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

Methyl orange turns pink/red in dilute sulphuric acid because sulphuric acid is an acidic solution.

Phenolphthalein:** Colourless in dilute HCl (acidic); pink in dilute NaOH (basic).

• Methyl Orange: Pink/red in dilute HCl (acidic); yellow in dilute NaOH (basic).
• Juice of Red Rose / Hibiscus: Dark pink/red in dilute HCl (acidic); green/yellowish-green in dilute NaOH (basic).

The characteristic odour of onion remains unchanged in the presence of dilute hydrochloric acid (first paper), but the odour completely disappears in the presence of dilute sodium hydroxide (second paper).

Pickles contain organic acids (such as acetic acid or citric acid). Acids react with metals like copper and aluminium to form toxic metal salts and release hydrogen gas, making the pickle poisonous and unfit for consumption.

Sabzi contains turmeric, which acts as a natural indicator. Soap solution is basic/alkaline in nature. Turmeric changes its colour from yellow to reddish-brown/red in the presence of a basic medium.

A visually disabled student can use Olfactory Indicators. These substances change their smell/odour in acidic or basic media rather than their colour. Examples include

1. Onion juice / onion paper
2. Vanilla extract / Clove oil

1. Reaction with Magnesium (Mg)

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂↑

1. Reaction with Zinc (Zn)

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂↑

1. Reaction with Iron (Fe)

Fe (s) + 2HCl (aq) → FeCl₂ (aq) + H₂↑

(i) Do you observe any changes on the surface of granulated zinc? What are the changes?**

(ii) How will you test the gas which evolves during the reaction?

(i) ? ?

(i) Bubbles of hydrogen gas are seen forming on the surface of granulated zinc.

(ii) Bring a burning matchstick or candle near the mouth of the test tube releasing the gas. The gas burns with a distinct ‘pop’ sound, confirming it is hydrogen gas (H₂).

• Equation

2NaOH (aq) + Zn (s) → [warm] Na_2ZnO₂ (aq) + H₂↑

(i) Do you notice any change?**

(ii) How will you test that the gas being produced is carbon dioxide?

(i) ?

(i) Rapid effervescence (foaming) occurs due to the release of carbon dioxide gas.

(ii) Pass the evolved gas through freshly prepared lime water (calcium hydroxide solution, Ca(OH)₂). The lime water turns milky due to the formation of insoluble calcium carbonate (CaCO₃).

• Equations

Na_2CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + H_2O (l) + CO₂↑

NaHCO₃ (s) + HCl (aq) → NaCl (aq) + H_2O (l) + CO₂↑

(i) What is the reason for the change in colour of the solution to light pink?**

(ii) As soon as we add another drop of alkali to the neutral solution it becomes pink. Can you tell what will be the nature of this solution?

(i) ?

(i) Initially, the added NaOH neutralizes the acid. Once all acid molecules have completely reacted, the solution becomes neutral. Adding an extra drop of NaOH turns the solution basic, causing phenolphthalein to turn light pink.

(ii) The nature of this solution is basic/alkaline.

• Equation

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

When metals react with dilute hydrochloric acid, they displace hydrogen to form metal chloride salts and evolve hydrogen gas.

• General Equation: Acid + Metal → Salt + Hydrogen
• Example: Zinc metal reacts with dilute hydrochloric acid to produce zinc chloride salt and hydrogen gas.

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂↑

Ca(HCO₃)₂ (aq) + 2HCl (aq) → CaCl₂ (aq) + 2H_2O (l) + 2CO₂↑

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

Non-metal oxides (like sulphur dioxide, SO₂) react with bases (like sodium hydroxide, NaOH) to form salt and water, similar to an acid-base neutralization reaction.

• Equation:

SO₂ (g) + 2NaOH (aq) → Na_2SO₃ (aq) + H_2O (l)

(Sulphur dioxide + Sodium hydroxide → Sodium sulphite + Water)

(i) Did the bulb light up and glow when hydrochloric acid solution was used?**

(ii) Did the bulb glow when you used sodium hydroxide solution?

(i) ?

(i) Yes, the bulb lit up and glowed brightly with dilute hydrochloric acid solution.

(ii) Yes, the bulb lit up and glowed with dilute sodium hydroxide solution as well.

• Reason: Both acids and alkalis undergo ionization in water, forming free ions (H^+ and OH^- respectively) that conduct electric current.

