Some liquids are acidic or soapy — Chemistry, 4–6 years
Liquids can have different chemical behaviours, even when they look alike. A safe colour test can show a difference without tasting or touching unknown substances.
Colour can reveal a liquid’s behaviour
Some liquids are acidic, like lemon juice, while others are more soapy, like a mild soap solution. They can look almost the same, but they behave differently in a colour test. A special indicator changes colour and gives us a clue; it is not a signal to taste the liquid.
Why compare liquids safely?
Long ago, people used sourness, slipperiness and plant colours to notice that liquids were not all alike. The problem was that tasting or touching could be unsafe and could not compare things carefully. Indicator colours gave people a safer, clearer way to sort liquids by their chemical behaviour.
Testing with red cabbage water
An adult prepares purple water by soaking red-cabbage leaves in warm water. Pour some into two clear cups. Add a little lemon juice to one: it turns pinker. Add a little mild soap solution to the other: it turns blue-green. The different colours show different chemical behaviour.
A colour is a clue, not a label for everything
It is tempting to think that one colour tells us exactly what a liquid is. That seems reasonable because traffic lights and signs use fixed colours. An indicator only gives information about a chemical behaviour, and different liquids can make similar colours. Never identify an unknown liquid by colour alone.
Food, soap and safety
Acidic foods give lemons and yoghurt their sharp taste, while soap solutions help lift grease from hands. Scientists and workers use indicators to check liquids in gardens, pools and factories. At home, ask an adult before using any test liquid, and never taste a substance to find out what it is.
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