In a gas the individual molecules move freely through space,
and do not touch except at the moments of collision, from which
they rebound. A gas has neither a fixed shape nor a fixed volume;
it adapts to the shape of its container and can be expanded or
The molecules of a liquid are in contact with one another, but
have enough energy to slip past one another and change their positions.
Therefore, a liquid has a relatively fixed volume, but no definite
In a crystalline
solid the molecules are packed against one another in a regular
pattern, and do not have enough energy to break that pattern and
slide from one place to another. Crystals have a definite volume
and shape, and work must be done to deform or break them.