CBSE Class 10 Science — Acids Bases and Salts

Chapter Overview & Weightage

Acids, Bases and Salts is Chapter 2 of CBSE Class 10 Science. It covers the properties of acids and bases, neutralization reactions, the pH scale, and preparation of important salts. This is consistently one of the highest-weightage chapters in Class 10 board exams.

Key Concepts You Must Know

Arrhenius definition:

• Acid: substance that produces $H^+$ (or $H_3O^+$) ions in water. Example: $\text{HCl} \to H^+ + Cl^-$
• Base: substance that produces $OH^-$ ions in water. Example: $\text{NaOH} \to Na^+ + OH^-$

Properties of acids:

• Sour taste (do not taste in lab!)
• Turn blue litmus red
• React with metals to produce $H_2$ gas: Zn + H₂SO₄ → ZnSO₄ + H₂↑
• React with metal carbonates/bicarbonates to produce $CO_2$: Na₂CO₃ + HCl → NaCl + H₂O + CO₂↑
• React with metal oxides to form salt + water: CuO + H₂SO₄ → CuSO₄ + H₂O

Properties of bases:

• Bitter taste, soapy feel
• Turn red litmus blue
• React with acids (neutralization): NaOH + HCl → NaCl + H₂O
• React with non-metallic oxides: NaOH + CO₂ → Na₂CO₃ + H₂O
• Strong bases react with certain metals: 2NaOH + Zn → Na₂ZnO₂ + H₂↑ (zinc dissolves in both acid and base — amphoteric)

pH scale: Measures acidity/basicity from 0 to 14.

• pH 0–6: Acidic (lower pH = more acidic)
• pH 7: Neutral
• pH 8–14: Basic/Alkaline (higher pH = more basic)
• $\text{pH} = -\log_{10}[H^+]$

Neutralization: Acid + Base → Salt + Water

$H^+ + OH^- \to H_2O$

Dilution rule: “Add acid to water, not water to acid” (exothermic process — adding water to concentrated acid causes explosive splattering; add acid slowly to water for safety)

Important Formulas

Solved Previous Year Questions

PYQ 1: (CBSE 2024, 3 marks)

Q: What happens when baking soda is heated? Write the chemical equation. List two uses of baking soda.

Solution: When baking soda ($\text{NaHCO}_3$) is heated, it decomposes to form sodium carbonate, water, and carbon dioxide:

Uses of baking soda:

1. Used as a leavening agent in baking — the CO₂ produced makes bread and cakes soft and fluffy
2. Used as an antacid — neutralizes excess stomach acid (HCl) to relieve acidity: NaHCO₃ + HCl → NaCl + H₂O + CO₂

PYQ 2: (CBSE 2023, 4 marks)

Q: Explain what happens when acid reacts with a metal, a base, and a metal carbonate. Give one equation for each.

Solution:

Acid + Metal → Salt + Hydrogen gas

$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$

Observation: Bubbles of H₂ gas evolved; zinc dissolves.

Acid + Base → Salt + Water (neutralization)

$\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

Observation: Heat is released (exothermic). This is called neutralization.

Acid + Metal carbonate → Salt + Water + Carbon dioxide

$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$

Observation: Brisk effervescence (CO₂ bubbles). CO₂ gas turns lime water milky.

PYQ 3: (CBSE 2022, 2 marks)

Q: What is the pH of a neutral solution? What is the role of pH in everyday life?

Solution: A neutral solution has pH = 7.

Role of pH in everyday life:

• Soil: Farmers test soil pH to determine whether lime (to increase pH) or fertilisers need to be added. Most crops grow best at pH 6-7.
• Digestive system: Stomach has pH ~2 (HCl for protein digestion). Intestines are slightly alkaline (pH ~8).
• Tooth decay: Bacteria in mouth produce acids that lower pH below 5.5, causing enamel erosion.
• Bee stings (acidic, pH~4) treated with baking soda; wasp stings (alkaline) treated with vinegar.

Difficulty Distribution

Expert Strategy

Common Traps