Matter on earth comes almost entirely in three basic forms:
The three phases of water, for example, are the solid ice, the liquid water, and the gaseous steam.
The earth, however, is not a representative place for the average conditions in the universe. Most of the matter in the universe, in fact, is inside hot stars and is in none of the above states. A gas is usually thought of as independent atoms or molecules that are mainly neutral, but the gases in stars are so hot that molecules can not exist, and the atoms are ionized. This is called a plasma. It consists of ions in a sea of free electrons, and has quite different properties from a gas. It is in plasma that we are trying to produce fusion power.
Can you describe solid, liquid and gas without using your hands? You'll find that this is difficult. That is because they are usually defined in terms of how they react when you squeeze them.
In this chapter, we will first deal with solids and then with liquids. We will return to gases in later chapters.
Let us be clear, though, that it does not make sense to talk of a given substance as a solid, liquid or gas. Which state of aggregation a substance is in depends entirely on the temperature and pressure. A substance like mercury, for example, which is a liquid at room temperature, will become a gas if we increase the temperature, and will become a solid if we lower the temperature. The following applet will make this clearer.
© MultiMedia Physics, 1999