Explain how density could be an indicator of a physical or chemical change? Give an example for each

The classical answer to this question involves the expansion of water when it freezes, thus increasing the volume of the same mass of water, lowering the density. This physical change is well illustrated by the fact that ice floats on water. The second part of your question is familiar to anyone who has measured the amount of acid in the electrolyte of an old style car battery. The densimeter is used to measure the density of the water/acid mixture to determine the strength of the diluted acid. The chemical change caused by adding more acid is determined by knowing the density.