Chemistry (5070)
Produce H⁺ ions in water pH less than 7 Taste sour (DON'T taste lab chemicals!) Examples: HCl (hydrochloric acid), H₂SO₄ (sulfuric acid), citric acid (lemons)
Bases neutralise acids. An alkali is a base that dissolves in water Produce OH⁻ ions in water pH greater than 7 Feel soapy/slippery Examples: NaOH (sodium hydroxide), Ca(OH)₂ (lime water), ammonia
pH 0-6: Acidic (lower = stronger acid) pH 7: Neutral (pure water) pH 8-14: Alkaline (higher = stronger alkali)
Litmus: Red in acid, Blue in alkali Universal indicator: Full colour spectrum (red → green → purple)
HCl + NaOH → NaCl + H₂O The key reaction: H⁺ + OH⁻ → H₂O
Acid + Metal → Salt + Hydrogen Acid + Carbonate → Salt + Water + CO₂ Acid + Base → Salt + Water
Topic 3 of 5Cambridge O Levels
Acids, Bases & Salts
pH scale, neutralisation, and everyday chemistry
Acids and bases are everywhere in daily life!
Acids:
Bases & Alkalis:
pH Scale: Measures acidity/alkalinity from 0-14
Indicators: Change colour to show pH
Neutralisation: Acid + Base → Salt + Water
Making Salts:
Key Points to Remember
- 1Acids: pH < 7, produce H⁺ ions
- 2Alkalis: pH > 7, produce OH⁻ ions
- 3Neutralisation: Acid + Base → Salt + Water
- 4Name of salt depends on acid: HCl→chloride, H₂SO₄→sulfate, HNO₃→nitrate
Pakistan Example
Lemon Water & Antacids
When you eat too much biryani at a dawat and get heartburn (acidity), you take an antacid tablet (ENO, or Gaviscon). Your stomach has hydrochloric acid (HCl, pH ~2) to digest food. Overeating produces too much acid. The antacid contains a base like magnesium hydroxide: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. It neutralises the acid, bringing pH closer to 7. Meanwhile, nimbu paani (lemon water) is acidic (pH ~3) — it doesn't cure acidity, but the cold water and salt can help settle your stomach!