An ideal gas is a theoretical idea – a gas in which there are no
attractive forces between the molecules, and in which the molecules
occupy no space. Both of these assumptions are incorrect, for:
- if there were no attractive forces between molecules, no
substances would ever condense from the gas state to become liquids
and solids. In fact, every substance does
condense when it is cooled and compressed enough. Therefore there
must be attractive forces between
molecules.
- if molecules took up no space, then we could compress
substances until they had no volume at all. In fact, solids and
liquids are almost incompressible, since their molecules are
basically touching each other. Since solids and liquids take up
space, the molecules of which they are made
must take up space also.