Adding a few drops of food colouring to a glass of water turns the water pink
Adding a few drops of food colouring to a glass of water turns the water pink. Kabir says this is a chemical change, because the colour changed. His sister says it is a physical change. Who is right?
Who do you think is right? Pick one.
The answer: His sister is right, no new substance formed — it is a physical change
Why
Kabir is using 'the colour changed' as the test for a chemical change, but colour is only one clue and not a reliable one on its own. The real test is whether a NEW substance has been made. Here, the water is still water and the dye is still dye — they are just mixed together, so the mixture looks pink; nothing new was created. You could even remove the water by evaporating it and be left with the dye again. Test it: compare this to burning paper, a real chemical change — the ash and smoke are new substances, and you cannot turn them back into paper; but pink water can be un-mixed. His sister is right: no new substance was formed, only a mixture, so colouring water is a physical change — a colour change by itself does not make something chemical.
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