gain electrons (Cu²⁺ + 2e⁻ → Cu) and are therefore reduced. Reason R is also true, as it correctly states that oxidation is the gain of oxygen, loss of hydrogen, or loss of electrons, and reduction is the loss of oxygen, gain of hydrogen, or gain of electrons. R is the correct explanation of A because the oxidation of iron and reduction of copper in this displacement reaction are explained directly by the electron-transfer definitions stated in R.
Q22MCQ1 mark

When silver bromide (AgBr) is exposed to sunlight, it undergoes decomposition. Which of the following correctly represents the products of this photolytic decomposition reaction?

Show answer
Option (A) is correct.

Explanation: Photolytic decomposition is a reaction in which a compound breaks down into simpler substances when exposed to light. When AgBr is exposed to sunlight, the reaction is: 2AgBr →(Sunlight)→ 2Ag + Br₂. The products are silver metal (Ag) and bromine gas (Br₂). Options (B), (C), and (D) introduce Ag₂O, AgBrO₃, and Ag₂S respectively, none of which are formed in this reaction. This reaction is the basis of black-and-white photography.
Q23Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper oxide is reduced to copper.
Reason (R): Reduction is defined as the gain of oxygen or loss of hydrogen during a chemical reaction.

Show answer
Option (c) is correct.

The Assertion is true: in the reaction CuO + H₂ → Cu + H₂O, copper oxide loses oxygen and is thus reduced to copper. However, the Reason is false. The Reason states that reduction is the gain of oxygen or loss of hydrogen — this is incorrect. Reduction is correctly defined as the loss of oxygen or gain of hydrogen. The Reason as stated actually describes oxidation (gain of oxygen), not reduction. Since the Assertion is true but the Reason is false, option (c) is correct.
Q24MCQ1 mark

Which of the following reactions represents a photolytic decomposition reaction?

Show answer
Option (A) is correct.

Explanation: Decomposition reactions are classified by the energy source that triggers them — thermal (heat), electrolytic (electricity), or photolytic (light). In Option (A), silver chloride (2AgCl) decomposes in the presence of sunlight to form silver (Ag) and chlorine gas (Cl₂), making it a photolytic decomposition reaction. Option (B) uses heat (Δ) to decompose CaCO₃, making it thermal decomposition. Option (C) uses electric current to decompose water, making it electrolytic decomposition. Option (D) also uses heat to decompose Pb(NO₃)₂, making it thermal decomposition.
Q25Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper is obtained and the reaction is considered a redox reaction.
Reason (R): In this reaction, hydrogen gas is oxidised to water while copper oxide is reduced to copper, involving simultaneous oxidation and reduction.

Show answer
Option (a) is correct.

The Assertion is true: when copper oxide (CuO) reacts with hydrogen gas (H₂), copper metal is obtained as a product. The Reason is also true and correctly explains the Assertion: in this reaction, hydrogen gas loses oxygen — meaning hydrogen is oxidised to form water (H₂O) — while copper oxide simultaneously gains electrons and loses oxygen, meaning copper oxide is reduced to copper (Cu). Since oxidation and reduction occur simultaneously in the same reaction, it is a redox reaction, which is precisely what the Assertion states. Therefore, R is the correct explanation of A.
Q26Short Answer1 mark

Assertion (A): When copper oxide reacts with hydrogen gas, copper is obtained and the black colour of copper oxide changes to brown.
Reason (R): In this reaction, hydrogen gas gains oxygen to form water, so hydrogen is oxidised, while copper oxide loses oxygen to form copper, so copper oxide is reduced.

Show answer
Option (a) is correct.

The Assertion is true: when copper oxide (black) reacts with hydrogen gas, copper (brown) is formed and water is produced — CuO + H₂ → Cu + H₂O. The Reason is also true: hydrogen gains oxygen to form water and is therefore oxidised, while copper oxide loses oxygen to form copper and is therefore reduced, making this a redox reaction. The Reason correctly and completely explains the Assertion because the observed colour change from black to brown is a direct consequence of CuO being reduced to Cu, which is precisely what R states. Hence R is the correct explanation of A.
Q27MCQ1 mark

When silver chloride (AgCl) is left exposed to sunlight, it turns grey. Select the correct statement that describes the type of decomposition involved in this change:

Show answer
Option (A) is correct.

