Measuring the density of air 2

Demonstration

This rough method can produce a difference of mass of at least 8 g. With enough apparatus it could be done as a class experiment, in small groups.

Apparatus and materials

Large plastic container with tap

Foot pump with gauge

Perspex box with internal dimensions 10 cm x 10 cm x 11 cm

Balance with a sensitivity of at least 1 g, possibly a lever-arm balance

Trough, filled with water

Health & Safety and Technical notes

You need to devise a means of coupling the pump to the container.

This might be a short length of glass tube, to fit the pump outlet, fitted with a rubber tube, to fit the tap of the container.

When the excess air is being released to measure its volume, you will need a piece of rubber tubing long enough to reach under the Perspex box in the water trough.

Procedure

a With the tap open, use the balance to measure the mass of the container (plus air at atmospheric pressure).

b Pump air into the container before closing the tap and re-weighing. The more air that can be pumped inside the better. As a rule of thumb, do not exceed 1.5 atmospheres (1.5 bars or 22 psi).

c Immerse the Perspex box in the water, invert and lift it so that 1000 cm3 of water is above the level of the water outside. Place the end of the rubber tubing under the box and carefully open the tap a little.

d When bubbles of air have replaced 1 litre of water in the box, close the tap. Refill the box with water and repeat the procedure until all the excess air has been released from the container. The last fractional filling will need to be estimated.

e Find the density of air from the mass of air expelled and the volume of air collected.

Teaching notes

1 It is essential that the volume of the container should not change significantly between the two weighings. Any increase in volume will increase the buoyancy (due to the external air), reducing the measured weight.

2 You should know from the mass of the excess air how many litres there should be. Do not overdo squeezing the container!

3 Getting students to help can enhance their feeling of expectation as the last of the air is released from the container.

This experiment was safety-checked in August 2007