Acids, bases and indicators — Chemistry, 7–10 years
Some liquids are acidic, some are basic, and many are neither strongly acidic nor strongly basic. Indicators change colour to give us clues about this, without needing to taste or touch a substance.
Colour gives a clue
An acid is a substance with acidic properties, such as lemon juice, while a base has basic properties, such as some soap solutions. An indicator is a substance that changes colour depending on which kind of liquid it meets. The colour is a clue about a range, not a label that tells everything about the liquid.
Why use an indicator?
You should never identify an unknown liquid by tasting it or touching it. Chemists needed a safer way to compare substances, so they used colour-changing indicators. These tests can show whether a liquid is acidic, basic or close to neutral, while keeping the sample away from the body.
Testing three liquids
Put a drop of red-cabbage indicator into lemon juice: it turns pink, showing an acidic liquid. Add it to a soap solution: it turns blue-green, showing a basic liquid. Add it to water: it stays close to purple, suggesting water is near neutral. The indicator colours are compared with a colour chart.
Acid does not mean dangerous
It is reasonable to think every acid burns, because strong acids can damage skin and materials. But the word acid covers a wide range: lemon juice is acidic yet safe to eat in ordinary amounts, while a laboratory acid may be dangerous. Never use taste as a test; follow safety instructions.
Soil, food and water
Gardeners test soil because different plants grow best in different acidity ranges. Food makers use acidity to affect flavour and help preserve some foods, and water scientists check water supplies for changes. These tests do not tell the whole story, but they provide useful information without guessing.
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