Permanent change in colour can indicate what?

Correct answer: Chemical change

The direct answer is option A: a chemical change. A permanent colour change can be evidence that a new substance has formed, which is the main idea of a chemical change. For example, a substance may react with oxygen or another substance and produce a product with a different colour. Option A is correct because a lasting colour change is a recognised possible sign of chemical reaction. It is important to say “can indicate,” not “always proves,” because some physical processes, mixtures, dyes, or changes in concentration may also alter appearance without forming a new substance. Option B is wrong because merely changing the container normally does not permanently change the substance. Option C is too narrow and unsupported: a colour change is not automatically only a measurement error. Option D is wrong because a permanent colour change is evidence that something may have changed. Check other signs too, such as gas formation, temperature change, precipitate, or difficulty of reversal. Memory cue: permanent colour change may mean a new substance, but use all evidence.