A gas always expands to fill a container. It is a physical
state for which neither the shape or the volume are rigid. Gases
can expand and contract over large temperature ranges. Gases are
very compressible.
A gas can be a pure substance or a mixture. You know several
elements which exist as gases and you know of at least one
compound which can be found as a gas (water). Gases can also be
mixtures. The interesting thing about mixtures of gases is that
they are always homogeneous.
What does a gas look like at the atomic level?
![](Defe.h8.gif)
The particles of a gas are well separated from each other and
are in constant motion.