Telling acids from bases with indicators

An indicator changes colour in an acid or a base, so it tells you which a solution is.

Activity 7.1 — telling acids from bases with indicators

You will need: blue and red litmus, methyl orange, phenolphthalein; lime juice, dilute hydrochloric acid, dilute sulphuric acid, vinegar, dilute sodium hydroxide, soap water

Add each indicator to each solution and record what you see.

Table 7.1 — the expected results

Solution	Litmus	Methyl orange	Phenolphthalein
Dilute hydrochloric acid	Red	Red	Colourless
Lime juice	Red	Red	Colourless
Dilute sulphuric acid	Red	Red	Colourless
Vinegar	Red	Red	Colourless
Dilute sodium hydroxide solution	Blue	Yellow	Pink
Soap water	Blue	Yellow	Pink

Table 7.2 — indicator colours

Indicator	In acid	In base
Litmus	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow

Only a rough test

Indicators only tell you roughly whether a substance is an acid or a base. They cannot tell you how strong it is.