The pH Scale
Acidity and basicity are measured on the pH scale, running from 0 to 14. Substances closer to 0 are more acidic; those closer to 14 are more basic (alkaline).
| Substance | pH |
|---|---|
| Stomach acid (HCl) | 1.5 |
| Vinegar | 3.0 |
| Tomato juice | 4.0 |
| Milk | 6.5 |
| Rain water | 6.6 |
| Distilled water | 7.0 |
| Blood | 7.4 |
| Baking soda | 8.3 |
| Toothpaste | 8.5 |
| Detergents | 11.0 |
- Acidic solution: pH less than 7.
- Neutral solution: pH equal to 7.
- Alkaline (basic) solution: pH greater than 7. (An alkali is specifically a basic hydroxide that dissolves in water.)
Acid-Base Indicators
An indicator changes colour depending on whether its environment is acidic or basic.
| Indicator | pH range | Colour in acid | Colour in base |
|---|---|---|---|
| Litmus | 5.0–8.0 | Red | Blue |
| Phenolphthalein | 8.3–10.0 | Colourless | Pink |
| Methyl orange | 3.1–4.6 | Red | Yellow |
Properties of Acidic Solutions
Physical: dilute acids taste sour, have a pH below 7, turn blue litmus red, and — when concentrated and strong — are corrosive.
Chemical:
- With bases (neutralisation): acid + base → salt + water. E.g. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
- With reactive metals (zinc, iron, magnesium, sodium): releases hydrogen gas. E.g. H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g).
- With metallic oxides: acid + metal oxide → salt + water. E.g. CuO(s) + 2HNO₃(aq) → Cu(NO₃)₂(aq) + H₂O(l).
- With carbonates: produces carbon dioxide. E.g. H₂SO₄(aq) + CaCO₃(s) → CaSO₄(s) + H₂O(l) + CO₂(g).
