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NCERT Solutions · Class 10 Science · Chapter 2

Chapter 2: Acids, Bases and Salts (Chemistry)

Answers to all in-text and exercise questions of Chapter 2, Acids, Bases and Salts (NCERT Class 10 Science, 2026-27 reprint): reactions of acids with metals and carbonates, hydronium and hydroxide ions, dilution, the pH scale, neutralisation, bleaching powder, baking soda, washing soda and Plaster of Paris. All 33 questions are answered, with the key answer highlighted.

Free NCERT solutions by Notes Bazar · www.notesbazar.in/ncert-solutions/class-10-science/chapter-2-acids-bases-and-salts

Acids give $H X^{+}$ ions (as hydronium, $H X_{3} O X^{+}$ ) in water; bases give $OH X^{-}$ ions, and bases soluble in water are called alkalis. On the pH scale (0 to 14), pH 7 is neutral, below 7 is acidic and above 7 is basic. The lower the pH, the higher the $H X^{+}$ concentration. Acid + base → salt + water is neutralisation.

In-text questions (Section 2.1)

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

Solution

Curd and sour foods contain acids (lactic acid, citric acid and so on). These acids react with copper and with the zinc in brass to form poisonous salts that spoil the food and can make us ill. They also corrode the vessel.

The acids in them react with copper and brass to form toxic compounds that contaminate the food.

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

Solution

Hydrogen gas. For example, zinc with dilute sulphuric acid:

$Zn (s) + H X_{2} SO X_{4} (aq) ZnSO X_{4} (aq) + H X_{2} (g)$

Test: bring a burning candle or matchstick near the gas. Hydrogen burns with a pop sound.

Hydrogen; it burns with a pop sound when a burning splint is brought near it.

Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved puts out a burning candle. Write a balanced equation if one product is calcium chloride.

Solution

The gas that puts out a flame is carbon dioxide, so A is a carbonate; since calcium chloride forms, A is calcium carbonate.

$CaCO X_{3} (s) + 2 HCl (aq) CaCl X_{2} (aq) + CO X_{2} (g) + H X_{2} O (l)$

A is calcium carbonate: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

In-text questions (Section 2.2)

Why do HCl, HNO₃, etc. show acidic character in water, while solutions of alcohol and glucose do not?

Solution

Acidic character comes from $H X^{+}$ ions in solution. HCl and HNO₃ ionise in water and give $H X^{+}$ ions ( $HCl + H X_{2} O H X_{3} O X^{+} + Cl X^{-}$ ). Alcohol and glucose also contain hydrogen, but they do not ionise to give $H X^{+}$ ions in water, so they are not acidic.

HCl and HNO₃ produce H⁺ ions in water; alcohol and glucose do not.

Why does an aqueous solution of an acid conduct electricity?

Solution

In water an acid produces ions ( $H X_{3} O X^{+}$ and the anion, such as $Cl X^{-}$ ). These ions are free to move and carry the electric current.

Because it contains ions (H⁺ or H₃O⁺ and the anions) that carry current.

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

Solution

Acids show their properties only when $H X^{+}$ ions are present. Dry HCl gas has no water, so it does not ionise and forms no $H X^{+}$ ions. Without them, litmus does not change colour. (Moist litmus paper does turn red.)

Without water, HCl does not form H⁺ ions, and only H⁺ ions change the colour of litmus.

While diluting an acid, why should the acid be added to water and not water to the acid?

Solution

Mixing a concentrated acid with water is highly exothermic. If water is added to the acid, the small amount of water heats up suddenly and may boil, splashing out the acid and causing burns; the glass container may also crack. Adding the acid slowly to a large amount of water, with constant stirring, spreads the heat safely.

Because the mixing gives out so much heat that water added to acid can boil and splash the acid out; adding acid to water keeps it safe.

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

Solution

Dilution adds more water while the number of $H X_{3} O X^{+}$ ions stays the same, so their concentration (per unit volume) decreases.

It decreases.

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

Solution

More base dissolved gives more $OH X^{-}$ ions in the same volume, so the concentration of $OH X^{-}$ increases (until the solution is saturated).

It increases.

In-text questions (Section 2.3)

Solution A has pH 6 and solution B has pH 8. Which has more hydrogen ion concentration? Which is acidic and which is basic?

