Board Previous Year Questions (Board PYQs)

About This Section

This section is a collection of questions asked in CBSE and other state board examinations over the past several years. Studying them will give you —

• An understanding of the pattern (what kinds of questions are asked)
• A sense of mark distribution (how many marks for which question)
• Practice in answer style (how to write effectively)
• Identification of frequently asked questions

Study Strategy:

1. Try to solve each question yourself first.
2. Match your answer length to the marks (1-mark = 1-2 sentences; 3-mark = 5-6 points; 5-mark = detailed explanation).
3. Show every step in numerical questions.
4. Write diagrams and equations clearly.
5. Solve all NCERT textbook exercises.

This collection is divided by mark levels — 1-mark (MCQ/short), 2-mark, 3-mark, and 5-mark.

1-Mark Questions (MCQ / Very Short)

Q1. [CBSE 2020] Which type of reaction is $A + B^- \rightarrow A^- + B$?

(a) Combination (b) Decomposition (c) Displacement (d) Double displacement

Solution: A is a free element displacing B. Answer: (c) Displacement.

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Q2. [CBSE 2019] The substance used for whitewashing walls is —

(a) Calcium carbonate (b) Calcium oxide (c) Calcium hydroxide (d) Calcium chloride

Solution: A solution of slaked lime is used — (c) $Ca(OH)_2$ (calcium hydroxide). It reacts with atmospheric $CO_2$ to form a shiny coat of $CaCO_3$.

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Q3. [CBSE 2018] The chemical formula of rust is —

(a) $FeO$ (b) $Fe_2O_3$ (c) $Fe_2O_3 \cdot xH_2O$ (d) $Fe_3O_4$

Solution: Rust = hydrated ferric oxide = (c) $Fe_2O_3 \cdot xH_2O$

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Q4. [CBSE 2017] Which gas is filled in chip packets?

(a) Hydrogen (b) Oxygen (c) Nitrogen (d) Helium

Solution: (c) Nitrogen — to prevent rancidity.

Q5. [CBSE 2016] Which of the following is a displacement reaction?

(a) $CaO + H_2O \rightarrow Ca(OH)_2$ (b) $Zn + 2HCl \rightarrow ZnCl_2 + H_2$ (c) $2KClO_3 \rightarrow 2KCl + 3O_2$ (d) $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$

Solution: (b) — Zn displaces H.

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Q6. [CBSE 2015] What is the white precipitate formed in Activity 1.10?

Solution: Barium sulphate ($BaSO_4$) — it is insoluble. Reaction: $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4\downarrow + 2NaCl$.

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Q7. [CBSE 2014] How is hydrogen gas identified?

Solution: Bring a burning matchstick close — hydrogen burns with a 'pop' sound. This is the famous test for $H_2$.

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Q8. [CBSE 2013] Why does a balanced chemical equation reflect the law of conservation of mass?

Solution: Because in a balanced equation, the number of atoms of each element is equal on both sides — mass is neither created nor destroyed.

2-Mark Questions

Q9. [CBSE 2020] Why is a magnesium ribbon cleaned with sandpaper before burning in air?

Solution:

1. Magnesium, when in air, develops a thin layer of MgO on its surface.
2. This MgO layer hinders combustion — it does not burn easily.
3. Sandpaper rubbing removes this layer.
4. Pure Mg can then react directly with oxygen.

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Q10. [CBSE 2019] What happens when slaked lime reacts with atmospheric $CO_2$? Write the chemical equation.

Solution:

1. Slaked lime ($Ca(OH)_2$) slowly reacts with atmospheric $CO_2$.
2. Product: Shiny white $CaCO_3$ (calcium carbonate).

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$

This is why a freshly whitewashed wall develops a shine after 2-3 days.

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Q11. [CBSE 2018] State the difference between exothermic and endothermic reactions.

Solution:

Aspect	Exothermic	Endothermic
Heat	Released	Absorbed
Temperature	Rises	Falls
Example	$CaO + H_2O$	$Ba(OH)_2 + 2NH_4Cl$

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Q12. [CBSE 2017] What happens when an iron nail is dipped in copper sulphate solution?

Solution:

1. Reaction: $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
2. Observations:

• The blue solution gradually turns pale green.
• A brown copper coating forms on the nail.

1. This is a displacement reaction.

Q13. [CBSE 2016] What is a double displacement reaction? Give one example.

Solution:

Definition: A reaction in which the ions of two reactants exchange partners to form two new compounds.

