Question

Why volume increases and density decreases when water is heated

Answer 1

This happens because the mass of the rod stays the same, but its volume increases. The increase in the volumeof matter with increasing temperature is called expansion. When cooleddown, most matter decreases involume and increases in density. Thisdecrease in volume is called contraction.

Answer 2

density = mass / volume
if volume will increase and mass remains the same the density will decrease this is because the same mass will occupy more volume does becoming less dense

because it is inversely proportional to density.