Characteristics of the gas phase

Gases :
Gases are the only state of matter that can be compressed very tightly or expanded to fill a very large space. There are four major phases of matter: solids, liquids, gases and plasmas.
Characteristics of the gas phase:
The gas phase of a substance has the following properties:
1. A gas is a collection of particles in constant, rapid, random motion
(sometimes referred to as ‘Brownian’ motion). The particles in a
gas are constantly undergoing collisions with each other and with
the walls of the container, which change their direction ? hence the
‘random’. If we followed the trajectory of a single particle within a
gas, it might look something like the figure on the right.
2. A gas fills any container it occupies. This is a result of the second law of thermodynamics i.e. gas expanding to fill a container is a spontaneous process due to the accompanying increase in entropy.
3. The effects of intermolecular forces in a gas are generally fairly small. For many gases over a fairly wide range of temperatures and pressures, it is a reasonable approximation to ignore them entirely. This is the basis of the ‘ideal gas’ approximation, of which more later.
4. The physical state of a pure gas (as opposed to a mixture) may be defined by four physical properties:
p – the pressure of the gas
T – the temperature of the gas
V – the volume of the gas