Explanation: Decomposition reactions are classified by the energy source that triggers them — heat (thermal), electricity (electrolytic), or light (photolytic). When AgCl is exposed to sunlight, it absorbs light energy, which breaks the Ag–Cl bond: 2AgCl →(Sunlight)→ 2Ag + Cl₂. The silver produced appears grey, which is the observed colour change. Since the energy source is light (not heat or electrical energy), this is photolytic decomposition. Option (B) is incorrect because no heat is applied; Option (C) is incorrect because no electrical energy is involved; Option (D) is incorrect because the grey colour is due to the formation of silver metal, not oxidation.
Q28MCQ1 mark

Which of the following is NOT an example of electrolytic decomposition?

Show answer
Option (C) is correct.

Explanation: Electrolytic decomposition is a type of decomposition reaction in which a compound is broken down into simpler substances by passing electric current through it. Decomposition of water into hydrogen and oxygen (Option A), decomposition of molten sodium chloride into sodium and chlorine (Option B), and decomposition of aluminium oxide into aluminium and oxygen (Option D) all require electric current as the driving force and are therefore examples of electrolytic decomposition. The decomposition of silver chloride (AgCl) into silver and chlorine, however, occurs in the presence of sunlight and is classified as photochemical decomposition — not electrolytic decomposition. Since Option (C) does not involve electric current, it is NOT an example of electrolytic decomposition.
Q29MCQ1 mark

When copper nitrate [Cu(NO₃)₂] is heated strongly, it decomposes to produce a black solid, a reddish-brown gas, and oxygen gas. This reaction can be categorised as:

Show answer
Option (A) is correct.

Explanation: A decomposition reaction in which a compound breaks down into simpler substances due to the application of heat is called a thermal decomposition reaction. Here, copper nitrate is heated strongly, supplying thermal energy that causes it to decompose: 2Cu(NO₃)₂ →(Heat)→ 2CuO + 4NO₂↑ + O₂↑. The products — black copper oxide (CuO), reddish-brown nitrogen dioxide gas (NO₂), and oxygen gas (O₂) — confirm that one compound has broken into simpler substances using heat as the trigger. Option (B) is incorrect because electrolytic decomposition requires electricity, not heat. Option (C) is incorrect because photolytic decomposition requires light energy. Option (D) is incorrect because a combination reaction involves two or more substances combining to form a single product, which is the opposite of what occurs here.
Q30MCQ1 mark

A student performs two experiments with calcium carbonate (marble chips):

Experiment 1: She adds dilute hydrochloric acid to marble chips in a test tube. Brisk effervescence occurs immediately at room temperature and the gas turns lime water milky.

Experiment 2: She strongly heats marble chips in a separate test tube. After prolonged heating, the same gas is produced and a white solid residue (quicklime) remains.

Which of the following CORRECTLY identifies both reactions and explains why Experiment 1 occurs at room temperature while Experiment 2 requires strong heating?

Show answer
Option (c) is correct.

Explanation: In Experiment 1, marble chips (CaCO₃) react with dilute HCl in a double displacement (acid–carbonate) reaction: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑. The H⁺ ions from the acid directly attack and break the carbonate (CO₃²⁻) bond, chemically supplying the activation energy needed to release CO₂. Because the acid provides this energy at the molecular level, no external heating is required and the reaction proceeds spontaneously at room temperature. The CO₂ evolved turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O).

In Experiment 2, no acid is present. The only way to break the strong ionic lattice of CaCO₃ and cleave the C–O covalent bonds within the carbonate ion is to supply thermal energy externally. This is a thermal decomposition reaction: CaCO₃ →(Heat)→ CaO + CO₂↑. The white residue is quicklime (CaO). Strong, prolonged heating is required because the lattice energy of CaCO₃ is large and cannot be overcome at room temperature without a chemical reactant supplying energy.

Option (a) is incorrect because HCl is a reactant, not a catalyst, and neither reaction is a double displacement in Experiment 2. Option (b) is incorrect because thermal decomposition of CaCO₃ is endothermic (heat is absorbed, not released) and requires no spark. Option (d) is incorrect because Experiment 1 is not a simple displacement of Ca from HCl (Ca remains as Ca²⁺ throughout), and Experiment 2 is decomposition, not combination.

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