A:

Hi Qihan,Water expands quite a lot when it turns from a liquid to a vapor. The density of liquid water is approximately 0.96 grams per cubic centimeter at 100 C at atmospheric pressure. The density of water vapor at 100 C at atmospheric pressure is about a factor of 1600 less, and so water expands by a factor of 1600 when it turns into steam at atmospheric pressure.It expands because the bonds holding water molecules to each other are broken in the transition from water to steam, and the steam behaves like a gas -- it takes up much more space.If you put the water in a strong box and heat it up, it will not expand because the box keeps it from doing so. This situation is very common -- steam engines rely on high pressure steam gotten by boiling water in a strong container.The steam will expand even more if you heat it up more, but it would take a big temperature change to make its volume change by another factor of 1600, and the molecules themselves would fall apart.Tom

(published on 10/22/2007)