Indicators & pH
Year 7 🧪 Acids, Bases & Salts Use litmus, universal indicator and pH meters to identify substances.
🌈 Types of Indicators
Indicators change colour at different pH values, allowing classification of solutions as acidic, neutral or alkaline.
Litmus mnemonic: RUBA — Red Under Bleached Acid, Blue Under Base (Alkali).
| Indicator | Acid (pH <7) | Neutral | Alkali (pH >7) |
|---|---|---|---|
| Red litmus | Stays red | Red | Turns blue |
| Blue litmus | Turns red | Blue | Stays blue |
| Universal indicator | Red/orange/yellow | Green | Blue/violet |
| Phenolphthalein | Colourless | Colourless | Pink |
🔬 Universal Indicator Colours by pH
| pH | 0–1 | 2–3 | 4–5 | 6 | 7 | 8–9 | 10–11 | 12–14 |
|---|---|---|---|---|---|---|---|---|
| Colour | Red | Orange | Yellow | Yellow-green | Green | Blue | Indigo | Violet |
A pH meter gives the most precise readings — accurate to 0.01 pH units.
🧪 Choosing the Right Indicator
Different indicators suit different titrations. Select one that changes sharply at the equivalence point.
Phenolphthalein: colourless → pink at pH 8.2–10. Best for strong acid + strong base.
Methyl orange: red → orange at pH 3.1–4.4. Best for strong acid + weak base.
Methyl orange: red → orange at pH 3.1–4.4. Best for strong acid + weak base.
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Interactive Demonstration — Indicators & pH
🌡️ pH & Indicator Calculator