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
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.