Jan 2021

The Kinetic Theory of Gases & Kinetic Energy of A Molecule

January 8, 2021

Everything around us that occupies space is made up of tiny particles. The air we breathe, the utensils we use to eat, and even the water we drink. Studying the kinetic theory of molecules can open our minds to why these things behave the way they do. To understand more, read on to find out more about the kinetic theory of gases and the kinetic energy of a molecule!

Kinetic molecular theory
According to kinetic molecular theory, all matter is made up of tiny particles which exist as atoms, ions, and molecules, that are constantly moving in random motions. When observed through a microscope, tiny particles in a fluid can be seen moving around in a random, erratic fashion. This movement is called the Brownian motion.

All molecules have kinetic energy, but they vary, depending on which state the matter is in. Matter can exist in three states – solids, liquids, and gases.

The particles in a solid are closely packed together in a regular pattern and they vibrate on the spot. Particles in a liquid are also closely packed together but in a random arrangement and they move around each other. Particles in gas are far apart from each other in a random arrangement and they move quickly in all directions. As the particles are constantly moving, they possess kinetic energy.

At a higher temperature, the particles move more vigorously. This causes the molecules to have greater kinetic energy and hence move at higher speeds. This is why molecules have the least amount of kinetic energy in the solid state, while the amount of kinetic energy of gas molecules is the largest.

The kinetic theory of gases
In the kinetic theory of gases, gas is made up of a huge number of tiny particles that are in constant, random, motion, displaying perfectly elastic collisions. This gives rise to a number of connections we can make between pressure and temperature of gas in a container. Charles’ Law and Boyle’s Law can be explained using kinetic theory of gases, but one crucial thing to note is this: this theory makes a few assumptions. It works on the basis that gases behave ideally (hence the name ‘ideal gases); but in reality, not all gases in all circumstances will work this way.

Pressure in gases
As mentioned above, air molecules are moving randomly at high speeds. As the molecules move around, they collide with one another or the surface of nearby walls. When the molecules bounce off the walls, a force is exerted. There are numerous collisions taking place at one time, and this produces a total average force on the walls that can be measured. The force per unit area is the pressure. Hence the formula:





P = pressure
F = force
A = area

Therefore, we can say that the gas pressure increases when the number of molecules increases, the speed of the molecules increases, or when the molecules have a larger mass.

The relationship between pressure, volume, and temperature

  • Pressure-temperature relationship of a gas: The pressure of a fixed mass of gas is directly proportionate to its temperature if the volume is constant. This is because when the temperature of the gas increases, the kinetic energy of the molecules will increase and hence the collisions will be of greater force and more frequent. The average force acting on the wall increases, and hence the pressure increases.
  • Volume-temperature relationship of a gas: The volume of a fixed mass of gas is directly proportional to its temperature if its pressure is constant. This is known as Charles’ Law, described by the following formula:



V1 is the initial volume
V2 is the final volume
T1 is the initial temperature
T2 is the final temperature

  • Pressure-volume relationship of a gas: The pressure of a fixed mass of gas is inversely proportional to its volume if its temperature is constant. This is because, when the volume of a gas decreases, the number of particles per unit volume increase. This will lead to a higher frequency of the number of collisions. Hence, the pressure will increase.

The average kinetic energy of a molecule can be deduced from the ideal gas equation relating pressure, volume, temperature, and speed. The ideal gas equation is:

PV = nRT


P = Pressure
V = Volume
n = number of moles of gas
R = ideal gas constant
T = Temperature

Now that you’ve got a better understanding of the kinetic model of matter, don’t you find it interesting? There are many more interesting theories and formulas in this field! If you’re looking for O level physics tuition to gain more knowledge and push up your grades, we have the best tutors who can provide the best physics tuition lessons and guidance you need!

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