Determining whether chemical change has occured or not can be tricky. Colour change might be given as one indication that change has occurred.

Solid potassium iodide is mixed with solid lead nitrate. A yellow solid is formed.

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What is the explanation for this?

The sign that a chemical reaction has occurred is the change in colour. When the ions in the two chemical compounds make contact with each other they rearrange themselves such that two new compounds are formed – lead iodide (yellow) and potassium nitrate (white).

A more complicated example can be seen when solid barium hydroxide is added to solid ammonium chloride. The temperature of the chemicals in the container drops dramatically, a liquid appears and a pungent gas is evolved:

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Here there is no colour change. 

What are the signs that a chemical reaction has occurred?

1. Evolution of ammonia gas. 2. Endothermic change. 3. Water is produced, hence the apparent change in state of the solid reactants to liquid.

So there are several observations which could indicate that a chemical reaction has occurred, amongst which are temperature change, evolution of gas and precipitation. Can you add any more?