Form: $AB + CD \rightarrow AD + CB$

Example: $$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s)\downarrow + NaNO_3(aq)$$

A white $AgCl$ precipitate is formed.

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Q14. [CBSE 2015] Why is respiration considered an exothermic reaction?

Solution:

Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

1. New substances are formed ($CO_2$, $H_2O$) → chemical reaction.
2. Energy is released (as ATP) → exothermic.
3. This energy powers all body functions.

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Q15. [CBSE 2014] What is the volume ratio of hydrogen to oxygen in the electrolysis of water? Give the reason.

Solution:

1. Ratio: $H_2 : O_2 = 2 : 1$
2. Reason: Reaction $2H_2O \rightarrow 2H_2 + O_2$ — 2 molecules $H_2$ : 1 molecule $O_2$.
3. By Avogadro's law, at the same conditions, volume ∝ number of molecules.
4. So the volume of $H_2$ is always twice that of $O_2$.

3-Mark Questions

Q16. [CBSE 2020] What are the observations on heating ferrous sulphate crystals (Activity 1.5)? Write the chemical equation.

Solution:

Observations:

1. Green crystals turn brown.
2. A pungent smell (sulphur oxides) is released.
3. Crystals first lose their water of crystallisation, then decompose.

Reactions (two steps):

Step 1: $FeSO_4 \cdot 7H_2O \xrightarrow{\Delta} FeSO_4 + 7H_2O$

Step 2: $2FeSO_4(s) \xrightarrow{\Delta} Fe_2O_3(s) + SO_2(g) + SO_3(g)$

This is a decomposition (thermal) reaction.

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Q17. [CBSE 2019] Why are iron objects painted? Write the chemical reaction of rusting.

Solution:

1. Problem: Iron, in contact with air and moisture, undergoes rusting.
2. Rusting reaction: $$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$
3. What does paint do? Paint forms a physical barrier between iron and atmospheric $O_2$/moisture.
4. Result: Without contact with $O_2$ and moisture — no rust.
5. Application: Doors, windows, gates — all painted.

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Q18. [CBSE 2018] Write one reaction each for decomposition by heat, light, and electricity.

Solution:

(i) Thermal: $$CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)$$

(ii) Electrical: $$2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$$

(iii) Photochemical: $$2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)$$

(1 mark each)

Q19. [CBSE 2017] Identify the oxidised and reduced substances in — $CuO + H_2 \rightarrow Cu + H_2O$

Solution:

Substance	Initial	Final	Change	Conclusion
Cu	in CuO, with O	Cu — O lost	O loss	Reduction
H₂	H₂ alone	in H₂O, with O	O gain	Oxidation

Answer:

• Oxidised substance: $H_2$ (hydrogen)
• Reduced substance: $CuO$ (copper oxide)
• It is a redox reaction.

Oxidising agent: $CuO$ ; Reducing agent: $H_2$

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Q20. [CBSE 2016] Verify the law of conservation of mass with one numerical example.

Solution:

Question: How many grams of $Ca(OH)_2$ are formed by the complete reaction of 5.6 g $CaO$? (Ca=40, O=16, $Ca(OH)_2$=74)

Reaction: $CaO + H_2O \rightarrow Ca(OH)_2$

Molecular masses: $CaO = 56$, $H_2O = 18$, $Ca(OH)_2 = 74$

Conservation: $56 + 18 = 74$ ✓

From 5.6 g $CaO$:

• $H_2O$ = $\frac{18}{56} \times 5.6 = 1.8$ g
• $Ca(OH)_2$ = $\frac{74}{56} \times 5.6 = 7.4$ g
• Verification: $5.6 + 1.8 = 7.4$ ✓

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Q21. [CBSE 2015] What are the impacts of corrosion and rancidity in daily life? Write two preventive measures for each.

Solution:

Effects of corrosion:

• Metal articles become weak
• Bridges and machine safety affected

Prevention:

1. Painting
2. Galvanisation (Zn coat)

Effects of rancidity:

• Foods become stale
• Harmful effects on health

Prevention:

1. Adding antioxidants (BHA, BHT)
2. Packaging with nitrogen gas

5-Mark Questions

Q22. [CBSE 2020 — Long Answer]

(a) What is a chemical reaction? Give an example. (b) State five signs of a chemical reaction. (c) Balance the following — $Fe + H_2O \rightarrow Fe_3O_4 + H_2$ (d) Identify the type of Activity 1.10.

