【练习】
Most people would describe water like a colorless liquid. They __1
would know that in very cold conditions it becomes a solid called
ice and that when heating on a fire it becomes a vapor called steam. __2
However, water, they would say, is a liquid. We have learned that
water consists of molecules composed with two atoms of hydrogen __3
and one atom of oxygen, which we describe by the formula H2O.
This is equally true of the solid called ice and the gas called steam.
Chemically there is no difference between the gas, the liquid, and
the solid, all of which is made up of molecules with the formula H2O. __4
This is true of other chemical substances; most of them can exist as
gases or as liquids or as solids. We may normally think of iron as a
solid, but if we will heat it in a furnace, it will melt and become a __5
liquid, and at very high temperatures it will become a gas. Nothing
very permanent occurs when a gas changes into a liquid or a solid.
Everyone knows that ice, which has been made by freezing water,
can be melted again by warmed and that steam can be condensed __6
on a cold surface to become liquid water. In fact, it is only because
water is so a familiar substance that different names are used for __7
the solid, liquid and gas. Most substances are only familiar with __8
us in one state, because the temperatures requiring to turn them __9
into gases are very high, or the temperatures necessary to turn them
into solids are so low. Water is an exception in this respect, which
is another reason why its three states have given three different names. __10