The pH scale
Every solution sits somewhere on the pH scale, from 1 to 14:
- pH below 7 — acidic. The lower the number, the more strongly acidic. Vinegar, lemon juice, dilute hydrochloric acid.
- pH exactly 7 — neutral. Pure water.
- pH above 7 — alkaline. The higher the number, the more strongly alkaline. Soap, baking soda solution, dilute sodium hydroxide.
Reading the scale with indicators
An indicator is a chemical that changes colour depending on whether it's in an acid, a neutral solution, or an alkali. You'll meet three kinds:
- Litmus paper — the simplest. Red litmus turns blue in an alkali; blue litmus turns red in an acid. Neither changes in a neutral solution. It tells you acid-or-alkali, but not how strong.
- Universal Indicator (UI) — a full spectrum from red (strongly acidic) through green (neutral) to purple (strongly alkaline), matched against a colour chart to read off an approximate pH number.
- Natural indicators — some plant pigments do the same job. Red cabbage juice, for instance, is pink-red in acid and blue-green in alkali.
Try it: test six substances
Pick a substance to "dip" a Universal Indicator strip and both litmus papers into.
Quick check
Q A solution turns red litmus paper blue. Is the solution acidic, neutral, or alkaline?
Red litmus only turns blue in an alkali — so the solution is alkaline.