Lemon juice makes you wince, soap feels slippery, and pure water tastes of nothing. Chemists put all three on a single ruler — the pH scale — that tells you exactly how acidic or alkaline anything is.
The pH scale: 0 to 14
The pH scale runs from 0 to 14 and sorts everything into three groups:
- Acids have a pH below 7. The lower the number, the stronger the acid. Examples: lemon juice, vinegar, hydrochloric acid.
- Neutral is exactly pH 7 — pure water sits right in the middle.
- Alkalis (soluble bases) have a pH above 7. The higher the number, the stronger the alkali. Examples: soap, sodium hydroxide.
You measure pH with an indicator — a substance that changes colour depending on the pH. Universal indicator turns red in strong acid, green at neutral, and purple in strong alkali, giving you a whole rainbow to read off.
Neutralisation: acid meets base
When an acid reacts with a base, they cancel each other out. This is neutralisation, and it always makes the same two things:
Analogy: think of an acid and a base as equal and opposite — like +1 and −1. Add them together and you land back at zero (neutral). The leftover atoms rearrange into a salt and water.
Two more acid reactions to know
| Reaction | Products | Gas given off |
|---|---|---|
| acid + base | salt + water | none |
| acid + metal | salt + hydrogen | hydrogen |
| acid + carbonate | salt + water + CO₂ | carbon dioxide |
The fizzing you see when you drop an indigestion tablet (a carbonate) into acid is the carbon dioxide escaping.
Memory hook: pH goes 0 → 7 → 14, from acid to neutral to alkali. And "acid + base makes salt and water" — the three reactions all start with acid, so learn the acid first, then just remember which gas fizzes off.