How does heating or cooling a substance affect density?

Answer 1

For most substances over all temperatures as you heat a substance it will become less dense and as you cool it the substance will become less dense. But as with any law there exceptions. Liquid sulphur for example will become less dense as it is heated up to a certain point and then as you continue to heat it the sulphur becomes more dense. But the most noteable exception is water. As water cools from a vapour to a liquid and is then further cooled as a liquid it becomes more and more dense until it reaches 2 degrees C. Then as water cools below this point it becomes less and less dense until it starts to solidify. At this point it becomes less dense. Water is one of very few substances whose density as a solid is less than when it is in a liquid state. It is a good thing to or we would never have invented hockey if the ice had been on the bottom of the pond.

Answer 2

Heating a substance decreases its density as the molecules have more energy and spread out, increasing the volume. Cooling a substance increases its density as the molecules lose energy and come closer together, decreasing the volume.