Describe acids in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange

Cambridge IGCSE Chemistry 0620

Topic: Acids, Bases and Salts – Characteristic Properties of Acids and Bases

Learning Objective

Describe acids in terms of their effect on:

1. Litmus
2. Thymolphthalein
3. Methyl orange

1. What is an Acid?

• Definition: An acid is a substance that donates a proton (H⁺) to water.
• In solution: an aqueous acid dissociates to give hydrogen ions (H⁺).
  Example: HCl → H⁺ + Cl⁻
• Result: the concentration of H⁺ ions in the solution increases, giving a pH < 7.

2. What is a Base / Alkali?

• Definition: A base is a substance that accepts a proton or produces hydroxide ions (OH⁻) in water.
• Typical bases: metal oxides (e.g., CaO) and metal hydroxides (e.g., NaOH, KOH).
• Alkalis: soluble bases. In aqueous solution an alkali dissociates to give hydroxide ions (OH⁻).
  Example: NaOH → Na⁺ + OH⁻
• Result: the concentration of OH⁻ ions increases, giving a pH > 7.

3. pH and the pH‑Scale

• pH = –log₁₀[H⁺]
• Acidic: pH < 7 Neutral: pH = 7 Basic (alkaline): pH > 7

4. Strength of Acids (supplementary)

• Strong acids dissociate completely (e.g., HCl, H₂SO₄).
• Weak acids dissociate only partially (e.g., CH₃COOH, H₂CO₃).
• Both strong and weak acids give the same colour changes with indicators; the difference is the amount of H⁺ released.

5. Characteristic Reactions of Acids (core syllabus)

• With metals: Acid + metal → salt + hydrogen gas
  e.g. 2 HCl + Zn → ZnCl₂ + H₂↑
• With carbonates (or bicarbonates): Acid + carbonate → salt + water + carbon dioxide
  e.g. 2 HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑
• With bases (neutralisation): Acid + base → salt + water
  General ionic equation: H⁺ + OH⁻ → H₂O

6. Indicators – How Acids Affect Their Colour

6.1 Litmus

• Transition range: pH ≈ 4.5 – 8.3 (around the neutral region).
• In an acidic solution (pH < 7): blue litmus turns red.
• In a neutral or basic solution (pH ≥ 7): litmus remains blue.

6.2 Thymolphthalein

• Transition range: pH 9.3 – 10.5 (well into the basic region).
• In an acidic solution (pH < 7): the indicator stays colourless because the pH never reaches its transition range.
• In a strongly basic solution (pH > 9.3): it turns blue.
• Mechanism: HIn (colourless) ⇌ In⁻ (blue) + H⁺; de‑protonation at high pH produces the blue colour.

6.3 Methyl orange

• Transition range: pH 3.1 – 4.4.
• In an acidic solution (pH < 3.1): the indicator appears red.
• In a neutral to slightly basic solution (pH > 4.4): it changes to orange‑yellow.
• Mechanism: HIn (red) ⇌ In⁻ (orange‑yellow) + H⁺; loss of a proton gives the orange‑yellow conjugate base.

7. Summary of Indicator Colour Changes

8. Suggested Classroom Diagram

9. Key Points to Remember

1. An acid is a proton donor; in water it produces H⁺ ions (e.g., HCl → H⁺ + Cl⁻).
2. A base is a proton acceptor; soluble bases (alkalis) produce OH⁻ ions (e.g., NaOH → Na⁺ + OH⁻).
3. Acidic solutions have pH < 7; basic solutions have pH > 7.
4. Blue litmus → red in an acidic solution; remains blue in neutral/basic media.
5. Thymolphthalein stays colourless in acids and only turns blue when the solution is strongly basic (pH > 9.3).
6. Methyl orange is red in strong acids (pH < 3.1) and changes to orange‑yellow as the solution becomes less acidic (pH > 4.4).
7. Acids react characteristically with:
   • Metals – producing a salt and H₂ gas.
   • Carbonates/bicarbonates – producing a salt, H₂O and CO₂.
   • Bases – neutralisation to give a salt and water (H⁺ + OH⁻ → H₂O).