Acids & Alkalis
Year 7 🧪 Acids, Bases & Salts Use the pH scale 1–14; identify common acids, alkalis and neutral substances.
🧪 The pH Scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a substance is.
| pH Range | Classification | Examples |
|---|---|---|
| 0–6 | Acidic | HCl (1), vinegar (3), cola (3.5) |
| 7 | Neutral | Pure water |
| 8–14 | Alkaline | Baking soda (9), bleach (12), NaOH (14) |
Each unit of pH = 10× change in H⁺ concentration. pH 3 is 10× more acidic than pH 4.
🔬 Common Acids and Alkalis
Common acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), ethanoic acid (vinegar), citric acid (lemons).
Common alkalis: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (limewater), ammonia solution (NH₃).
Key Definition
$$\text{Acids produce H}^+ \text{ ions in water. Alkalis produce OH}^- \text{ ions.}$$🌈 Indicators
An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline.
| Indicator | In acid | In neutral | In alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Universal indicator | Red/orange | Green | Blue/violet |
| Phenolphthalein | Colourless | Colourless | Pink |
Ready to test yourself? Click the Quiz tab above to practise questions on this topic!
Interactive Demonstration — Acids & Alkalis
🌡️ pH Calculator