NCERT Solutions for Class 10 Science Chapter 2: Acids, Bases and Salts (NCERT 2026–27)

These Class 10 Science Chapter 2 solutions cover Acids, Bases and Salts with every in-text “Questions” set and all 15 end-of-chapter Exercises reproduced verbatim and solved step by step. You will learn how acids and bases react with metals, carbonates and each other, what the pH scale means, and how common salts like baking soda, washing soda, bleaching powder and Plaster of Paris are made and used — all updated for session 2026–27.

Class: 10 Subject: Science (Chemistry) Chapter: 2 Chapter Name: Acids, Bases and Salts Type: In-text + 15 Exercises Session: 2026–27
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Class 10 Science Chapter 2 Solutions – Overview

Chapter 2, Acids, Bases and Salts, builds on the idea that the sour taste of food comes from acids and the bitter taste from bases. Acids turn blue litmus red; bases turn red litmus blue. The chapter studies the chemical properties of acids and bases — their reactions with metals, metal carbonates/hydrogencarbonates, metal oxides and with each other (neutralisation) — and shows that all acids release H+(aq) ions and all bases release OH−(aq) ions in water, which is why their aqueous solutions conduct electricity. It introduces the pH scale (0–14) as a measure of hydrogen-ion concentration and its importance in our body, soil, teeth and self-defence by plants/animals. Finally, it explores the family of salts, the pH of salts, and useful chemicals from common salt — sodium hydroxide (chlor-alkali process), bleaching powder, baking soda, washing soda — along with water of crystallisation and Plaster of Paris.

Key Concepts, Reactions & Formulae

Acid + Metal → Salt + Hydrogen gas

Metal carbonate/hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

Metal oxide (basic) + Acid → Salt + Water

Non-metallic oxide (acidic) + Base → Salt + Water

Neutralisation: Base + Acid → Salt + Water; ionically H+(aq) + OH−(aq) → H2O(l)

Acidic / basic nature: Acids give H+(aq) (hydronium, H3O+) in water; bases give OH−(aq). H+ cannot exist alone — it combines with water: H+ + H2O → H3O+.

pH scale (0–14): 7 = neutral, <7 = acidic, >7 = basic. Higher H3O+ concentration means lower pH. Strong acids/bases give more ions; weak ones give fewer.

Strong vs weak: HCl is a strong acid (more H+); CH3COOH (acetic acid) is a weak acid (fewer H+).

Chlor-alkali process: 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g). Cl2 at anode, H2 at cathode, NaOH near cathode.

Important salts: Bleaching powder Ca(ClO)2; Baking soda NaHCO3; Washing soda Na2CO3·10H2O; Plaster of Paris CaSO4·½H2O; Gypsum CaSO4·2H2O; hydrated copper sulphate CuSO4·5H2O.

In-text “Questions” — Answers

These are the in-text “Questions” sets given within the chapter, reproduced verbatim and solved.

In-text Questions (page 18)

1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

ANSWER Dip a piece of red litmus paper into each test tube and observe the colour change. • In the basic solution, the red litmus paper turns blue. • In the acidic solution and in distilled water, red litmus shows no change (stays red). To distinguish the last two, take the blue litmus paper formed in the basic test tube and dip it into the remaining two tubes: it turns red in the acidic solution and shows no change in distilled water. The tube giving no change with either paper is distilled water.

In-text Questions (page 22)

1. Why should curd and sour substances not be kept in brass and copper vessels?

ANSWER Curd and sour substances contain acids (such as lactic acid). These acids react with the metals of brass and copper vessels, forming toxic metal compounds (salts). This spoils the food and can make it harmful to health, so curd and sour foods are not stored in such vessels.

2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

ANSWER Hydrogen gas (H2) is usually liberated when an acid reacts with a metal. Example: Zn(s) + H2SO4(dil.) → ZnSO4(aq) + H2(g)↑ Test: Pass the gas through soap solution and bring a burning candle near the gas-filled bubbles. The gas burns with a “pop” sound, which confirms hydrogen.

3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

ANSWER The gas that extinguishes a burning candle is carbon dioxide (CO2), so compound A is a carbonate. Since calcium chloride is formed, A is calcium carbonate (CaCO3). Balanced equation: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

In-text Questions (page 25)

1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

ANSWER HCl and HNO3 dissociate in water to give H+(aq) (hydronium) ions, which are responsible for acidic character. Alcohol and glucose contain hydrogen but do not ionise in water to release H+ ions, so they do not show acidic behaviour.

