Board Previous Year Questions (Board PYQs)

About This Section

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

• An understanding of the pattern
• A sense of mark distribution
• Practice in answer style
• Identification of frequently asked questions

Study strategy:

1. Try to solve each question yourself first.
2. Match your answer length to the marks.
3. Show every step in numerical questions.
4. Write equations clearly.

This collection is divided by mark levels — 1-mark, 2-mark, 3-mark, and 5-mark.

1-Mark Questions (MCQ / Very Short)

Q1. [CBSE 2020] What is the source of litmus?

(a) Lichen (b) Algae (c) Flowers (d) Mushroom

Solution: (a) Lichen — Litmus is a purple dye obtained from lichen (Thallophyta).

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Q2. [CBSE 2019] The pH of pure water is —

(a) 0 (b) 1 (c) 7 (d) 14

Solution: (c) 7 — Pure water is neutral. $[H^+] = [OH^-] = 10^{-7}$ M.

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Q3. [CBSE 2018] The acid found in the stomach is —

(a) HCl (b) $H_2SO_4$ (c) $HNO_3$ (d) $CH_3COOH$

Solution: (a) HCl — hydrochloric acid — pH 1-2.

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Q4. [CBSE 2017] The chemical formula of baking soda is —

(a) $Na_2CO_3$ (b) $NaHCO_3$ (c) $NaOH$ (d) $NaCl$

Solution: (b) $NaHCO_3$ — sodium hydrogen carbonate.

Q5. [CBSE 2016] The formula of gypsum is —

(a) $CaSO_4$ (b) $CaSO_4 \cdot 2H_2O$ (c) $CaSO_4 \cdot \frac{1}{2}H_2O$ (d) $Ca(OH)_2$

Solution: (b) $CaSO_4 \cdot 2H_2O$ — calcium sulphate dihydrate.

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Q6. [CBSE 2015] A solution has pH 12. The solution is —

Solution: A strong base — pH > 7 is basic; pH 12 is moderately strong base.

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Q7. [CBSE 2014] Where is $Cl_2$ released in the chlor-alkali process?

Solution: At the anode (positive electrode).

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Q8. [CBSE 2013] How many water molecules are present in $CuSO_4 \cdot 5H_2O$?

Solution: 5 water molecules — responsible for its blue colour.

2-Mark Questions

Q9. [CBSE 2020] What are indicators? Name three types and give one example of each.

Solution:

Indicators: substances that show the presence of an acid/base by changing colour or odour.

Three types:

1. Natural: litmus
2. Synthetic: phenolphthalein
3. Olfactory: onion

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Q10. [CBSE 2019] What happens when baking soda is heated? Write the reaction.

Solution:

Reaction:

$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2\uparrow$$

Observation: $CO_2$ is released — that's why cakes become spongy.

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Q11. [CBSE 2018] What is the correct method for diluting a concentrated acid? Why?

Solution:

Method: Always add concentrated acid slowly to water — never water to acid!

Reason:

• Dilution is exothermic — a lot of heat is released.
• If water is added to acid — the solution can splash out (risk of burns).
• Adding acid to a large volume of water — safe.

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Q12. [CBSE 2017] What is the ionic form of the neutralisation reaction?

Solution:

$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$

This is the basis of every acid-base neutralisation.

Q13. [CBSE 2016] How does tooth decay happen? Give one preventive measure.

Solution:

Mechanism:

1. We eat sugar.
2. Mouth bacteria convert sugar into lactic acid.
3. Mouth pH drops below 5.5.
4. Tooth enamel begins to dissolve.
5. Cavities develop.

Prevention: Brushing with basic toothpaste — neutralises mouth acid.

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Q14. [CBSE 2015] Why do we apply baking soda on an ant sting?

Solution:

An ant sting contains formic acid ($HCOOH$) — acidic. Baking soda is basic — it neutralises the acid, reducing burning.

$$NaHCO_3 + HCOOH \rightarrow HCOONa + H_2O + CO_2$$

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Q15. [CBSE 2014] What happens when dilute $HCl$ is added to black $CuO$?

Solution:

Observations:

• Black $CuO$ dissolves.
• The solution turns bluish-green.

