Which statement about Plaster of Paris is incorrect?

Its chemical formula is CaSO₄·2H₂O.

It is obtained by heating gypsum at 373 K.

It sets into a hard mass on mixing with water.

Its chemical formula is CaSO₄·2H₂O.

It is used for making casts and moulds.

Assertion (A): A solution with a pH of 7 is considered neutral. Reason (R): The pH scale quantifies hydrogen ion concentration, and a value of 7 corresponds to equal H+ and OH- concentrations.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

Write a balanced chemical equation for the reaction between sodium bicarbonate and dilute hydrochloric acid. What is the nature of the gas evolved?

NaHCO₃ + HCl → NaCl + H₂O + CO₂. The gas evolved (CO₂) is acidic in nature.

A solution has a pH of 5. What will happen to its pH if a small amount of solid sodium bicarbonate is added to it? Explain the reason.

The pH will increase (become greater than 5) because sodium bicarbonate is basic in nature and will neutralize some of the acid, reducing the hydrogen ion concentration.

Dry hydrogen chloride gas does not change the colour of dry blue litmus paper, whereas its aqueous solution turns blue litmus red. Explain why.

Dry HCl gas does not produce H⁺ ions, so it does not show acidic behaviour. In water, HCl dissociates to give H⁺ and Cl⁻ ions; H⁺ ions are responsible for the colour change of blue litmus to red.

A solution has a pH of 3. It is gradually diluted with water. Explain how its pH changes and why the pH does not become exactly 7 even after adding a large amount of water.

On dilution, the concentration of H⁺ ions decreases, so the pH increases and approaches 7. However, due to the self‑ionisation of water, the H⁺ ions from water (10⁻⁷ mol/L) become significant compared to those from the acid at extreme dilution, preventing the pH from reaching exactly 7.

Rohan had three unknown salt samples: A, B, and C. He dissolved each in distilled water and tested with pH paper. Sample A gave pH 7, sample B gave pH around 5, and sample C gave pH around 8.5. Identify the nature of the acid and base from which each salt might have been formed (strong/weak). Justify your answer by explaining the reaction of the constituent ions with water. Write ionic equations for the hydrolysis reactions responsible for the acidic and basic nature.

A: salt of strong acid and strong base (e.g., NaCl) → no hydrolysis, pH 7. B: salt of strong acid and weak base (e.g., NH4Cl) → cation hydrolysis, pH 7: CH3COO⁻ + H2O ⇌ CH3COOH + OH⁻ (basic). Complete ionization in water: A: Na⁺, Cl⁻ do not react with water; B: NH4⁺ releases H⁺; C: CH3COO⁻ releases OH⁻.

What is water of crystallization? Describe an activity to demonstrate that copper sulphate crystals contain water of crystallization. Draw the changes observed. Explain why plaster of Paris hardens when mixed with water, giving the relevant chemical equation. Also, comment on how the number of water molecules affects the color of a salt.

Water of crystallization is the fixed number of water molecules chemically bound to a salt in its crystalline form. Activity: Heat a few blue copper sulphate crystals in a dry test tube. Water droplets appear on the cooler parts and the blue color fades to white. When water is added to the white powder, it turns blue again with evolution of heat. Equation: CuSO4.5H2O (blue) → heat → CuSO4 (white) + 5H2O. Plaster of Paris is CaSO4.½H2O; on mixing with water, it forms gypsum (CaSO4.2H2O) a hard solid: CaSO4.½H2O + 1½ H2O → CaSO4.2H2O. Color change: CuSO4.5H2O is blue, CuSO4 is white. In general, hydration state affects d-d electronic transitions and hence color.

A student mixes 100 mL of a strong acid solution of pH 4 with 100 mL of a strong base solution of pH 10. Both solutions are fully dissociated. (1) Calculate the concentration of H⁺ ions in the pH 4 acid solution. (2) Calculate the concentration of OH⁻ ions in the pH 10 base solution. (3) Predict the pH of the mixture and justify your answer.

H⁺ concentration is 10⁻⁴ M, OH⁻ concentration is also 10⁻⁴ M. Mixing equal volumes results in exact neutralisation, so the mixture is neutral with pH 7.

Class 10 Science Chapter 2: Acids, Bases and Salts — Important Questions & Sample Paper

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Yes — this page has 44+ original Class 10 Science Chapter 2 (“Acids, Bases and Salts”) important questions with answers (Multiple Choice (MCQ), Assertion–Reason, Short Answer, Short Answer, Long Answer, Case Study). Practise them free, or generate a full CBSE board-pattern sample paper (80 marks) and export it to PDF or Word — in English & Hindi, for 2026-27.

