Notes Class 10 Science Chapter 2 Acids Bases and Salts

Notes For All Chapters Science Class 10 CBSE

1. Introduction

Substances taste sour due to acids and bitter due to bases.

Acids and bases neutralise each other’s effects.

Indicators are used to identify acids and bases.

• Natural indicators: Litmus, turmeric, red cabbage, Hydrangea, Petunia, Geranium.
• Synthetic indicators: Phenolphthalein, methyl orange.
• Olfactory indicators: Substances that change smell in acidic or basic medium (onion, vanilla, clove).

2. Chemical Properties of Acids and Bases

(a) Reaction with Metals

Acid + Metal → Salt + Hydrogen gas

• Example:
• Zn + H₂SO₄ → ZnSO₄ + H₂↑

Bases also react with certain metals:
2NaOH + Zn → Na₂ZnO₂ (sodium zincate) + H₂↑

(b) Reaction with Metal Carbonates & Hydrogencarbonates

Acid + Metal carbonate → Salt + CO₂ + H₂O

Acid + Metal hydrogencarbonate → Salt + CO₂ + H₂O

• Example:
• Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

(c) Reaction between Acids and Bases (Neutralisation)

Acid + Base → Salt + Water

• Example:
• NaOH + HCl → NaCl + H₂O

(d) Reaction with Metallic Oxides

Metal oxide + Acid → Salt + Water

• Example:
• CuO + 2HCl → CuCl₂ + H₂O

Metallic oxides are basic in nature.

(e) Reaction with Non-metallic Oxides

Non-metal oxide + Base → Salt + Water

• Example:
• CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

Non-metallic oxides are acidic in nature.

3. What Do All Acids and Bases Have in Common?

Acids produce H⁺ (aq) or H₃O⁺ ions in water.

Bases produce OH⁻ ions in water.

Acids and bases conduct electricity in aqueous solution (due to ions).

Acids show their properties only in aqueous solution, not in dry state.

Example: Dry HCl gas does not change litmus colour, but HCl solution does.

Important Reactions

• HCl + H₂O → H₃O⁺ + Cl⁻
• NaOH → Na⁺ + OH⁻

4. Dilution of Acids and Bases

Mixing acids/bases with water is highly exothermic.

Always add acid to water slowly, never the reverse.

Dilution decreases H₃O⁺/OH⁻ ion concentration per unit volume.

5. Strength of Acids and Bases – pH Scale

pH scale ranges from 0 to 14.

pH = 7 → Neutral solution

pH < 7 → Acidic solution pH > 7 → Basic solution

Universal indicator is used to measure pH.

Strong acids/bases: Produce more H⁺/OH⁻ ions.

Weak acids/bases: Produce fewer ions.

6. Importance of pH in Everyday Life

Human body: Works in narrow pH range (7.0 – 7.8).

Acid rain: pH < 5.6 harms aquatic life.

Soil pH: Affects plant growth.

Digestion: Stomach produces HCl; antacids (like Mg(OH)₂) neutralise excess acid.

Tooth decay: Occurs if mouth pH < 5.5.

Self-defense:

• Bee sting → Acidic (methanoic acid), relieved by mild base (baking soda).
• Nettle sting → Methanoic acid, treated by dock plant leaf (basic in nature).

7. Salts

(a) Family of Salts
Salts with same positive or negative radical belong to one family.

• Example: NaCl and Na₂SO₄ → Sodium salts.

(b) pH of Salts
Salt of strong acid + strong base → Neutral (pH = 7).

Salt of strong acid + weak base → Acidic (pH < 7). Salt of strong base + weak acid → Basic (pH > 7).

8. Chemicals from Common Salt (NaCl)

(i) Sodium Hydroxide (NaOH)

Produced by electrolysis of brine (Chlor-alkali process).

Products: NaOH, Cl₂, H₂.

Uses: Soap, paper, textiles, detergents, etc.

(ii) Bleaching Powder (CaOCl₂)

Produced by Cl₂ acting on Ca(OH)₂.

Uses: Disinfectant, bleaching, oxidising agent.

(iii) Baking Soda (NaHCO₃)

Mild base.

Uses: Baking powder, antacids, fire extinguishers.

(iv) Washing Soda (Na₂CO₃·10H₂O)

Obtained by recrystallisation of sodium carbonate.

Uses: Glass, soap, paper industries, cleaning agent, removes hardness of water.

(v) Plaster of Paris (CaSO₄·½H₂O)

Obtained by heating gypsum (CaSO₄·2H₂O).

Used in plastering, making toys, decoration, smooth surfaces.

9. Water of Crystallisation

Some salts contain fixed number of water molecules in their structure.

Examples:

• CuSO₄·5H₂O (Blue vitriol) – 5 water molecules.
• Na₂CO₃·10H₂O (Washing soda) – 10 water molecules.
• CaSO₄·2H₂O (Gypsum) – 2 water molecules.

10. Key Points to Remember

Acids → produce H⁺ in solution.

Bases → produce OH⁻ in solution.

Acid + Base → Salt + Water (Neutralisation).

Indicators help identify acids and bases.

pH scale measures strength of acids/bases.

Salts and their derivatives (NaOH, baking soda, washing soda, bleaching powder, POP) have important daily uses.