Reaction:

$$CuO + 2HCl \rightarrow CuCl_2 + H_2O$$

3-Mark Questions

Q16. [CBSE 2020] What is acid rain? State its causes and two effects.

Solution:

Definition: Rain with pH less than 5.6 is called acid rain.

Causes:

• $SO_2$ and $NO_2$ — from burning coal/oil and from vehicle engines.
• They dissolve in water to form acids:

$$SO_3 + H_2O \rightarrow H_2SO_4$$ $$2NO_2 + H_2O \rightarrow HNO_3 + HNO_2$$

Two effects:

1. Aquatic life harmed: river/lake pH drops — fish die.
2. Marble buildings dissolve: monuments like the Taj Mahal damaged.

$$CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$$

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Q17. [CBSE 2019] What is the chlor-alkali process? Name the three products and one use of each.

Solution:

Definition: Electrolysis of an aqueous $NaCl$ solution.

Reaction:

$$2NaCl + 2H_2O \xrightarrow{\text{electricity}} 2NaOH + Cl_2 + H_2$$

Three products and uses:

Product	Location	Use
$NaOH$	In solution	Soap industry
$Cl_2$	Anode	Water purification; PVC
$H_2$	Cathode	Ammonia synthesis; fuel

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Q18. [CBSE 2018] What is a salt? Three pH types and examples.

Solution:

Salt: A compound formed by neutralisation of an acid and a base.

Three pH types:

Type	Parents	Example	pH
Neutral	Strong acid + strong base	$NaCl$	7
Acidic	Strong acid + weak base	$NH_4Cl$	< 7
Basic	Weak acid + strong base	$Na_2CO_3$	> 7

Q19. [CBSE 2017] How is bleaching powder prepared? Three uses.

Solution:

Formula: $CaOCl_2$

Preparation:

$$Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$$

Three uses:

1. Bleaching cotton/linen.
2. Bleaching paper pulp.
3. Water purification (drinking water, swimming pools).

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Q20. [CBSE 2016] What is acidity? Three home remedies and one chemical reaction.

Solution:

Acidity: Excess $HCl$ in the stomach — burning, acidic burping.

Three remedies (all basic):

1. Baking soda ($NaHCO_3$)
2. Milk of magnesia ($Mg(OH)_2$)
3. Calcium carbonate ($CaCO_3$)

Reaction (milk of magnesia):

$$Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$$

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Q21. [CBSE 2015] What is the water of crystallisation? Three examples.

Solution:

Definition: A definite number of water molecules chemically bound in the crystal of a salt.

Three examples:

1. $CuSO_4 \cdot 5H_2O$ (blue vitriol)
2. $Na_2CO_3 \cdot 10H_2O$ (washing soda)
3. $CaSO_4 \cdot 2H_2O$ (gypsum)

5-Mark Questions

Q22. [CBSE 2020 — Long Answer]

(a) What are indicators? Three types and one example each. (b) What colour does phenolphthalein turn in dilute $HCl$ and $NaOH$? (c) The pH range and colours of methyl orange. (d) Application — a student has only red litmus. How to identify three solutions (acid, base, water)?

Solution:

(a) Indicators — substances that show the presence of an acid/base by changing colour or odour.

Three types: natural (litmus), synthetic (phenolphthalein), olfactory (onion).

(b) Phenolphthalein: in dilute $HCl$ — colourless; in $NaOH$ — pink.

(c) Methyl orange — pH 3.1 (red) to 4.4 (yellow). Neutral — orange.

(d) Step 1: identify the base with red litmus (turns blue). Step 2: with this blue litmus, test the other two — turns red in acid; stays blue in water.

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Q23. [CBSE 2019 — Long Answer]

(a) Complete reaction of the chlor-alkali process. (b) Three products and their locations. (c) Two uses of each product. (d) Importance in industry.

Solution:

(a) $2NaCl + 2H_2O \xrightarrow{\text{electricity}} 2NaOH + Cl_2 + H_2$

(b)

• $NaOH$ — in solution
• $Cl_2$ — at the anode
• $H_2$ — at the cathode

(c) Uses:

• NaOH: soap industry; paper manufacture.
• Cl₂: water purification; PVC.
• H₂: ammonia synthesis; fuel.