2. Why does an aqueous solution of an acid conduct electricity?

ANSWER An acid dissociates in water to produce free ions — H+(aq) (cations) and the corresponding anions (such as Cl−, NO3−). These mobile ions carry electric current through the solution, so an aqueous acid conducts electricity.

3. Why does dry HCl gas not change the colour of the dry litmus paper?

ANSWER Dry HCl gas does not release H+ ions because the separation of H+ from HCl molecules occurs only in the presence of water. With dry litmus paper there is no water, so no H+ ions form and the colour does not change.

4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

ANSWER Mixing a concentrated acid with water is highly exothermic (releases a lot of heat). When acid is added slowly to water with stirring, the heat is absorbed and spread out by the large amount of water. If water is added to concentrated acid, the heat generated can make the mixture splash out and cause burns, and may even crack the glass container.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

ANSWER On dilution, the number of hydronium ions per unit volume decreases, so the concentration of H3O+ ions decreases.

6. How is the concentration of hydroxide ions (OH−) affected when excess base is dissolved in a solution of sodium hydroxide?

ANSWER When more base is dissolved, the number of OH− ions per unit volume increases, so the concentration of hydroxide ions increases.

In-text Questions (page 28)

1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

ANSWER A lower pH means a higher hydrogen-ion concentration. Since pH of A (6) is less than pH of B (8), solution A has more hydrogen-ion concentration. pH 6 < 7, so solution A is acidic; pH 8 > 7, so solution B is basic.

2. What effect does the concentration of H+(aq) ions have on the nature of the solution?

ANSWER The higher the concentration of H+(aq) ions, the more acidic (and stronger) the solution, and the lower its pH. As the H+ concentration decreases, the solution becomes less acidic and more basic.

3. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

ANSWER Yes, basic solutions also contain some H+(aq) ions. However, the concentration of OH−(aq) ions is much greater than that of H+(aq) ions, so the solution is basic in nature.

4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

ANSWER When the soil is too acidic (low pH), the farmer treats it with quick lime, slaked lime or chalk. These are basic in nature and neutralise the excess acid, raising the soil pH to a level suitable for healthy plant growth.

In-text Questions (page 33)

1. What is the common name of the compound Ca(ClO)2?

ANSWERThe common name of Ca(ClO)2 is bleaching powder (calcium oxychloride / calcium hypochlorite).

2. Name the substance which on treatment with chlorine yields bleaching powder.

ANSWERDry slaked lime, Ca(OH)2 (calcium hydroxide), on treatment with chlorine yields bleaching powder.

3. Name the sodium compound which is used for softening hard water.

ANSWERWashing soda, Na2CO3·10H2O (sodium carbonate decahydrate), is used for softening hard water.

4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

ANSWER On heating, sodium hydrogencarbonate (baking soda) decomposes to give sodium carbonate, water and carbon dioxide. 2NaHCO3 ⟶[Heat] Na2CO3 + H2O + CO2

5. Write an equation to show the reaction between Plaster of Paris and water.

ANSWER CaSO4·½H2O + 1½H2O → CaSO4·2H2O Plaster of Paris (calcium sulphate hemihydrate) takes up water and sets into a hard solid mass of gypsum (calcium sulphate dihydrate).

End-of-Chapter Exercises (Q1–Q15)

1. A solution turns red litmus blue, its pH is likely to be(a) 1    (b) 4    (c) 5    (d) 10

ANSWER (d) 10. A solution that turns red litmus blue is basic, so its pH must be greater than 7. Of the options, only 10 is greater than 7.

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains(a) NaCl    (b) HCl    (c) LiCl    (d) KCl

ANSWER (b) HCl. Egg-shells are made of calcium carbonate (CaCO3). They react with an acid (HCl) to release CO2, which turns lime-water milky. NaCl, LiCl and KCl are neutral salts and do not react. CaCO3 + 2HCl → CaCl2 + H2O + CO2.

