Boyle's Law

Pressure in Fluids

We know that particles in liquids and gases can move freely. They exert a pressure on the walls of their container by the particles  colliding with them. It is the forces that these collisions produce that are responsible for the pressure that they exert. 

The faster the particles are moving, then the more frequent and forceful these collisions will be, so the pressure exerted will increase.

Boyle's Law

Boyle's Law describes the relationship between pressure and volume of a fixed mass of gas at a constant temperature. 

It is important to note that the mass of the gas and the temperature must not change.

Boyle noticed that when he halved the volume of the gas, the pressure doubled; he saw that the pressure of the gas was inversely proportional to its volume. This can be expressed in the equation:

P1V1 = P2V2

(P1 = starting pressure, V1 = starting volume; P2 = final pressure, V2 = final volume)

Worked example:

This relationship can be explained in terms of particles; if we reduce the amount of space the particles have to move around in they will collide with each other and the walls of the container more frequently. This increase in the number of collisions will result in an increase in the pressure of the gas.

More collisions per second means a greater average force on the wall and therefore a greater pressure.

You should know:

  • that gases exert a pressure because of the collisions that gas particles make with other matter
  • when you change the volume of a gas its pressure changes because the particles hit the walls of the container at a different rate
  • how to describe these changes in terms of the movement of the particles
  • Boyle's Law equation: p1V1 = p2V2 

Answer the following questions:

  1. As the volume decreases, the number of collisions

    increases
    		2
    decreases
    		1
    stays the same
    		0
    Total 3

    Click an answer to vote.

  2. As the pressure of a gas is doubled, the volume

    halves
    		0
    doubles
    		1
    stays the same
    		1
    Total 2

    Click an answer to vote.

  3. Which 2 properties of the gas must remain constant?

    mass and volume
    		0
    volume and temperature
    		1
    mass and temperature
    		0
    weight and size
    		0
    Total 1

    Click an answer to vote.