Solution:

(a) Chemical reaction — A process in which atomic bonds in the reactants break, new bonds form, and new substances are produced. Example: $C + O_2 \rightarrow CO_2$ (combustion of coal).

(b) Five signs:

1. Change in state
2. Change in colour
3. Evolution of gas
4. Change in temperature
5. Formation of precipitate

(c) Balancing:

• Balance O: $H_2O$ × 4 → $Fe + 4H_2O \rightarrow Fe_3O_4 + H_2$
• Balance H: $H_2$ × 4 → $Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$
• Balance Fe: $Fe$ × 3 → $3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$

Final answer: $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$

(d) Activity 1.10: $$Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl$$ This is a double displacement (precipitation) reaction.

Q23. [CBSE 2019 — Long Answer]

(a) State the difference between combination and decomposition. (b) Give one example of each. (c) Is it correct to say decomposition is the opposite of combination? Prove. (d) Name the three types of decomposition with examples.

Solution:

(a) Difference:

Aspect	Combination	Decomposition
Form	$A + B \rightarrow AB$	$AB \rightarrow A + B$
Reactants	2 or more	1
Products	1	2 or more
Energy	Usually exothermic	Usually endothermic

(b) Examples:

• Combination: $CaO + H_2O \rightarrow Ca(OH)_2$
• Decomposition: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$

(c) Yes, proof:

• Combination: A + B → AB
• Decomposition: AB → A + B
• These are reverse of each other.
• Paired example: $CaO + H_2O \rightleftarrows Ca(OH)_2$ (combination one way, decomposition the other).

(d) Three types of decomposition:

1. Thermal: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$

2. Electrical: $2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$

3. Photochemical: $2AgBr \xrightarrow{\text{light}} 2Ag + Br_2$

Q24. [CBSE 2018 — Long Answer]

(a) Define oxidation and reduction (in terms of gain/loss of oxygen). (b) What is a redox reaction? (c) Identify the oxidised and reduced substances in — $ZnO + C \rightarrow Zn + CO$ (d) Give two examples each of oxidising and reducing agents.

Solution:

(a)

• Oxidation: Gain of oxygen (or loss of hydrogen). Example: $2Cu + O_2 \rightarrow 2CuO$ — Cu oxidised.
• Reduction: Loss of oxygen (or gain of hydrogen). Example: $CuO + H_2 \rightarrow Cu + H_2O$ — CuO reduced.

(b) Redox reaction: A reaction in which oxidation of one substance and reduction of another happen simultaneously.

(c) $ZnO + C \rightarrow Zn + CO$:

• C gained O → C oxidised
• ZnO lost O → ZnO reduced
• This is a displacement + redox reaction.

(d) Oxidising agents: $O_2$, $KMnO_4$ Reducing agents: $H_2$, $C$

Q25. [CBSE 2017 — Long Answer]

(a) What is corrosion? Give one example. (b) State the two conditions necessary for rusting. (c) State four methods of preventing corrosion. (d) Define rancidity and state two methods of prevention.

Solution:

(a) Corrosion: Slow chemical destruction of a metal by air, moisture, and chemicals. Example: Rusting of iron ($4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$).

(b) Conditions for rusting:

1. Oxygen (atmospheric)
2. Water/moisture If even one is missing — no rust.

(c) Four methods of preventing corrosion:

1. Painting or oiling — physical barrier.
2. Galvanisation — Zn coating.
3. Alloying — stainless steel.
4. Chrome plating — Cr coating.

(d) Rancidity: Oxidative deterioration of fat-containing foods, making them go stale.

Two prevention methods:

1. Antioxidants (Vitamin E, BHA, BHT)
2. Packaging with nitrogen gas

Additional Multi-Topic Questions

Q26. [CBSE — Mock 2021] Balance the following reactions and identify the type: (i) $Fe + H_2SO_4 \rightarrow FeSO_4 + H_2$ (ii) $NaOH + HCl \rightarrow NaCl + H_2O$ (iii) $C_2H_4 + O_2 \rightarrow CO_2 + H_2O$

Solution:

(i): Already balanced — $Fe(s) + H_2SO_4(aq) \rightarrow FeSO_4(aq) + H_2(g)\uparrow$ Type: Displacement.

(ii): Already balanced — $NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$ Type: Neutralisation (a kind of double displacement).

(iii): $C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O$ Type: Combustion (exothermic).

