Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Weighing air

Demonstration

Weighing air by pumping it into a container and then releasing it into a vessel under water.

Apparatus and materials

Plastic container with tap (at least 30 cm x 30 cm x 30 cm)

Foot pump with pressure-gauge

Measuring cylinder, 1 litre

Lever-arm balance

Large transparent trough (glass or plastic)

Health & Safety and Technical notes


Weighing contrainer

The container should be tested carefully beforehand to find out how much extra pressure it will take. It should take about 0.5 atmospheres (= 51K Ρa = 7.3 psi) extra.

1 The container may be a water carrier, or of the type in which beer is sold in bulk. It should be sufficiently rigid to retain its shape when filled with air at atmospheric pressure. An alternative could be a large ball and needle valve.

2 Two plastic containers are recommended for this experiment, so that there is a spare one available if the first should leak.

 

Procedure


Weighing air

a Weigh the plastic container on the pan of the lever-arm balance.

b Attach a 1m length of rubber tubing to the outlet, the air inside being at normal atmospheric pressure.

c Pump air into the container using the foot pump. Close the tap. The more air that can be got inside the better.

d Weigh the container again. With the containers provided, a difference of about 8 g is possible (corresponding to an excess pressure of about half an atmosphere) and this can be measured on the lever-arm balance.

e Fill the measuring cylinder with water and immerse, mouth downwards, in a large trough of water with the open side downwards.

f Put the rubber tubing in the water with the end of the tubing well under the inverted cylinder.

g Open the tap until the cylinder is filled with air. Close the tap, refill the cylinder with water and repeat until the container is empty.

 

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 Students may be amazed by the large volume of air, at atmospheric pressure, which will come out of the pumped-up container.

3 Knowing the mass of the excess air and its volume at atmospheric pressure, the density can be found.

4 As soon as the density is known, you can ask how much mass the air in the room has. For a room 10 m x 10 m x 2 m it is 2 x 105 litres x 1.2 g = 240 kg, or about ¼ of a tonne. Not something to dismiss.

5 If students have now measured the density of solids, liquids and gases, they could draw up a table of their results and include the values of other common substances from a data book. The differences could lead to a discussion on why solids and gases have such different densities. Atomic structure and separation of atoms are points to draw out.

This experiment was safety-checked in July 2007