Solution

The lower the pH, the higher the $H X^{+}$ concentration. So A (pH 6) has more hydrogen ions.

A has more H⁺ ions. A (pH 6) is acidic; B (pH 8) is basic.

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

Solution

The more $H X^{+}$ ions there are, the more acidic the solution (the lower its pH). As the $H X^{+}$ concentration falls, the solution becomes less acidic, then neutral, then basic.

A higher H⁺ concentration makes the solution more strongly acidic; a lower one makes it less acidic or basic.

Do basic solutions also have H⁺(aq) ions? If yes, why are they basic?

Solution

Yes. Water itself always gives a few $H X^{+}$ ions, so every aqueous solution has some. A solution is basic because its $OH X^{-}$ ions outnumber its $H X^{+}$ ions.

Yes; they are basic because the concentration of OH⁻ ions is greater than that of H⁺ ions.

Under what soil condition would a farmer treat the fields with quicklime, slaked lime or chalk?

Solution

When the soil is too acidic (low pH). Quicklime (CaO), slaked lime (Ca(OH)₂) and chalk (CaCO₃) are basic, so they neutralise the excess acid and raise the pH to the range most crops need.

When the soil is too acidic; these basic substances neutralise the acidity.

In-text questions (Section 2.4)

What is the common name of Ca(ClO)₂?

Solution

Bleaching powder (calcium hypochlorite; written by NCERT as CaOCl₂)

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

Solution

Dry slaked lime, calcium hydroxide: $Ca (OH) X_{2} + Cl X_{2} CaOCl X_{2} + H X_{2} O$

Slaked lime, Ca(OH)₂

Name the sodium compound used for softening hard water.

Solution

Washing soda, sodium carbonate decahydrate (Na₂CO₃·10H₂O)

What happens when a solution of sodium hydrogencarbonate is heated? Give the equation.

Solution

It decomposes to give sodium carbonate, water and carbon dioxide:

$2 NaHCO X_{3} heat Na X_{2} CO X_{3} + H X_{2} O + CO X_{2}$

It decomposes into sodium carbonate, water and carbon dioxide gas.

Write an equation for the reaction between Plaster of Paris and water.

Solution

Plaster of Paris takes up water and sets into hard gypsum:

$CaSO X_{4} \cdot \frac{1}{2} H X_{2} O + 1 \frac{1}{2} H X_{2} O CaSO X_{4} \cdot 2 H X_{2} O$

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum)

Exercises

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

Solution

Turning red litmus blue means the solution is basic, so its pH is above 7.

(d) 10

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

Solution

Egg-shells are calcium carbonate. An acid releases $CO X_{2}$ from them, which turns lime-water milky. Only HCl is an acid.

(b) HCl

10 mL of an NaOH solution is neutralised by 8 mL of an HCl solution. How much of the same HCl is needed to neutralise 20 mL of the NaOH? (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Solution

Twice the base needs twice the acid: $2 \times 8 = 16$ mL.

(d) 16 mL

Which type of medicine is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Solution

Indigestion is caused by excess acid in the stomach; a mild base (antacid) such as milk of magnesia neutralises it.

Write word and balanced equations for the reactions of: (a) dilute sulphuric acid with zinc granules (b) dilute hydrochloric acid with magnesium ribbon (c) dilute sulphuric acid with aluminium powder (d) dilute hydrochloric acid with iron filings.

Solution

Zinc + sulphuric acid → zinc sulphate + hydrogen: $Zn + H X_{2} SO X_{4} ZnSO X_{4} + H X_{2}$
Magnesium + hydrochloric acid → magnesium chloride + hydrogen: $Mg + 2 HCl MgCl X_{2} + H X_{2}$
Aluminium + sulphuric acid → aluminium sulphate + hydrogen: $2 Al + 3 H X_{2} SO X_{4} Al X_{2} (SO X_{4}) X_{3} + 3 H X_{2}$
Iron + hydrochloric acid → iron(II) chloride + hydrogen: $Fe + 2 HCl FeCl X_{2} + H X_{2}$

In each case a salt and hydrogen gas are formed (equations above).

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

Solution

Fix two iron nails in a rubber cork and place the cork in a beaker.
Connect the nails, through a switch and a bulb, to a 6 V battery.
Pour dilute HCl into the beaker and switch on: the bulb glows.
Repeat with dilute H₂SO₄ (the bulb glows), then with glucose solution and alcohol solution: the bulb does not glow.

