Why should a colour change alone not be considered conclusive evidence of a chemical reaction?

Correct answer: Because a colour change can also result from a physical change, so other observations or reaction conditions must also be considered.

A colour change can be an indication of a chemical reaction, but it is not always conclusive. For example, dissolving a coloured substance in water or mixing coloured substances can change the observed colour without necessarily forming a new substance. Therefore, other observations, such as precipitate formation, gas evolution, temperature change, or the nature of reactants and products, should also be considered. Option D is incorrect because many reactions show a colour change without producing a gas.