Acids
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. Acids have pH less than 7.
Common Acids
| Name | Formula | Use |
|---|---|---|
| Hydrochloric acid | HCl | Stomach acid, cleaning metals |
| Sulfuric acid | H₂SO₄ | Car batteries, fertilisers |
| Nitric acid | HNO₃ | Fertilisers, explosives |
| Ethanoic acid | CH₃COOH | Vinegar |
Bases and Alkalis
A base is a substance that reacts with an acid to form salt and water. An alkali is a base that is soluble in water — it produces hydroxide ions (OH⁻). Alkalis have pH greater than 7.
Examples: NaOH (caustic soda), Ca(OH)₂ (lime), KOH (potash)
Neutralisation
Acid + Base → Salt + Water
HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
pH Scale
pH 0–6: Acidic | pH 7: Neutral | pH 8–14: Alkaline
Indicators
- Litmus: red in acid, blue in alkali
- Phenolphthalein: colourless in acid, pink in alkali
- Universal indicator: gives a range of colours across the pH scale