Most people would describe water like a colorless liquid. They __1__would know that in very cold conditions it becomes a solid calledice 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 thatwater 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, andthe 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 asgases or as liquids or as solid. We may normally think of iron as a __5__ solid, but if we heat it in a furnace, it will melt and become a 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 becausewater is so familiar a substance that different names are used to __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 theminto 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__