why does water not expand when heated
water does expand when it freezes because water is polar and so pushing the molecules close together actually makes them feel a force of repulsion, hence they move apart, so it expands. In general, materials expand when heated and contract when cooled...you are trying to apply the logic that if water expands when its cooled then it must contract when heated...but this is only true between the liquid and solid states. If you heat water to be a gas (steam) then it expands just like normal materials do when heated. When heated you put more energy into the atoms so if you put them in a box it will push harder against that box (which you see as pressure) or if you allow the box to expand then it will to equalize the pressure (i.e. expanding makes the pressure less).