Acids, alkalis and the pH scale
What makes something an acid or an alkali, how the pH scale measures it, and what happens when the two meet. The reliable middle-school chemistry that turns up every year.
Acids and alkalis are two opposite families of chemicals, and the pH scale is how we measure where any solution sits between them. It's one of the most dependable topics in school science — learn the scale and the one reaction that matters, and you've got it.
The pH scale
pH runs from to and tells you how acidic or alkaline a solution is:
| pH | Type | Examples |
|---|---|---|
| 0–6 | Acidic | Lemon juice, vinegar, stomach acid |
| 7 | Neutral | Pure water |
| 8–14 | Alkaline | Soap, bleach, oven cleaner |
The lower the number, the stronger the acid; the higher the number, the stronger the alkali. Exactly is neutral — neither.
Measuring pH with indicators
An indicator is a dye that changes colour with pH. Universal indicator is the most useful because it gives a whole range of colours — red/orange for acids, green for neutral, blue/purple for alkalis. Litmus is simpler: it just turns red in acid, blue in alkali.
Neutralisation — the reaction to know
When an acid meets an alkali, they cancel each other out. This is neutralisation, and the general rule is:
The pH moves towards as they react. It explains a lot of everyday life: indigestion tablets (a mild alkali) neutralise excess stomach acid; lime is spread on acidic soil to bring its pH up for crops.
Strong vs concentrated — not the same thing
A subtle but examinable point: strong describes how fully an acid splits up in water (a property of the acid itself), while concentrated describes how much acid there is in the water (how much you dissolved). A strong acid can be dilute, and a weak acid can be concentrated.
pH below 7 is acid, above 7 is alkali, exactly 7 is neutral. Anchor those three facts first; every indicator colour and reaction in the topic hangs off the pH number.
Last revised 8 October 2025.