CG Board Class 10 Science Chapter 2 Solutions: Acids, Bases and Salts (अम्ल, क्षारक एवं लवण) (2026-27)

(i) In lemon juice:**

• (a) H^+ ions are more, OH^- ions are less
• (b) H^+ ions are less, OH^- ions are more
• (c) H^+ ions and OH^- ions are equal
• (d) Only H^+ ions are present

(a) H^+ ions are more, OH^- ions are less

Explanation: Lemon juice is acidic (pH ≈ 2.2 < 7). In acidic solutions, the concentration of H^+ ions is greater than that of OH^- ions.

Answer: (b) Salt and water and carbon dioxide

Explanation: Metal carbonate + Acid → Salt + Water + Carbon dioxide.

Answer: (c) CH_3COOH

Explanation: Acetic acid (CH_3COOH) is an organic weak acid that ionizes partially in water. HCl, HNO₃, and H_2SO₄ are strong mineral acids.

Answer: (d) 7

Explanation: At 25°C, a neutral solution has equal amounts of H^+ and OH^- ions (10^-7 mol/L), making its pH equal to 7.

Answer: (c) A weak base

Explanation: Excess stomach acid causing acidity is neutralized safely using antacids, which are mild/weak bases like magnesium hydroxide or baking soda.

Answer: (c) Being less than 5.5

Explanation: Tooth enamel (calcium phosphate) corrodes and starts decaying when the pH of saliva drops below 5.5 due to acid produced by mouth bacteria.

Answer: (c) NH_4Cl

Explanation: Ammonium chloride (NH_4Cl) is formed from a strong acid (HCl) and a weak base (NH_4OH), making it acidic in nature. NaCl, Na_2SO₄, and KNO₃ are neutral salts.

1. Tartaric acid (found in tamarind)
2. Citric acid / Malic acid (found in citrus fruits / apples)
3. Acetic acid (found in vinegar)

Olfactory indicators (such as onion juice, vanilla extract, or clove oil) change their characteristic smell/odour in basic media but retain their smell in acidic media.

• In Acids: The characteristic odour of the indicator remains unchanged.
• In Bases: The characteristic odour of the indicator vanishes completely.

When fresh milk sets into curd, bacteria convert lactose sugar into lactic acid. The formation of lactic acid increases the concentration of H^+ ions, making the curd more acidic. Therefore, the pH value will decrease below 6 (typically between 4.5 and 5.5).

1. Identify the Base: Dip the red litmus paper into each of the three test tubes one by one. The test tube that turns the red litmus blue contains the basic solution.
2. Identify Acid and Distilled Water: Divide the newly formed blue litmus paper into two strips

• Dip one strip into the remaining two test tubes. The test tube that turns the blue litmus back to red contains the acidic solution.
• The remaining third test tube, which causes no colour change on either red or blue litmus paper, contains distilled water (neutral solution).

Chemical Reaction:**

2NaCl (s) + H_2SO₄ (conc.) → Na_2SO₄ (aq) + 2HCl (g)↑

Dry hydrogen chloride (HCl) gas is produced during this reaction.

• Dry Litmus Test: Dry HCl gas does not ionize in the absence of water to release H^+ ions, so it does not show acidic behavior on dry blue litmus paper.
• Moist Litmus Test: When HCl gas contacts moist blue litmus paper, it dissolves in water to form H^+ ions (HCl + H_2O → H_3O^+ + Cl^-). These free H^+ ions turn the moist blue litmus paper red.

(a) Basic substances (pH > 7): Solution of baking soda (pH = 8.2) and Sodium hydroxide solution (pH = 13).

• (b) Acidic substances (pH < 7): Lemon juice (pH = 2.2) and Vinegar (pH = 5.5).
• (c) Neutral substance (pH = 7): Water (pH = 7).

(a) Acid “B” is a strong acid (ionizes completely) and Acid “A” is a weak acid (ionizes partially).

• (b) Weak Acid: An acid whose molecules ionize only partially in aqueous solution, yielding a lower concentration of hydrogen ions (H^+), is called a weak acid.
• (c) Strong Acid: An acid whose molecules ionize completely in aqueous solution, yielding a high concentration of hydrogen ions (H^+), is called a strong acid.
• (d) Examples:
• Strong Acids: Hydrochloric acid (HCl), Sulphuric acid (H_2SO₄), Nitric acid (HNO₃).
• Weak Acids: Acetic acid (CH_3COOH), Carbonic acid (H_2CO₃).

1. Gas Displaced: Hydrogen gas (H₂) is displaced when acids react with metals.
2. Chemical Equation with Magnesium:

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂↑

(Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen gas)

1. Test for Gas: Bring a burning candle or matchstick near the mouth of the test tube where the gas is evolving. The gas burns with a popping sound (‘pop’ test), confirming the gas is hydrogen (H₂).

(a) Procedure:

1. Take crushed egg shells (which are composed primarily of calcium carbonate, CaCO₃) in a test tube.
2. Add 2-3 mL of dilute hydrochloric acid (HCl) using a dropper.
3. Observe effervescence/foaming caused by the evolving gas.
4. Place a lighted agarbatti/matchstick inside the mouth of the test tube after foaming subsides.

• (b) Observations / Experimental set-up:
• Fast effervescence occurs as carbon dioxide gas (CO₂) is generated.
• Carbon dioxide does not support combustion, so the lighted agarbatti gets extinguished immediately.
• (c) Balanced Chemical Equation:

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H_2O (l) + CO₂↑

(Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide)

Analysis:** Paddy crops grow ideally in soil with a pH between 5 and 8. A pH of 4.2 indicates that Tikeshweri’s field soil is highly acidic.

• Remedy: To neutralize excess acidity and increase soil pH to the optimal range (5 to 8), Tikeshweri should add basic substances like slaked lime / quicklime (Ca(OH)₂ / CaO) or wood ash to her field.

Definition: The chemical reaction between an acid and a base/alkali to form salt and water is called a neutralization reaction**.

• General Equation: Acid + Base → Salt + Water
• Example 1:

HCl (aq) + NaOH (aq) → NaCl (aq) + H_2O (l)

(Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water)

• Example 2:

HNO₃ (aq) + KOH (aq) → KNO₃ (aq) + H_2O (l)

(Nitric acid + Potassium hydroxide → Potassium nitrate + Water)

Fresh milk has a pH of 6 (slightly acidic). When baking soda (a basic salt, NaHCO₃) is added, the milk becomes alkaline with a pH of 8. During curd formation, bacteria produce lactic acid which neutralizes the added alkali first before lowering the pH to make the medium acidic enough for curdling. Therefore, alkaline milk takes a longer time to set into curd.

1. Definition of Salt: A salt is an electrically neutral ionic compound formed during the neutralization reaction between an acid and a base. It consists of a positive basic radical (from base) and a negative acidic radical (from acid).
2. Determination of Nature: The chemical nature (acidic, basic, or neutral) of a salt depends on the strengths of the acid and base from which it is derived

• Strong Acid + Weak Base → Acidic Salt (pH < 7)
• Weak Acid + Strong Base → Basic Salt (pH > 7)
• Strong Acid + Strong Base → Neutral Salt (pH = 7)

1. Examples:

• Ammonium Nitrate (NH_4NO₃):
• Basic radical: NH₄^+ from weak base NH_4OH
• Acidic radical: NO₃^- from strong acid HNO₃
• Nature: Acidic salt (pH < 7)
• Sodium Carbonate (Na_2CO₃):
• Basic radical: Na^+ from strong base NaOH
• Acidic radical: CO₃^2- from weak acid H_2CO₃
• Nature: Basic salt (pH > 7)