The glowing bulb shows that the acid solutions contain ions ( $H X^{+}$ ), which carry the current. Glucose and alcohol solutions have no $H X^{+}$ ions, so they do not conduct and are not acids.

In a conductivity test, acid solutions light the bulb but glucose and alcohol solutions do not, so they do not give H⁺ ions.

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

Solution

Distilled water is pure and has almost no ions, so it does not conduct electricity. Rain water dissolves gases such as carbon dioxide (and sulphur dioxide) from the air as it falls, forming acids that produce ions. These ions conduct electricity.

Rain water contains dissolved acids (from CO₂ and other gases) that give ions; distilled water has almost no ions.

Why does dry HCl gas not show acidic behaviour in the absence of water?

Solution

Acidic behaviour is due to $H X^{+}$ ions. HCl forms these ions only when it dissolves in water: $HCl + H X_{2} O H X_{3} O X^{+} + Cl X^{-}$ . Dry HCl gas does not ionise, so it does not behave as an acid.

Without water, HCl does not ionise to give H⁺ ions.

Solutions A, B, C, D and E show pH 4, 1, 11, 7 and 9. Which is (a) neutral (b) strongly alkaline (c) strongly acidic (d) weakly acidic (e) weakly alkaline? Arrange the pH values in increasing order of hydrogen-ion concentration.

Solution

Neutral: D (pH 7)
Strongly alkaline: C (pH 11)
Strongly acidic: B (pH 1)
Weakly acidic: A (pH 4)
Weakly alkaline: E (pH 9)

The $H X^{+}$ concentration rises as the pH falls, so in increasing order of $H X^{+}$ : C (11) < E (9) < D (7) < A (4) < B (1).

(a) D (b) C (c) B (d) A (e) E; increasing H⁺: 11 < 9 < 7 < 4 < 1 (C, E, D, A, B)

Equal lengths of magnesium ribbon are put in test tubes A (hydrochloric acid) and B (acetic acid), with the same amount and concentration of acid. In which test tube will fizzing be more vigorous, and why?

Solution

In test tube A. HCl is a strong acid: it ionises completely and gives many more $H X^{+}$ ions than acetic acid, a weak acid that ionises only partly. More $H X^{+}$ ions react faster with magnesium and release hydrogen more vigorously.

A, because hydrochloric acid is a strong acid that gives more H⁺ ions than acetic acid.

Fresh milk has pH 6. How will the pH change as it turns into curd?

Solution

The pH falls below 6. Bacteria turn the milk sugar into lactic acid as curd forms, so the curd is more acidic than milk.

It decreases, because lactic acid is formed.

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

Solution

Slightly alkaline milk does not turn sour quickly, so it stays fresh for a longer time.
The acid formed during curdling first has to neutralise the baking soda before the milk can become acidic enough to set, so curd formation takes longer.

(a) So that the milk does not turn sour quickly. (b) The lactic acid formed is first used up neutralising the baking soda.

Why should Plaster of Paris be stored in a moisture-proof container?

Solution

Plaster of Paris reacts with water (even moisture from the air) and sets into hard gypsum: $CaSO X_{4} \cdot \frac{1}{2} H X_{2} O + 1 \frac{1}{2} H X_{2} O CaSO X_{4} \cdot 2 H X_{2} O$ . If it absorbed moisture during storage, it would become a hard lump and be useless.

It absorbs moisture and sets into hard gypsum, which makes it useless.

What is a neutralisation reaction? Give two examples.

Solution

The reaction between an acid and a base to form a salt and water is called neutralisation.

$NaOH (aq) + HCl (aq) NaCl (aq) + H X_{2} O (l)$
$Mg (OH) X_{2} + 2 HCl MgCl X_{2} + 2 H X_{2} O$ (an antacid neutralising stomach acid)

Acid + base → salt + water, e.g. NaOH + HCl → NaCl + H₂O.

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

Solution

Washing soda (Na₂CO₃·10H₂O): (1) removes the permanent hardness of water; (2) used in the glass, soap and paper industries and as a cleaning agent at home.
Baking soda (NaHCO₃): (1) in baking powder, where it releases CO₂ to make cakes and bread soft and spongy; (2) as an antacid to relieve acidity, and in soda-acid fire extinguishers.

See the uses listed above.

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