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be(a) 4 mL    (b) 8 mL    (c) 12 mL    (d) 16 mL

ANSWER (d) 16 mL. Reasoning: 10 mL NaOH is neutralised by 8 mL HCl. The amount of HCl needed is directly proportional to the volume of NaOH (same concentrations). For 20 mL NaOH (which is 2 × 10 mL), HCl required = 2 × 8 = 16 mL.

4. Which one of the following types of medicines is used for treating indigestion?(a) Antibiotic    (b) Analgesic    (c) Antacid    (d) Antiseptic

ANSWER (c) Antacid. During indigestion the stomach produces excess acid. Antacids are mild bases (e.g. milk of magnesia, Mg(OH)2) that neutralise this excess acid and give relief.

5. Write word equations and then balanced equations for the reaction taking place when – (a) dilute sulphuric acid reacts with zinc granules. (b) dilute hydrochloric acid reacts with magnesium ribbon. (c) dilute sulphuric acid reacts with aluminium powder. (d) dilute hydrochloric acid reacts with iron filings.

ANSWER (a) Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) (b) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) (c) Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
2Al(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2(g) (d) Iron + Hydrochloric acid → Ferrous chloride + Hydrogen
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

ANSWER Fix two iron nails on a cork and place the cork in a beaker. Connect the nails through a bulb and switch to the two terminals of a battery. Pour dilute HCl into the beaker and switch on the current — the bulb glows, showing HCl produces ions (H+). Now repeat with glucose solution and then alcohol solution — the bulb does not glow. Conclusion: Glucose and alcohol do not ionise to give H+ ions in water, so their solutions do not conduct electricity and are not acidic, even though they contain hydrogen. Acidic character is due to H+(aq) ions, not merely the presence of hydrogen.

7. Why does distilled water not conduct electricity, whereas rain water does?

ANSWER Distilled water is pure and contains no dissolved ions, so it cannot conduct electricity. Rain water, while falling, dissolves carbon dioxide and other gases from the air to form weak acids (such as carbonic acid), which release ions. These ions make rain water a conductor of electricity.

8. Why does dry HCL gas not show acidic behaviour in the absence of water?

ANSWER Dry HCl gas does not ionise in the absence of water, so it cannot release H+ ions. Acidic behaviour is caused by H+(aq) ions, which form only when HCl dissolves in water (HCl + H2O → H3O+ + Cl−). Without water, no H+ ions are produced, so dry HCl gas shows no acidic behaviour.

9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration.

ANSWER Given: A = 4, B = 1, C = 11, D = 7, E = 9. (a) Neutral: D (pH = 7). (b) Strongly alkaline: C (pH = 11) — highest pH. (c) Strongly acidic: B (pH = 1) — lowest pH. (d) Weakly acidic: A (pH = 4). (e) Weakly alkaline: E (pH = 9). Increasing order of hydrogen-ion concentration (H+ increases as pH decreases): C (11) < E (9) < D (7) < A (4) < B (1).

10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

ANSWER Fizzing will be more vigorous in test tube A (with HCl). HCl is a strong acid and ionises completely, producing more H+ ions, whereas acetic acid is a weak acid that ionises only partially, giving fewer H+ ions. More H+ ions react faster with magnesium to release hydrogen gas, so the fizzing (effervescence) is more vigorous in test tube A.

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

ANSWER As milk turns into curd, its pH decreases (becomes less than 6). During curdling, bacteria convert lactose into lactic acid, which increases the H+ ion concentration. More acid means a lower pH, so the curd is more acidic than fresh milk.

12. A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?

ANSWER (a) Baking soda is a mild base. By making the milk slightly alkaline, it neutralises the small amount of acid already present, so the milk does not turn sour quickly and stays fresh for a longer time. (b) Curd forms when enough lactic acid is produced to lower the pH. Since the milk is now slightly alkaline, the bacteria first have to neutralise this extra base before the milk becomes acidic enough to set. Hence the milk takes a longer time to set as curd.

13. Plaster of Paris should be stored in a moisture-proof container. Explain why?

ANSWER Plaster of Paris (CaSO4·½H2O) readily absorbs moisture (water). On reacting with water it changes into hard, solid gypsum (CaSO4·2H2O): CaSO4·½H2O + 1½H2O → CaSO4·2H2O This would make it set into a useless hard lump. To prevent it from absorbing moisture and setting, it must be stored in a moisture-proof container.

