4. How do gases behave?
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Volume and pressure P.9 of 13 

Imagine a bicycle pump full of air. You put your finger over the end of the pump and push the plunger in slowly. What happens to the pressure of the gas inside?

The pressure goes up. Pushing the plunger in reduces the volume available to the gas and increases the pressure. Let’s see why this happens in terms of air particles.

Reducing volume increases pressure
Picture 4.1 The particles in a pump collide with the walls. If the volume is halved, each particle collides twice as often. (The temperature (and therefore the speed) of the particles is the same throughout.

Moving particles
The air particles are moving around inside the pump. They collide with the walls, making a pressure (see page 7). If the volume of the pump is halved, then each particle will collide with the walls more often. The particles have not sped up, they simply have less distance to travel between each collision.
Taking it further
If we halve the volume available to a gas, its pressure will go up. However, we can say more than this: its pressure will double. This is because, on average, each particle will make twice as many collisions with the walls.
When we divide the volume by 2 the pressure is multiplied by 2
If we divide the volume by 3 the pressure will be multiplied by 3
If we divide the volume by 4 the pressure will be multiplied by 4
and so on

However much the volume goes down, the pressure goes up by the same amount.
Expanding
The same thing happens in reverse. If we increase the volume, the pressure will decrease:
doubling the volume half the pressure
tripling the volume a third the pressure
and so on

We say that:

the pressure is inversely proportional to the volume

We call this Boyle’s Law after the scientist Robert Boyle.

See question 9
Picture 4.2 Using a compressed gas propellant. As the product is used up, the volume available to the propellant increases.
Aerosols cans are under pressure. Never puncture a can or try to look inside.

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Compressed gases
Some aerosols use a compressed gas propellant, like nitrogen or air. The gas is put into the can under pressure and, after the can is sealed, it pushes down on the product. Compressed gas propellants have some advantages (see page 12). However, because of Boyle's Law, they also have a disadvantage; work through question 9 to find out what it is (then have a look at page 12).
Question 9
Some aerosols use a compressed gas propellant. Picture 4.2 shows how the level changes as the product is used up. We have assumed that none of the gas propellant escapes. The initial pressure is 600,000 Pa.

a) What is the pressure when the product is at level:

A:
B:
b) As the product is used up, the volume available to the propellant gas and its pressure . This is an effect and means that compressed gas propellants are less widely used than liquefied gases.

Summary                                           Close
  • if the volume of a fixed amount of gas goes down, the pressure goes up
  • because the particles don't travel so far between collisions; so there are more collisions every second
  • the pressure is inversely proportional to the volume
  • doubling the volume halves the pressure (and vice versa)