Indicators and tests: limewater, hydrogencarbonate, universal indicator and litmusSpec S6
In short
Indicators are substances that change colour to show a chemical change, and you must give the colour before and after. Limewater turns from colourless to milky with carbon dioxide. Hydrogencarbonate indicator is red at the level in air, yellow when carbon dioxide increases and purple when it decreases. Universal indicator gives the pH number; litmus shows only acid or alkali.
Indicators change colour to show a chemical change. You must know the colours for each one, and always give the colour before and after.
| Indicator | What it shows | Colours |
|---|---|---|
| Limewater | carbon dioxide | colourless → milky (cloudy white) when carbon dioxide is bubbled through |
| Hydrogencarbonate indicator | carbon dioxide concentration | red at the level in air; yellow when carbon dioxide increases; purple when carbon dioxide decreases |
| Universal indicator | pH (a number) | red/orange in acid, green at pH 7 (neutral), blue/purple in alkali |
| Litmus | acid or alkali | turns red in acid, blue in alkali |
Testing pH
- Add a few drops of universal indicator solution to the sample, or dip universal indicator paper into it.
- Compare the colour with the pH colour chart.
- Read off the pH number.
Litmus only tells you whether a solution is acidic or alkaline. Universal indicator gives the pH number, so it shows how strongly acidic or alkaline it is.
Where they are used
- Limewater: comparing inspired and expired air, and showing that yeast or germinating seeds respire.
- Hydrogencarbonate indicator: gas exchange in an aquatic plant in light and dark, or respiration of small organisms in a sealed tube.
- Universal indicator or litmus: the pH falling as lipase digests fat into fatty acids, or testing soil and water samples.
(opens full size in a new tab)Food tests
| Test | For | Positive result |
|---|---|---|
| Iodine solution | starch | orange-brown → blue-black |
| Benedict's solution, heated in a water bath | reducing sugars | blue → green, yellow, orange or brick-red |
| Biuret test | proteins | blue → purple |
| Ethanol emulsion test | fats and oils | cloudy white emulsion |
| DCPIP | vitamin C | blue → colourless |
Wear eye protection for all these tests. Heat Benedict's in a water bath, not over a flame, and keep ethanol away from flames because it is flammable.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026