No, the bulb did not glow. Although glucose contains hydrogen atoms (C_6H₁₂O₆), it does not ionize in water to produce free H^+ ions, so it does not conduct electricity and is not acidic.

(i) Did you notice any gas coming out of the test tube?**

(ii) Did the dry blue litmus paper change its colour?

(iii) Did the moist blue litmus paper change its colour?

(i) ?

(iii) ?

(i) Yes, dry hydrogen chloride (HCl) gas evolves.

(ii) No, the dry blue litmus paper shows no change in colour because dry HCl gas cannot ionize to release H^+ ions in the absence of moisture.

(iii) Yes, moist blue litmus paper turns red because HCl gas dissolves in moisture to form H^+ ions, showing acidic behavior.

Acids:** HNO₃ (Nitric acid) and HCl (Hydrochloric acid).

• (Note: Na_2CO₃ is a basic salt and Ca(OH)₂ is a base).

Sulphuric acid (H_2SO₄)** is a strong acid because its molecules ionize completely in aqueous solution to yield a high concentration of hydrogen ions (H^+).

• Ammonium hydroxide (NH_4OH) is a weak base because its molecules ionize only partially in aqueous solution, yielding a low concentration of hydroxide ions (OH^-).

Sodium hydroxide (NaOH) pellets are solid and dry initially, so they cannot release free OH^- ions to show basic nature. After some time, NaOH absorbs moisture from the surrounding air (hygroscopic nature), forms an aqueous film, ionizes to release OH^- ions, and turns red litmus blue.

Sulphuric acid (H_2SO₄) and acetic acid (CH_3COOH) ionize in aqueous solution to produce hydrogen ions (H^+), which are responsible for acidic properties. Glucose and starch dissolve in water without ionizing to form H^+ ions, so they do not show acidic properties.

Yes, H^+ ions are present in basic solutions due to the partial ionization of water (H_2O ⇌ H^+ + OH^-). However, the solution is basic because the concentration of hydroxide ions (OH^-) is significantly higher than the concentration of hydrogen ions (H^+).

1. Hydrogen ion concentration: Solution “A” (pH = 6, [H^+] = 10^-6 mol/L) has a higher H^+ concentration than Solution “B” (pH = 8, [H^+] = 10^-8 mol/L) because lower pH indicates higher [H^+].
2. Nature:

• Solution “A” (pH = 6 < 7) is acidic.
• Solution “B” (pH = 8 > 7) is basic.

(a) Weak acid: Solution E (pH = 6)

• (b) Weak base: Solution A (pH = 9)
• (c) Strong acid: Solution C (pH = 1)
• (d) Strong base: Solution D (pH = 13)
• (e) Neutral: Solution B (pH = 7)

Hydrogen ion concentration [H^+] increases as pH decreases.

• pH values: Solution D (13) < Solution A (9) < Solution B (7) < Solution E (6) < Solution C (1)
• Increasing order of [H^+]:

D (pH 13) < A (pH 9) < B (pH 7) < E (pH 6) < C (pH 1)

(10^-13 mol/L < 10^-9 mol/L < 10^-7 mol/L < 10^-6 mol/L < 10^-1 mol/L)

Basic radicals** (positive part coming from base)

1. NH_4Cl → NH₄^+ (Ammonium ion)
2. KNO₃ → K^+ (Potassium ion)
3. (NH₄)_2CO₃ → NH₄^+ (Ammonium ion)
4. CuSO₄ → Cu²⁺ (Copper ion)

• Acidic radicals (negative part coming from acid)

1. NH_4Cl → Cl^- (Chloride ion)
2. KNO₃ → NO₃^- (Nitrate ion)
3. (NH₄)_2CO₃ → CO₃^2- (Carbonate ion)
4. CuSO₄ → SO₄^2- (Sulphate ion)

The aqueous solution of potassium chloride (KCl) will be neutral (pH = 7).

• Explanation: KCl is formed by the neutralization reaction between a strong base, potassium hydroxide (KOH), and a strong acid, hydrochloric acid (HCl). Salts formed from a strong acid and a strong base are neutral.

These acidic gases dissolve in rain water to form carbonic acid (H_2CO₃), sulphuric acid (H_2SO₄), and nitric acid (HNO₃), causing Acid Rain (pH < 5.6).

• Effects:

1. Damages plants, crops, and forests by turning soil acidic.
2. Harms aquatic animals and marine life as water bodies become acidic.
3. Corrodes historical monuments, buildings, and metallic structures.

(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.

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)