4. How do gases behave?
Pressure and temperature P.10 of 13 
Hazard warning
Picture 4.3 Aerosol cans carry an important warning: keep away from heat.

You should never throw an aerosol can onto a fire or leave it in direct sunlight – even if it is empty. This is because the pressure will build so up so much that the can will burst (and explode if there is a naked flame nearby). There are two reasons for the pressure going up:
  1. the gas pressure increases
  2. more of the liquefied propellant turns into a gas.
    Let's look at these in more detail.
    Pressure from gases
    Picture 4.4 When a gas is heated its particles move faster.

    Absolute temperature

    We call kelvin temperatures absolute temperatures. At zero kelvin, all particles stop moving; this is absolute zero.

    Ice melts at 0 ºC. This is equivalent to 273 K. To find out the temperature in kelvins, we need to add 273 to the celsius value. So the boiling point of water is 100 ºC or 373 K (100 + 273).

    Absolute temperature scale

    1. Why does gas pressure increase?
    The particles in a gas are moving. They bump into the walls creating a pressure. When a gas is heated, its particles speed up. There are two ways that this increases the pressure:
    • the faster particles bump into the container walls more often
    • each collision is harder because the particles are moving faster.
      Taking it further
      The pressure of a gas increases if the temperature increases. We get a stronger relationship between pressure and temperature as long as we measure the temperature on the absolute temperature scale (see box on the left).

      In that case, the pressure is proportional to temperature. If we:

      • double the absolute temperature, the pressure will double
      • triple the absolute temperature, the pressure will triple

      and so on.

      2. More propellant turns into gas

      The increase in gas pressure is not enough on its own to be dangerous. It would take a big rise in temperature to have any noticeable effect on gas pressure. If you heat a gas from 27 ºC up to 127 ºC, you might think that the temperature has gone up by a factor of about 5. However, this is not the case. The absolute temperature has risen from 300 K to 400 K. So the temperature has not even doubled - it has gone up by a third. Therefore the pressure will go up by only a third. There must be something else that causes the pressure to become dangerously high.

      Even more pressure
      As well as the gas pressure increasing, pressure will also increase because more of the liquefied propellant turns into a gas. You can see this at home when a saucepan of water starts to boil. The water turns into steam and produces a huge increase in pressure - enough to lift up the saucepan lid.

      Aerosols are tested before leaving the factory. The filled can is immersed in a water bath at 50 ºC for several minutes. This increases the pressure in the can, revealing any leaks and assuring its strength.

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      Question 10
      Look at the table below and fill in the gaps.
      ºC K Pressure
      27 2

      Summary                                           Close
      • as the temperature of a fixed mass of gas increases, the pressure increases
        Warning - do not try heating aerosols at home - it is extremely dangerous
      • because the particles make more collisions and each one is more forceful
      • pressure is proportional to absolute temperature
      • the absolute temperature scale is measured in kelvins
      • absolute zero is -273 ºC