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Q27. [CBSE — Mock 2020] A student mixed solutions of $H_2SO_4$ and $BaCl_2$. (a) What happens? (b) Write the reaction. (c) What type of reaction is it?

Solution:

(a) A white precipitate ($BaSO_4$) is formed.

(b) Reaction: $$BaCl_2(aq) + H_2SO_4(aq) \rightarrow BaSO_4(s)\downarrow + 2HCl(aq)$$

(c) Type: Double displacement (precipitation) reaction.

(This is a famous test for the $SO_4^{2-}$ ion.)

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Q28. [CBSE 2014 — Practical] If a silver spoon is placed in a blue vitriol ($CuSO_4$) solution, what happens? Explain.

Solution:

Activity series: $\ldots Cu > Hg > Ag > Au$

• Silver (Ag) is below copper (Cu).
• Rule: A metal below cannot displace one above.
• So no reaction will occur.
• The blue colour of the vitriol stays the same.

Numerical PYQs

Q29. [CBSE 2019 — Numerical] How much $C$ and $O_2$ are needed to form 22 g of $CO_2$? (C=12, O=16)

Solution:

1. Reaction: $C + O_2 \rightarrow CO_2$
2. Molecular masses: C=12, O₂=32, CO₂=44
3. Ratio: 12 g C + 32 g O₂ → 44 g CO₂
4. For 22 g CO₂:

• C = $\frac{12}{44} \times 22 = 6$ g
• O₂ = $\frac{32}{44} \times 22 = 16$ g

1. Verification: $6 + 16 = 22$ ✓

Answer: 6 g of carbon and 16 g of oxygen are needed.

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Q30. [CBSE 2018 — Numerical] How much $NH_3$ is formed from 14 g of $N_2$ and excess $H_2$? (N=14, H=1)

Solution:

1. Reaction: $N_2 + 3H_2 \rightarrow 2NH_3$
2. Molecular masses: $N_2 = 28$, $NH_3 = 17$, $2NH_3 = 34$
3. Ratio: 28 g N₂ → 34 g NH₃
4. From 14 g N₂: $\frac{34}{28} \times 14 = 17$ g
5. Answer: 17 g of ammonia is formed.

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Q31. [CBSE 2016 — Numerical] What volume of $H_2$ gas is liberated at STP from 32.5 g of Zn reacting with excess $H_2SO_4$? (Zn=65, molar volume=22.4 L)

Solution:

1. Reaction: $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$
2. Moles of Zn: $\frac{32.5}{65} = 0.5$ mol
3. Moles of $H_2$: 0.5 mol (1:1 ratio)
4. Volume at STP: $0.5 \times 22.4 = 11.2$ L
5. Answer: 11.2 L of hydrogen at STP.

Higher-Order-Thinking Questions (HOTS)

Q32. [CBSE HOTS] An unknown metal M displaces zinc from $ZnSO_4$ but does not displace iron from $FeSO_4$. Where does M lie in the activity series?

Solution:

Analysis:

• M displaces Zn → M is more reactive than Zn.
• M does not displace Fe → M is less reactive than Fe.

Conclusion: M's reactivity lies between Fe and Zn.

Activity series: $K > Na > Ca > Mg > Al > Zn > \textbf{M (?)} > Fe > Pb > H > Cu > ...$

This appears to place M between Zn and Fe — but in the standard series Zn > Fe; so the precise placement is M is more reactive than Fe but less than Zn.

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Q33. [CBSE HOTS] Why is a small 'oxygen absorber' sachet placed in milk powder packets?

Solution:

1. Problem: Milk powder contains fat — which can become rancid by oxidation.
2. Solution: An oxygen scavenger sachet — usually iron powder.
3. How does it work?

• Iron reacts with $O_2$: $4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$
• This absorbs the $O_2$.
• The milk powder is protected.

1. Final result: Milk powder stays fresh longer.

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Q34. [CBSE HOTS] A blue solution of copper sulphate becomes colourless on heating, and turns blue again on cooling. What kind of change is this? Explain.

Solution:

1. On heating: $CuSO_4 \cdot 5H_2O$ (blue, hydrated) loses water to become $CuSO_4$ (white/colourless, anhydrous).
2. On cooling and adding water: It reverts to $CuSO_4 \cdot 5H_2O$ (blue).
3. What kind of change is it?

• Reversible change.
• Chemical change — hydrated/anhydrous transformation.

1. Use: This is the most common qualitative test for the presence of water.