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)
