Indicators: Methyl Orange & Thymolphthalein
15 Questions · 23 Total Marks · Cambridge IGCSE Chemistry 0620
Section A: Methyl Orange
Acid → Red | Alkali → Yellow
State the colour of methyl orange in hydrochloric acid (HCl).
State the colour of methyl orange in sulfuric acid (H₂SO₄).
State the colour of methyl orange in aqueous sodium hydroxide (NaOH).
State the colour of methyl orange in aqueous ammonia (NH₃).
A solution turns methyl orange red. Is the solution acidic, neutral, or alkaline?
Section B: Thymolphthalein
Acid → Colourless | Alkali → Blue
State the colour of thymolphthalein in sulfuric acid (H₂SO₄).
State the colour of thymolphthalein in hydrochloric acid (HCl).
State the colour of thymolphthalein in aqueous ammonia (NH₃).
State the colour of thymolphthalein in aqueous sodium hydroxide (NaOH).
A solution turns thymolphthalein colourless. What can you conclude about the solution?
Section C: Application
Combining both indicators to identify pH
A solution turns methyl orange yellow AND thymolphthalein blue. Is the solution an acid, neutral, or an alkali? Explain your reasoning.
A solution turns methyl orange red AND thymolphthalein colourless. Identify the solution as acid, neutral, or alkali.
A solution turns methyl orange yellow AND thymolphthalein colourless. What is the pH range of this solution?
Explain why a chemist would use BOTH methyl orange AND thymolphthalein together rather than just one indicator.
Complete the table: Fill in the colours of methyl orange and thymolphthalein for: (a) dilute HCl, (b) pure water, (c) aqueous KOH.
📊 Quick Reference: The Colour Matrix
| Solution | Methyl Orange | Thymolphthalein |
|---|---|---|
| Acid (pH < 7) | Red | Colourless |
| Neutral (pH = 7) | Yellow | Colourless |
| Alkali (pH > 7) | Yellow | Blue |
💡 The Detective Strategy: Use both indicators together. Only an alkali gives yellow + blue. Only a neutral gives yellow + colourless. Only an acid gives red + colourless.
🎯 Top 3 Examiner Traps (Paper 6)
- 1. Writing "orange" for methyl orange in acid → must be "red"
- 2. Writing "clear" instead of "colourless" → Cambridge accepts colourless only
- 3. Saying thymolphthalein turns "purple" in alkali → it turns "blue"
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