14. What is a neutralisation reaction? Give two examples.

ANSWER A neutralisation reaction is a reaction between an acid and a base to form a salt and water. In general: Acid + Base → Salt + Water. Example 1: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Example 2: Ca(OH)2(aq) + H2SO4(aq) → CaSO4(aq) + 2H2O(l)

15. Give two important uses of washing soda and baking soda.

ANSWER Uses of washing soda (Na2CO3·10H2O): (i) It is used in glass, soap and paper industries; (ii) it is used for removing permanent hardness of water (and as a cleaning agent for domestic purposes). Uses of baking soda (NaHCO3): (i) It is used in the kitchen for making crispy, soft and spongy foods (as part of baking powder); (ii) it is used as an antacid to neutralise excess acid in the stomach (and in soda-acid fire extinguishers).

Extra Practice Questions

Short Answer Type Questions

Q1. What are olfactory indicators? Give two examples.

ANSWERSubstances whose odour changes in acidic or basic media are called olfactory indicators. Examples: onion and vanilla essence (clove oil can also be used).

Q2. What is water of crystallisation? Give one example.

ANSWERIt is the fixed number of water molecules present in one formula unit of a salt. For example, hydrated copper sulphate CuSO4·5H2O has five molecules of water of crystallisation.

Q3. Why does tooth decay start when the pH of the mouth is lower than 5.5?

ANSWERBelow pH 5.5, the tooth enamel (calcium hydroxyapatite) gets corroded by the acids produced by bacteria from food sugars. This corrosion of enamel is what starts tooth decay.

Q4. What happens chemically when quick lime is added to water?

ANSWERQuick lime (CaO) reacts vigorously with water in a highly exothermic reaction to form slaked lime (calcium hydroxide): CaO(s) + H2O(l) → Ca(OH)2(aq) + heat.

Q5. Why is bleaching powder used to make drinking water free from germs?

ANSWERBleaching powder, Ca(ClO)2, acts as a strong oxidising and disinfecting agent. The chlorine it releases kills germs and micro-organisms, making the water safe to drink.

Long Answer Type Questions

Q1. Explain the chlor-alkali process. Name the three products formed and give one use of each.

ANSWERWhen electricity is passed through a concentrated aqueous solution of sodium chloride (brine), it decomposes. This is called the chlor-alkali process (chlor for chlorine, alkali for sodium hydroxide): 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g). Chlorine gas is released at the anode, hydrogen gas at the cathode, and sodium hydroxide forms near the cathode. Uses: (i) NaOH — making soaps, detergents and paper; (ii) Cl2 — water treatment and making bleaching powder; (iii) H2 — fuel and making hydrogenated vegetable oil (margarine).

Q2. Describe the importance of pH in everyday life with four examples.

ANSWER(i) In our body: living organisms survive only within a narrow pH range (about 7.0–7.8); when rain water pH falls below 5.6 it becomes acid rain and harms aquatic life. (ii) In digestion: the stomach makes HCl for digestion; during acidity, antacids (mild bases like milk of magnesia) neutralise the excess acid. (iii) In tooth decay: decay begins below pH 5.5; basic toothpastes neutralise mouth acids and protect enamel. (iv) Self-defence: a bee-sting injects acid causing pain; rubbing mild base like baking soda gives relief, and nettle stings (methanoic acid) are eased by the basic dock-plant leaf.

Q3. How are baking soda and Plaster of Paris obtained? Give one equation and two uses of each.

ANSWERBaking soda (NaHCO3): made using common salt: NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3. Uses: as an antacid; in baking powder to make food soft and spongy. Plaster of Paris (CaSO4·½H2O): made by heating gypsum at 373 K: CaSO4·2H2O ⟶[373 K] CaSO4·½H2O + 1½H2O. Uses: doctors use it as a plaster to support fractured bones; it is used for making toys, decorative materials and smooth surfaces.

