Particle Motion

The particles in a gas are in constant random motion. Compared to the particles of solids and liquids, gas particles are more spaced out and they move at higher speeds (on average).

Pressure in a Gas

Gas pressure is the force exerted by gas particles on the walls of the container, as the particles collide with it.

If we put gas particles in a sealed container, the particles will move around quickly in random directions. Whenever the particles collide with the walls of the container, they bounce off and move in a different direction.

Diagram of a transparent container filled with blue gas particles. The particles are shown in motion with red arrows indicating their random paths of movement. Some particles are bouncing off the walls of the container, exemplifying the typical behaviour of gas molecules in a confined space. Labels identify individual elements like "Gas particle" and "Container".

Even though the particles are tiny, they create a force when colliding with the walls of the container and this creates pressure. As pressure is just the force that is exerted per unit of area:

Equation showing the formula to calculate pressure. The formula states that pressure is equal to the force divided by the area, represented as "Pressure = Force over Area".

Formula representing the calculation of pressure. The equation reads "P equals F divided by A", where P stands for pressure, F for force, and A for area.

  • P = Pressure in pascals (Pa)
  • F = Force in newtons (N)
  • A = Area in metres squared (m²)

There are two ways that we can increase the pressure:

1. Increasing the number of collisions per second

2. Increasing the energy of each collision

We can use these two factors to understand how temperature, volume and concentration affect gas pressure.

Temperature and Pressure

The temperature of a gas is related to the average kinetic energy of its particles. If we heat a gas, the kinetic energy of its particles increases, so the average speed of the particles will increase.

Below, there are two containers containing gases at different temperatures.

Side-by-side illustration of two laboratory setups. On the left, a flask filled with cold water is shown with several molecules moving slowly within. The label below reads "Cold Water". On the right, a similar flask, placed on a blue flame, contains boiling water with molecules moving rapidly and bubbles forming. The label below reads "Boiling Water". Both flasks have a stopper and a tube attached to their tops.

  • Gas particles at a lower temperature have lower kinetic energy. This results in fewer collisions per second, and the average energy of these collisions is also lower. Therefore, at lower temperatures, the gas pressure is relatively low.
  • Gas particles at a higher temperature have higher kinetic energy, leading to more collisions per second and a higher average energy for these collisions. Therefore, at higher temperatures, the gas pressure is relatively high.

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