What causes pressure in a container full of gas?

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The pressure of a contained gas is caused by collisions of the gas particles with the walls of the container. The gas molecules are in constant, rapid motion. If gas is enclosed in a flexible container such as a balloon the volume will increase or decrease until the pressure caused by collisions of gas molecules with the inside wall equals the pressure caused by collisions of atmospheric molecules with the outside wall. 

The pressure of a gas in a rigid container will be greater than the exterior pressure if there are more collisions with the inside wall than on the outside. The gas pressure in a rigid container can be increased in three ways:

  1. Increase the temperature of the gas. The molecules will move faster and have more frequent collisions with the wall of the container.
  2. Decrease the volume of the container. As the molecules get closer together the number of collisions per unit of surface area increases.
  3. Increase the amount of gas. If there are more gas molecules then there will be more collisions. 
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