Class 10 Science Chapter 2 – Acids, Bases and Salts on the NCERT textbook (2025-26). These are written in a simple, explanatory way for easy understanding.
📘 Chapter 2: Acids, Bases and Salts
🔷 2.1 Understanding the Chemical Properties of Acids and Bases
▶️ Indicators
- Indicators are substances that change color in acidic or basic media.
- Natural indicators: Litmus, turmeric, red cabbage.
- Synthetic indicators: Phenolphthalein (turns pink in base), methyl orange (red in acid, yellow in base).
- Olfactory indicators: Change smell in acid/base — e.g., onion, vanilla, clove.
▶️ Reaction of Acids with Metals
Acid + Metal → Salt + Hydrogen gas
Example:
H₂SO₄ + Zn → ZnSO₄ + H₂↑
- Hydrogen gas can be tested by bringing a burning candle near it → it burns with a ‘pop’ sound.
▶️ Reaction of Bases with Metals
Base + Metal → Salt + Hydrogen gas (with reactive metals like zinc)
Example:
2NaOH + Zn → Na₂ZnO₂ + H₂↑
(The salt formed is called sodium zincate.)
▶️ Reaction of Acids with Metal Carbonates and Bicarbonates
Metal carbonate/Metal hydrogen carbonate + Acid → Salt + CO₂ + H₂O
Examples:
- Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
- NaHCO₃ + HCl → NaCl + CO₂ + H₂O
Test for CO₂: Turns lime water milky (forms CaCO₃).
▶️ Reaction of Acid with Base (Neutralisation)
Acid + Base → Salt + Water
Example:
NaOH + HCl → NaCl + H₂O
This reaction neutralizes both acid and base.
▶️ Reaction of Metallic Oxides with Acids
Metal oxide + Acid → Salt + Water
→ Shows that metallic oxides are basic in nature.
Example:
CuO + HCl → CuCl₂ + H₂O
▶️ Reaction of Non-metallic Oxides with Bases
Non-metallic oxide + Base → Salt + Water
→ Non-metallic oxides (like CO₂) are acidic.
Example:
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
🔷 2.2 What Do All Acids and Bases Have in Common?
▶️ Acids produce H⁺ ions in water.
Example:
HCl → H⁺ + Cl⁻
But in water: H⁺ + H₂O → H₃O⁺ (Hydronium ion)
▶️ Bases produce OH⁻ ions in water.
Examples:
- NaOH → Na⁺ + OH⁻
- KOH → K⁺ + OH⁻
- Mg(OH)₂ → Mg²⁺ + 2OH⁻
🔔 Note: Acids and bases show their properties only in aqueous solution.
▶️ Exothermic Process
- Mixing acid or base with water releases a lot of heat.
- Always add acid to water slowly while stirring — never the other way.
🔷 2.3 How Strong Are Acid or Base Solutions?
▶️ pH Scale (0–14)
- Measures strength of acids and bases.
- pH < 7 → Acidic
- pH = 7 → Neutral
- pH > 7 → Basic
🧪 Measured using universal indicator or pH paper.
- Strong acid: HCl, HNO₃ (pH 1–2)
- Weak acid: Acetic acid (pH ~4–5)
- Strong base: NaOH, KOH (pH 12–14)
- Weak base: NH₄OH (pH ~9)
🔷 2.3.1 Importance of pH in Everyday Life
- Human body: Works in pH 7.0 to 7.8
- Acid rain: pH < 5.6 — harms aquatic life
- Soil pH: Affects plant growth
- Digestive system: Excess HCl causes indigestion → use antacids (e.g., milk of magnesia)
- Tooth decay: pH < 5.5 damages enamel — use basic toothpaste
- Bee stings/Nettle stings: Contain acids — use mild base like baking soda to neutralize
🔷 2.4 More About Salts
▶️ Salts are formed by neutralisation reaction of acids and bases.
Families of Salts:
- Same positive ion → same family
- NaCl and Na₂SO₄ → Sodium salts
- NaCl and KCl → Chloride salts
▶️ pH of Salts
- Strong acid + Strong base → Neutral salt (pH = 7)
- Strong acid + Weak base → Acidic salt (pH < 7)
- Weak acid + Strong base → Basic salt (pH > 7)
▶️ Chemicals from Common Salt (NaCl)
1. Sodium Hydroxide (NaOH)
- Formed by electrolysis of brine (saltwater)
- Products: NaOH, Cl₂, H₂ → called Chlor-alkali process
- Uses: Soap, paper, cleaning agents
2. Bleaching Powder (Ca(ClO)₂)
- Made by reacting Cl₂ with slaked lime
- Used for disinfecting drinking water, bleaching fabrics
3. Baking Soda (NaHCO₃)
- Mild base, non-corrosive
- Uses:
- Antacid
- Fire extinguisher
- Baking (releases CO₂ to make cakes soft)
4. Washing Soda (Na₂CO₃·10H₂O)
- Prepared by recrystallising sodium carbonate
- Used for softening water, in glass and soap industry
5. Plaster of Paris (POP) – CaSO₄·½H₂O
- Made by heating gypsum (CaSO₄·2H₂O)
- Uses: Support fractured bones, making idols, decoration
- When mixed with water → converts back to gypsum
📌 Important Reactions Summary
| Type of Reaction | General Equation |
|---|---|
| Acid + Metal | Acid + Metal → Salt + H₂ |
| Base + Metal | Base + Metal → Salt + H₂ |
| Acid + Carbonate | Acid + Metal Carbonate → Salt + CO₂ + H₂O |
| Neutralisation | Acid + Base → Salt + Water |
| Metal Oxide + Acid | Basic oxide + Acid → Salt + Water |
| Non-metallic oxide + Base | Acidic oxide + Base → Salt + Water |