Everyday Acids and Alkalis
Acids and alkalis are not just found in chemistry labs — they appear in food, household products, medicines, and the human body.
Common acids:
| Substance | Acid it contains | pH (approx) |
|---|---|---|
| Lemon juice | Citric acid | 2–3 |
| Vinegar | Ethanoic (acetic) acid | 3 |
| Stomach acid | Hydrochloric acid | 1–2 |
| Coffee | Chlorogenic acid | 5 |
| Rainwater | Carbonic acid | 5.5 |
Common alkalis:
| Substance | pH (approx) |
|---|---|
| Baking soda (sodium hydrogencarbonate) | 8–9 |
| Milk of magnesia (antacid) | 10 |
| Soap | 9–10 |
| Bleach | 12–13 |
| Drain cleaner (oven cleaner) | 13–14 |
The pH Scale
pH runs from 0 to 14:
- 0–6: acidic (lower = stronger acid)
- 7: neutral (pure water)
- 8–14: alkaline (higher = stronger alkali)
pH is measured using indicators (substances that change colour with pH), pH paper, or a digital pH meter.
Universal indicator changes through a rainbow of colours — red/orange for acids, green for neutral, blue/purple for alkalis. It gives a quick estimate of pH.
Neutralisation
Acids and alkalis react together in a neutralisation reaction:
This is how antacid medicines work: they contain a weak alkali (e.g. magnesium hydroxide or calcium carbonate) that neutralises excess stomach acid, relieving heartburn.
Safety
Strong acids and alkalis are corrosive — they cause chemical burns.
- Handle with gloves and eye protection
- If spilled on skin, wash away immediately with plenty of cold water
- Never mix bleach with acidic cleaners — toxic chlorine gas can be produced
Gardeners add lime (calcium oxide or calcium carbonate) to acidic soil to raise the pH and make conditions better for crops. This is a practical example of neutralisation in action.
Real world: Limescale removers in the kitchen
Limescale (calcium carbonate, CaCO₃) builds up in kettles, coffee machines, and taps in hard water areas. Descalers contain citric acid or acetic acid (vinegar). The acid reacts with the calcium carbonate: CaCO₃ + 2CH₃COOH → Ca(CH₃COO)₂ + H₂O + CO₂. The bubbling you see is CO₂ gas being released. A household acid (food-safe vinegar) dissolves a stubborn mineral deposit — practical acid-carbonate chemistry in your kitchen.
Real world: Vinegar preserving food
Vinegar (acetic acid, pH ~3) has been used for thousands of years to preserve food — pickles, chutneys, and marinades all rely on acidity. The low pH inhibits the growth of bacteria and moulds that cause food to spoil. The same principle explains why stomach acid (HCl, pH 1–2) kills most bacteria in food before they reach the intestines. Acidity is one of nature's most effective preservatives.
Real world: Mixing bleach with acidic cleaners is dangerous
Many households have both bleach (alkaline, pH 12–13) and acidic toilet cleaners. Mixing them releases chlorine gas (Cl₂) — a toxic yellow-green gas that was used as a chemical weapon in World War I. The acid reacts with sodium hypochlorite in bleach to release the Cl₂. This is why cleaning product labels warn never to mix different chemicals. Even at low concentrations in a confined bathroom, chlorine gas causes eye and lung irritation.
Where do you see this in real life? Vinegar descaling your kettle (acid dissolves limestone), vinegar preserving pickles (low pH stops bacteria), antacid tablets in your medicine cabinet (base neutralises stomach acid), warning labels on bleach and acidic cleaners (dangerous if mixed) — acids and alkalis are everywhere in the home, useful but requiring care.