(d) Industrial importance:

• $NaCl$ — cheap and abundant.
• All three products useful.
• Foundation of the chemical industry.

Q24. [CBSE 2018 — Long Answer]

(a) Introduction to the pH scale. (b) Universal indicator colour code. (c) Three daily-life examples of pH. (d) How much more acidic is pH 2 than pH 5?

Solution:

(a) pH scale = 0 to 14. pH < 7 acidic; 7 neutral; > 7 basic. $\text{pH} = -\log[H^+]$.

(b) Universal indicator colours:

• pH 1-3 → red
• pH 4-6 → orange/yellow
• pH 7 → green
• pH 8-10 → blue
• pH 11-14 → violet

(c) Daily life:

• Stomach: pH 1-2
• Blood: pH 7.4
• Acid rain: pH < 5.6

(d) Difference = $5 - 2 = 3$; ratio = $10^3 = 1000$ times.

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Q25. [CBSE 2017 — Long Answer]

State five daily-life applications of neutralisation. Give a chemical reaction for each.

Solution:

1. Treating acidity: $$Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$$

2. On an ant sting: $$NaHCO_3 + HCOOH \rightarrow HCOONa + H_2O + CO_2$$

3. Adding lime to acidic soil: $$Ca(OH)_2 + 2H^+ \rightarrow Ca^{2+} + 2H_2O$$

4. Industrial wastewater: $$H_2SO_4 + Ca(OH)_2 \rightarrow CaSO_4 + 2H_2O$$

5. Preventing tooth decay: Toothpaste is basic — neutralises mouth acid.

Principle: base on acid, acid on base — neutralisation.

Additional Mixed-Topic Questions

Q26. [CBSE — Mock 2021] For the following chemicals, give the formula, common name, and one use each: (i) $NaOH$ (ii) $CaOCl_2$ (iii) $NaHCO_3$ (iv) $Na_2CO_3 \cdot 10H_2O$ (v) $CaSO_4 \cdot \frac{1}{2}H_2O$

Solution:

Chemical	Common Name	One use
$NaOH$	Caustic soda	Soap industry
$CaOCl_2$	Bleaching powder	Water purification
$NaHCO_3$	Baking soda	Cakes/biscuits
$Na_2CO_3 \cdot 10H_2O$	Washing soda	Laundry
$CaSO_4 \cdot \frac{1}{2}H_2O$	Plaster of Paris	Plaster casts for broken bones

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Q27. [CBSE — Mock 2020] A student has three test tubes — vinegar, lime water, baking soda solution. How to identify them?

Solution:

Method: Use red and blue litmus.

Tube	Red Litmus	Blue Litmus	Conclusion
Vinegar (acid)	Unchanged	Red	Acid
Lime water (base)	Blue	Unchanged	Base
Baking soda solution (mild base)	Blue (faint)	Unchanged	Base

Distinguishing vinegar from baking soda/lime water: vinegar has a pungent smell; baking soda solution is less basic to taste.

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Q28. [CBSE 2014 — Practical] A river's water has pH 5. What is its nature? State the effect on aquatic life.

Solution:

Analysis:

• Normal river pH ~ 7.
• pH 5 = 100 times more acidic (from acid rain or pollution).

Effects on aquatic life:

1. Fish begin to die (gill damage).
2. Eggs/larvae do not develop.
3. Plankton decline — food chain disrupted.
4. Calcium-shelled creatures (snails) — shells dissolve.
5. Long term: river ecosystem destroyed.

Prevention: Stop acid rain; treat wastewater.