MCQs & Assertion–Reason

1. Which of the following turns blue litmus red?

(a) NaOH solution    (b) Soap solution    (c) Lemon juice    (d) Baking soda solution

2. The pH of a neutral solution at room temperature is:

(a) 0    (b) 7    (c) 10    (d) 14

3. The gas evolved when a metal reacts with a dilute acid is:

(a) oxygen    (b) carbon dioxide    (c) hydrogen    (d) chlorine

4. The chemical formula of washing soda is:

(a) NaHCO3    (b) Na2CO3·10H2O    (c) NaCl    (d) NaOH

5. Which acid is present in a bee sting?

(a) Acetic acid    (b) Citric acid    (c) Methanoic (formic) acid    (d) Tartaric acid

6. The number of water molecules of crystallisation in gypsum (CaSO4·xH2O) is:

(a) ½    (b) 2    (c) 5    (d) 10

7. Brine is the common name for an aqueous solution of:

(a) sodium hydroxide    (b) sodium chloride    (c) sodium carbonate    (d) calcium hydroxide

8. On heating blue copper sulphate crystals, the colour changes to:

(a) green    (b) black    (c) white    (d) yellow

9. Which one is a weak acid?

(a) HCl    (b) H2SO4    (c) HNO3    (d) CH3COOH

10. Tooth enamel begins to corrode when the pH of the mouth falls below:

(a) 7.0    (b) 6.5    (c) 5.5    (d) 3.0

Answer key: 1-(c), 2-(b), 3-(c), 4-(b), 5-(c), 6-(b), 7-(b), 8-(c), 9-(d), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Dry HCl gas does not turn dry blue litmus paper red.

Reason: Acidic character is due to H+ ions, which are produced only in the presence of water.

A-R 2. Assertion: While diluting an acid, acid is added to water and not water to acid.

Reason: Dilution of a concentrated acid is a highly endothermic process.

A-R 3. Assertion: Fizzing is more vigorous with HCl than with acetic acid for the same metal.

Reason: HCl is a strong acid and gives more H+ ions than the weak acetic acid.

A-R 4. Assertion: An aqueous solution of glucose conducts electricity.

Reason: Glucose ionises in water to release H+ ions.

A-R 5. Assertion: A solution with pH 2 has a higher hydrogen-ion concentration than a solution with pH 5.

Reason: Lower the pH value, higher is the hydronium-ion concentration.

Answer key: 1-(A), 2-(C), 3-(A), 4-(D), 5-(A).

Common Mistakes & Exam Tips

Common mistakes to avoid

  • Forgetting to balance acid–metal equations — check H atoms (e.g. 2HCl for Mg, 3H2SO4 for 2Al).
  • Confusing the products: acid + metal gives H2, but carbonate/hydrogencarbonate + acid gives CO2.
  • Mixing up formulae: baking soda is NaHCO3, washing soda is Na2CO3·10H2O — do not interchange them.
  • Writing the “pop” test for CO2 — the pop test is for H2; CO2 turns lime-water milky.
  • Saying glucose/alcohol are acids because they contain hydrogen — they do not give H+ ions in water.
  • Reversing the pH rule — lower pH means more acidic / higher H+, not less.

How to score full marks in this chapter

Always write equations with correct symbols, balancing and state symbols (s/l/aq/g). Use sub/superscripts correctly (H2SO4, H+). For pH numericals, remember: lower pH = stronger acid = more H+. Learn the formulae and one use each for bleaching powder, baking soda, washing soda and Plaster of Paris — these are repeatedly asked. For the gas test, link H2 to the “pop” sound and CO2 to milky lime-water.

Frequently Asked Questions

What is Class 10 Science Chapter 2 Acids, Bases and Salts about?

Chapter 2 studies the chemical properties of acids and bases — their reactions with metals, carbonates, oxides and each other (neutralisation), the pH scale and its importance in everyday life, and important salts such as sodium hydroxide, bleaching powder, baking soda, washing soda and Plaster of Paris.

Are all in-text and exercise questions of Chapter 2 solved here?

Yes. Every in-text “Questions” set (pages 18, 22, 25, 28 and 33) and all 15 end-of-chapter Exercises are reproduced verbatim from the NCERT textbook and solved step by step with balanced equations.

Why is acid added to water and not water to acid during dilution?

Diluting a concentrated acid is highly exothermic. Adding acid slowly to water (with stirring) lets the large amount of water absorb the heat safely. Adding water to acid can make the mixture splash out, cause burns and crack the container.

Are these Class 10 Science Chapter 2 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 10 Science are free and follow the official NCERT textbook for session 2026–27.

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