Pressure & Volume (Edexcel GCSE Physics)

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Pressure Changes in a Gas

  • If the temperature of a gas remains constant, the pressure of the gas changes when it is:
    • Compressed – decreases the volume which increases the pressure
    • Expanded – increases the volume which decreases the pressure

Gas Volumes at Low Temperatures & High Pressures, downloadable IB Chemistry revision notes

Pressure increases when a gas is compressed

  • Similarly, a change in pressure can cause a change in volume
  • A vacuum pump can be used to remove the air from a sealed container
  • The diagram below shows the change in volume to a tied up balloon when the pressure of the air around it decreases:

Vacuum pump, downloadable IGCSE & GCSE Physics revision notes

  • Therefore, if the gas is compressed, the molecules will hit the walls of the container more frequently
  • This creates a larger overall net force on the walls which increases the pressure

Pressure on Surfaces

  • As the gas particles move about randomly they collide with the walls of their containers
  • These collisions produce a net force at right angles to the wall of the gas container (or any surface)
  • Therefore, a gas at high pressure has more frequent collisions with the container walls and a greater force
    • Hence the higher the pressure, the higher the force exerted per unit area

molecules-&-force, IGCSE & GCSE Physics revision notes

Gas molecules bouncing off the walls of a container

  • It is possible for someone to experience this force by closing their mouth and forcing air into their cheeks
  • The strain on the cheeks is due to the force of the gas particles pushing at right angles to the cheeks

Pressure and Volume

  • In a gas, the molecules are widely spread
  • This makes the gas easy to expand and compress
  • Changing the pressure acting on the gas will compress it or allow it to expand if the temperature is kept constant
  • When a gas is compressed, the volume is decreased
    • The density of the gas increases, since the size of the container has decreased but the number of molecules has remained the same
    • This allows more frequent collisions of the molecules on the container wall
    • This means they hit the walls with a greater force and therefore increases the pressure

  • When a gas expands, the volume is increased
    • This causes a decrease in pressure

States of Matter Volume and Pressure, downloadable IB Chemistry revision notes

Decreasing the volume increases the pressure of molecules at the same temperature

  • Therefore, in summary:
    • When the volume decreases (compression), the pressure increases
    • When the volume increases (expansion), the pressure decreases

  • The key assumption is that the temperature and the mass (and number) of the particles remains the same

Worked example

A deodorant can contains a highly pressurised fluid that pushes the deodorant out as a fine mist.WE Pressure & Volume Deodorant can, downloadable IGCSE & GCSE Physics revision notesWhich of the following is not a true statement about this situation?

A   The total number of particles remains constant throughout

B   The temperature of the deodorant remains constant throughout

C   The pressure of the deodorant decreases as it leaves the can

D   The total volume of the deodorant increases as it leaves the can

ANSWER:  B

    • A is true because the particles only spread about, but there is no chemical change
    • C is true because the particles have a larger volume, which means the collide less frequently with any surfaces
      • the pressure therefore decreases

    • D is true because the deodorant is able to spread out as it leaves the can
    • B is not true because as gases expand their temperatures decrease

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.