Class 10 Science Chapter 2, ‘Acids, Bases and Salts,’ builds on your previous knowledge of chemical substances and introduces fundamental concepts like the Arrhenius definition of acids and bases, the pH scale, and the behavior of salts in aqueous solutions. You’ll learn how acids and bases react with each other in neutralization reactions, how to safely dilute concentrated acids, and why some acids and bases are classified as strong or weak. The chapter explores the characteristics of salts, including water of crystallization, and how the pH of a salt solution depends on the strengths of its parent acid and base. Concepts such as hydrolysis of salts and the exothermic nature of neutralization are critical for CBSE exams. Typical questions range from predicting pH changes when a substance is added to a solution, to identifying the nature of unknown salts using pH paper, to writing balanced chemical equations for reactions like that of magnesium ribbon with hydrochloric acid. You’ll also encounter application-based questions on household substances like baking soda and Plaster of Paris, making it essential to connect theory with everyday life.

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Science — Acids, Bases and Salts

Class 10Time: 3 hrsMax Marks: 80

SECTION A

1.
Baking soda is chemically known as:
(a) Sodium carbonate(b) Sodium hydrogencarbonate(c) Calcium oxychloride(d) Calcium sulphate hemihydrate
1
2.
Which statement about Plaster of Paris is incorrect?
(a) It is obtained by heating gypsum at 373 K.(b) It sets into a hard mass on mixing with water.(c) Its chemical formula is CaSO₄·2H₂O.(d) It is used for making casts and moulds.
1
3.
Which of the following is a strong acid?
(a) Acetic acid(b) Citric acid(c) Hydrochloric acid(d) Carbonic acid
1

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In a CBSE exam, this chapter typically contributes questions across the following types. The last column shows how many original questions of each type we have ready in our bank for this chapter:

Question type	Marks each	In our bank
Multiple Choice (MCQ)	1 mark	13
Assertion–Reason	1 mark	6
Short Answer	2 marks	8
Short Answer	3 marks	6
Long Answer	5 marks	5
Case Study	4 marks	6

44 original, exam-style questions in our bank for this chapter — with answers.

