Acids, Bases and Salts
Introduction
Acids and bases are fundamental classes of chemical compounds. In ICSE Class 10 Chemistry, you study their definitions, properties, the pH scale, types of salts, and preparation methods.
Theories of Acids and Bases
Arrhenius Theory
- Acid: A substance that dissociates in water to give H⁺ ions (H₃O⁺).
- Example: HCl → H⁺ + Cl⁻
- Base: A substance that dissociates in water to give OH⁻ ions.
- Example: NaOH → Na⁺ + OH⁻
Bronsted-Lowry Theory
- Acid: A proton (H⁺) donor.
- Base: A proton (H⁺) acceptor.
- Conjugate acid-base pair: An acid and its corresponding base that differ by one H⁺.
| Acid | Conjugate base | Base | Conjugate acid |
|---|---|---|---|
| HCl | Cl⁻ | NH₃ | NH₄⁺ |
| H₂O | OH⁻ | H₂O | H₃O⁺ |
| CH₃COOH | CH₃COO⁻ | OH⁻ | H₂O |
pH Scale
The pH scale measures the concentration of H⁺ ions in a solution.
pH = −log[H⁺]
| pH value | Nature | Example |
|---|---|---|
| 0−3 | Strongly acidic | Stomach acid (pH ~1) |
| 4−6 | Weakly acidic | Lemon juice (pH ~2), Vinegar (pH ~3) |
| 7 | Neutral | Pure water |
| 8−10 | Weakly alkaline | Baking soda (pH ~9) |
| 11−14 | Strongly alkaline | NaOH solution (pH ~14) |
Types of Salts
| Salt type | Definition | Example |
|---|---|---|
| Normal salt | All H⁺ of acid replaced by metal | NaCl, Na₂SO₄ |
| Acid salt | Contains replaceable H⁺ | NaHSO₄, NaHCO₃ |
| Basic salt | Contains OH⁻ group | Cu(OH)Cl, Zn(OH)Cl |
| Double salt | Two different cations/anions in a crystal | KAl(SO₄)₂·12H₂O (alum) |
| Complex salt | Contains complex ion | K₄[Fe(CN)₆] |
| Mixed salt | More than one cation/anion (not as complex) | NaClO·Na₂CO₃ |
Preparation of Salts
| Method | Suitable for | Example |
|---|---|---|
| Acid + Metal | Reactive metals | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| Acid + Base (Neutralisation) | All salts | NaOH + HCl → NaCl + H₂O |
| Acid + Carbonate | Carbonates | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ |
| Acid + Metal oxide | Metal oxides | CuO + 2HCl → CuCl₂ + H₂O |
| Direct combination | Some salts | 2Fe + 3Cl₂ → 2FeCl₃ |
| Double decomposition | Insoluble salts | BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl |
Action of Dilute Acids on Salts
| Salt | Dilute acid | Product | Observation |
|---|---|---|---|
| Carbonate | Dilute HCl | CO₂ gas | Effervescence, lime water turns milky |
| Sulphite | Dilute HCl | SO₂ gas | Pungent smell, turns K₂Cr₂O₇ green |
| Sulphide | Dilute HCl | H₂S gas | Rotten egg smell, turns lead acetate black |
| Nitrite | Dilute HCl | NO gas | Brown fumes in air |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing acid salt with acidic solution | An acid salt still has a replaceable H (e.g., NaHSO₄) |
| Thinking all bases are soluble in water | Only alkalis (NaOH, KOH) are soluble bases |
| Using the wrong indicator for pH | Universal indicator gives a range of colours at different pH |
| Forgetting the difference between normal and acid salts | Normal salt has NO replaceable H; acid salt has at least one |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: Arrhenius vs Bronsted-Lowry theories, pH scale, salt types, salt preparation methods.
Marks Blueprint: Theories — 2 marks, pH scale — 2 marks, Salt types/Preparation — 3 marks, Reactions — 2 marks.
Self-Test Questions
-
State the Arrhenius definition of acids and bases. Give one example of each.
-
Define pH. What is the pH of pure water? What does a pH of 2 indicate?
-
Differentiate between normal salt and acid salt with examples.
-
Describe how you would prepare sodium chloride from sodium hydroxide.
-
Write balanced equations for the action of dilute HCl on (a) CaCO₃, (b) Na₂SO₃.
-
What is a conjugate acid-base pair? Identify the conjugate base of H₂O and the conjugate acid of NH₃.