Numerical PYQs

Q29. [CBSE 2019 — Numerical] From 100 g of $NaCl$ via the chlor-alkali process, how much $NaOH$, $Cl_2$, and $H_2$ are obtained? (Na=23, Cl=35.5, O=16, H=1)

Solution:

Reaction:

$$2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2$$

Molar masses:

• $NaCl = 58.5$
• $NaOH = 40$
• $Cl_2 = 71$
• $H_2 = 2$

Ratio (from 2 mol NaCl):

• $2 \times 58.5 = 117$ g NaCl → $2 \times 40 = 80$ g NaOH + 71 g Cl₂ + 2 g H₂

From 100 g NaCl:

• NaOH = $\frac{80}{117} \times 100 \approx 68.4$ g
• Cl₂ = $\frac{71}{117} \times 100 \approx 60.7$ g
• H₂ = $\frac{2}{117} \times 100 \approx 1.7$ g

Answer: ~ 68.4 g NaOH, ~ 60.7 g Cl₂, ~ 1.7 g H₂.

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Q30. [CBSE 2018 — Numerical] How much $NaOH$ is needed to fully neutralise 49 g of $H_2SO_4$? ($H_2SO_4 = 98$, $NaOH = 40$)

Solution:

Reaction:

$$H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$$

Mole ratio: 1 mol $H_2SO_4$ → 2 mol $NaOH$

Mass ratio: 98 g → 80 g

From 49 g $H_2SO_4$:

$$NaOH = \frac{80}{98} \times 49 = 40 \text{ g}$$

Answer: 40 g $NaOH$ needed.

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Q31. [CBSE 2016 — Numerical] How much $CO_2$ is released on heating 84 g of $NaHCO_3$? ($NaHCO_3 = 84$, $CO_2 = 44$)

Solution:

Reaction:

$$2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$$

Mole ratio: 2 mol NaHCO₃ → 1 mol CO₂

Masses: 168 g NaHCO₃ → 44 g CO₂

From 84 g:

$$CO_2 = \frac{44}{168} \times 84 = 22 \text{ g}$$

Answer: 22 g $CO_2$.

Higher-Order-Thinking Questions (HOTS)

Q32. [CBSE HOTS] Two solutions — A with pH 4 and B with pH 10. (i) Which is more acidic? (ii) By how much? (iii) If equal volumes are mixed, what is the resulting pH?

Solution:

(i) A is more acidic (pH 4 < 7); B is basic (pH 10 > 7).

(ii) Difference = 10 - 4 = 6 units. A is $10^6$ times more acidic (in $H^+$ concentration) than B.

(iii) On mixing:

• A has $[H^+] = 10^{-4}$ M
• B has $[H^+] = 10^{-10}$ M, $[OH^-] = 10^{-4}$ M

The acid in A is neutralised by the base in B (equal volumes and equal concentrations, with opposite ions).

Resulting solution: approximately neutral — pH ~ 7.

(Approximate — the exact pH depends on the strengths of the acid/base.)

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Q33. [CBSE HOTS] Why do some metals (like Zn) react with both acids and bases? Explain.

Solution:

These metals are 'amphoteric'.

Zinc's amphoteric nature:

With acid: $$Zn + 2HCl \rightarrow ZnCl_2 + H_2$$ (Zn behaves like a metal — displaces H.)

With base: $$Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2$$ (Zn behaves 'acid-like' — reacts with $OH^-$ to form sodium zincate.)

Other amphoteric metals: aluminium (Al), tin (Sn), lead (Pb).

Explanation:

• These metals can act both as acid-like and base-like.
• Their oxides are also amphoteric ($ZnO, Al_2O_3$).

In daily life: care should be taken with both acidic and basic foods in aluminium utensils.

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Q34. [CBSE HOTS] Why is $H_2$ obtained from dilute $H_2SO_4$ but not from concentrated $H_2SO_4$?

Solution:

Dilute $H_2SO_4$: releases $H_2$ from active metals (displacement).

$$Zn + H_2SO_4 (\text{dilute}) \rightarrow ZnSO_4 + H_2$$

Concentrated $H_2SO_4$: acts as an oxidising agent — gives $SO_2$ instead of $H_2$.

$$Cu + 2H_2SO_4 (\text{conc.}) \rightarrow CuSO_4 + 2H_2O + SO_2$$

Explanation:

• In concentrated $H_2SO_4$, sulphur has an oxidation state of +6 — a strong oxidising agent.
• It directly oxidises the metal.
• Instead of $H_2$, $SO_2$ is released.

(Class 11 deeper context — at Class 10 level, just know the behaviours differ.)