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Q1. Baking soda is chemically known as:
1 mark
Multiple Choice (MCQ)
(A) Sodium carbonate(B) Sodium hydrogencarbonate(C) Calcium oxychloride(D) Calcium sulphate hemihydrate
▸ Answer▾ Answer
Sodium hydrogencarbonate
Q2. Which statement about Plaster of Paris is incorrect?
1 mark
Multiple Choice (MCQ)
(A) It is obtained by heating gypsum at 373 K.(B) It sets into a hard mass on mixing with water.(C) Its chemical formula is CaSO₄·2H₂O.(D) It is used for making casts and moulds.
▸ Answer▾ Answer
Its chemical formula is CaSO₄·2H₂O.
Q3. Which of the following is a strong acid?
1 mark
Multiple Choice (MCQ)
(A) Acetic acid(B) Citric acid(C) Hydrochloric acid(D) Carbonic acid
▸ Answer▾ Answer
Hydrochloric acid
Q4. Which of the following salts does not contain water of crystallization?
1 mark
Multiple Choice (MCQ)
(A) Gypsum(B) Washing soda(C) Baking soda(D) Blue vitriol
▸ Answer▾ Answer
Baking soda
Q5. Assertion (A): A solution with a pH of 7 is considered neutral. Reason (R): The pH scale quantifies hydrogen ion concentration, and a value of 7 corresponds to equal H+ and OH- concentrations.
1 mark
Assertion–Reason
(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) A is true but R is false.(D) A is false but R is true.
▸ Answer▾ Answer
Both A and R are true and R is the correct explanation of A.
Q6. Write a balanced chemical equation for the reaction between sodium bicarbonate and dilute hydrochloric acid. What is the nature of the gas evolved?
2 marks
Short Answer
▸ Answer▾ Answer
NaHCO₃ + HCl → NaCl + H₂O + CO₂. The gas evolved (CO₂) is acidic in nature.
Q7. A solution has a pH of 5. What will happen to its pH if a small amount of solid sodium bicarbonate is added to it? Explain the reason.
2 marks
Short Answer
▸ Answer▾ Answer
The pH will increase (become greater than 5) because sodium bicarbonate is basic in nature and will neutralize some of the acid, reducing the hydrogen ion concentration.
Q8. Dry hydrogen chloride gas does not change the colour of dry blue litmus paper, whereas its aqueous solution turns blue litmus red. Explain why.
3 marks
Short Answer
▸ Answer▾ Answer
Dry HCl gas does not produce H⁺ ions, so it does not show acidic behaviour. In water, HCl dissociates to give H⁺ and Cl⁻ ions; H⁺ ions are responsible for the colour change of blue litmus to red.
Q9. A solution has a pH of 3. It is gradually diluted with water. Explain how its pH changes and why the pH does not become exactly 7 even after adding a large amount of water.
3 marks
Short Answer
▸ Answer▾ Answer
On dilution, the concentration of H⁺ ions decreases, so the pH increases and approaches 7. However, due to the self‑ionisation of water, the H⁺ ions from water (10⁻⁷ mol/L) become significant compared to those from the acid at extreme dilution, preventing the pH from reaching exactly 7.
Q10. Rohan had three unknown salt samples: A, B, and C. He dissolved each in distilled water and tested with pH paper. Sample A gave pH 7, sample B gave pH around 5, and sample C gave pH around 8.5. Identify the nature of the acid and base from which each salt might have been formed (strong/weak). Justify your answer by explaining the reaction of the constituent ions with water. Write ionic equations for the hydrolysis reactions responsible for the acidic and basic nature.
5 marks
Long Answer
▸ Answer▾ Answer
A: salt of strong acid and strong base (e.g., NaCl) → no hydrolysis, pH 7. B: salt of strong acid and weak base (e.g., NH4Cl) → cation hydrolysis, pH < 7: NH4⁺ + H2O ⇌ NH4OH + H⁺ (acidic). C: salt of weak acid and strong base (e.g., CH3COONa) → anion hydrolysis, pH > 7: CH3COO⁻ + H2O ⇌ CH3COOH + OH⁻ (basic). Complete ionization in water: A: Na⁺, Cl⁻ do not react with water; B: NH4⁺ releases H⁺; C: CH3COO⁻ releases OH⁻.
Q11. What is water of crystallization? Describe an activity to demonstrate that copper sulphate crystals contain water of crystallization. Draw the changes observed. Explain why plaster of Paris hardens when mixed with water, giving the relevant chemical equation. Also, comment on how the number of water molecules affects the color of a salt.
5 marks
Long Answer
▸ Answer▾ Answer
Water of crystallization is the fixed number of water molecules chemically bound to a salt in its crystalline form. Activity: Heat a few blue copper sulphate crystals in a dry test tube. Water droplets appear on the cooler parts and the blue color fades to white. When water is added to the white powder, it turns blue again with evolution of heat. Equation: CuSO4.5H2O (blue) → heat → CuSO4 (white) + 5H2O. Plaster of Paris is CaSO4.½H2O; on mixing with water, it forms gypsum (CaSO4.2H2O) a hard solid: CaSO4.½H2O + 1½ H2O → CaSO4.2H2O. Color change: CuSO4.5H2O is blue, CuSO4 is white. In general, hydration state affects d-d electronic transitions and hence color.
Q12. A student mixes 100 mL of a strong acid solution of pH 4 with 100 mL of a strong base solution of pH 10. Both solutions are fully dissociated.
4 marks
Case Study
1. (i) Calculate the concentration of H⁺ ions in the pH 4 acid solution.1 mark
2. (ii) Calculate the concentration of OH⁻ ions in the pH 10 base solution.1 mark
3. (iii) Predict the pH of the mixture and justify your answer.2 marks
▸ Answer▾ Answer
H⁺ concentration is 10⁻⁴ M, OH⁻ concentration is also 10⁻⁴ M. Mixing equal volumes results in exact neutralisation, so the mixture is neutral with pH 7.

Frequently asked questions

Why is the dilution of concentrated acid done by adding acid to water and not the reverse?

The dilution of concentrated acid is highly exothermic. If water is added to acid, the heat released can cause the water to boil and splash, leading to severe burns. Adding acid slowly to water allows the heat to be absorbed and dissipated safely.

How can we predict whether an aqueous salt solution will be acidic, basic, or neutral using its formula?

A salt's nature depends on the strengths of the parent acid and base. A salt from a strong acid and strong base (e.g., NaCl) gives a neutral solution. A salt from a strong acid and weak base (e.g., NH4Cl) gives an acidic solution due to hydrolysis of the cation. A salt from a weak acid and strong base (e.g., CH3COONa) gives a basic solution due to hydrolysis of the anion.

When magnesium ribbon reacts with dilute hydrochloric acid and dilute acetic acid, why is the fizzing more vigorous in hydrochloric acid?

Hydrochloric acid (HCl) is a strong acid that dissociates completely in water, providing a high concentration of H⁺ ions. Acetic acid is weak and dissociates partially, so its H⁺ ion concentration is lower. Hence, the reaction rate with magnesium (which produces hydrogen gas) is faster with HCl, leading to more vigorous fizzing.

What is the correct way to write the dissociation equation for a strong base like sodium hydroxide?

A strong base like NaOH dissociates completely in water: NaOH(s) → Na⁺(aq) + OH⁻(aq). For CBSE, it’s important to show the single arrow and state the ions are aqueous.

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