Let's look at the density of water at 25 deg C and compare that to a higher temperature, 80 deg C. The density decreases from 0.9970 g/mL to 0.9718 as it is heated. This makes sense because, as heat is added to the liquid water, there is greater kinetic energy of the molecules and there are also more vibrations of the water molecules. Together these mean that each H2O unit in liquid water takes up more space as the temperature increases.
