Gas particles are the most widely spaced among all the three states of matter.
- They have a comparatively lower density.
- They expand or contract under appropriate influences like heat or pressure.
Kinetic Theory of Gases
- An ideal gas consists of a large number of identical molecules that are in constant motion.
- As for the volume occupied by the molecules themselves is negligible compared to the volume occupied by the gas.
- The molecules also obey Newton’s laws of motion, and they move in random motion.
- The molecules experience forces only during collisions. The gas molecules strike against the wall of the containing vessel and exert pressure. Any collisions are completely elastic, and take a negligible amount of time.
- The average kinetic energy of the molecule is related to the temperature on the Kelvin scale.
Ideal gases
- Ideal gases are gases that obey all the assumptions of the kinetic theory of gases. They have identical particles of zero volume, with no intermolecular forces between them. The gas that behaves closest to an ideal gas is helium. The helium molecule consists of a single small atom, and the van der Waals dispersion forces are low.
Real gases
- A real gas deviates from the ideal gas law, and it only shows the ideal gas behaviours at very high temperatures and low pressures.
Boyle’s law
It states that the volume of gas is inversely proportional to the pressure exerted by the gas at a constant temperature.
Charles’ law
It states that the volume of the gas is directly proportional to the temperature when pressure is assumed to be constant. Due to an increase in temperature, the average kinetic energy of the molecules increases. This increases the speed of the molecules and as a result, causes an increase in intermolecular distance.
The volume of gas is directly proportional to the number of moles of the gas (n). As the number of molecules increases, the number of collisions with the wall of the container also increases, hence the pressure in the container increases as well.
The volume of gas is directly proportional to the temperature and the number of moles of gas, while it is inversely proportional to applied pressure.
- V ∝ T
- V ∝ n
- V ∝ 1/P
The ideal gas equation
pV = nRT
where, R = 8.314 J K^-1 mol^-1
Numerical problem:
A 20 litres box contains a fixed amount of gas at a temperature of 200 Kelvin and 100 kilopascals of pressure. What amount of moles of gas is in the box?
Formula to calculate the molecular weight using the ideal gas law
Formula to calculate the density of gas using the ideal gas law

4.01 - The Gaseous State

Learn Step-By-Step Video Explanation For This Topic

Explore our Revision Resources

Mock Exams

Tutor Support

Create an account