Ideal Gas Propellant Answer Page


1. What is an inert gas?

2. Why is nitrogen used when air would also work?

3. At stage 2 the nitrogen pressure is 2 bar. It fills most of the can. What will happen to the pressure of the nitrogen when the bag is filled with the product at stage 4?

 4. The can has a diameter of 4.5 cm and is 10cm high. If the nitrogen pressure is 2 bar at stage 2 and the space left for the nitrogen is only 40cm3 once the bag is filled, what will be the pressure when the bag is full?

5. What will be the pressure in the can when all the product has gone?

6. What two factors affect the pressure inside a bag on valve aerosol?


1. An inert gas is one that will not react chemically with with anything. Nitrogen is an inert gas. Other inert gases are the noble gases such as argon and helium.

2. Air contains oxygen that could react with the product, the can, or the bag over time.

3. The volume left for the nitrogen is decreased. So we have the same mass of gas squeezed into a smaller space. The pressure will go up.

4. This question is asking about changes in volume, V and pressure, P for an ideal gas. The temperature is constant. The Gas Law we need  is Boyle's Law:

P1V1 = P2V2

V1 is the volume of the cylinder. We can ignore the volume of the empty bag.

V1 = πr2h = 3.142 x 2.252 x 10 cm3 = 160 cm3

If the space between the bag and the can is reduced to 40 cm3 after filling, then the pressure P will be

P2 = P1 x V1/V2

P2 = 2 x 160/40 = 8 bar

5. When all the product has gone the pressure will return to 2 bar. The nitrogen behaves like a compressed spring forcing the product out of the bag.

6. The volume of the nitrogen around the bag is the biggest factor. The temperature of the can is the second factor. In normal household use the temperature will vary between about 5 and 20 degrees. Cans are tested during manufacture to the pressure experienced at 50 ºC.