Chemistry 3.1 - 3.3 - Acids, alkalis and indicators
Use ions, pH and indicator colours to identify acidic, neutral and alkaline solutions, and recognise what one indicator result can and cannot show.
Ions and the pH scale
The characteristic acidic or alkaline behaviour discussed here occurs in aqueous solution. At the particle level:
- an acid in solution is a source of hydrogen ions, ;
- an alkali in solution is a source of hydroxide ions, .
The pH scale describes the solution as a whole. It does not have a unit.
| pH | Classification |
|---|---|
| below 7 | acidic |
| exactly 7 | neutral |
| above 7 | alkaline |
For example, pH 2 is acidic, pH 7 is neutral and pH 11 is alkaline. Neutral does not mean that the solution contains no particles or no ions; it means that its measured pH is 7 in this GCSE model.
The direction matters: lower pH means more acidic, while higher pH means more alkaline. Avoid calling a low-pH solution a "strong acid" from its pH alone. Acid strength has a separate Higher-tier meaning that is taught later.
What indicators reveal
An indicator is a substance that changes colour depending on whether the conditions are acidic or alkaline. The colour is an observation; using it to classify the solution is an inference.
| Indicator | Usual acidic colour | At neutral pH | Usual alkaline colour |
|---|---|---|---|
| litmus | red | purple | blue |
| methyl orange | red | yellow | yellow |
| phenolphthalein | colourless | colourless | pink |
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