Measuring the density of air 3

Demonstration

This simple experiment gives a reasonable value for the density of air.

Apparatus and materials

Vacuum pump, electronically operated rotary type

Top-pan chemical balance with a sensitivity of no less than 0.01 g

Measuring cylinder (1 litre capacity would be useful)

Bung, plastic tube and short length of rubber vacuum hose

Hoffman clip

Water trough

Safety screens

Cork ring (see technical notes)

Health & Safety and Technical notes

Remember that vacuum pumps of this type are heavy. Two persons will probably be needed to move one from trolley to bench.

Safety screens should surround the flask before it is evacuated.

The bung should fit the mouth of the flask snugly so that it is secure but unlikely to be forced into the flask when that is evacuated.

The glass tube through the bung should be just long enough to allow the rubber tubing to be securely fitted. The rubber tube connecting the glass tube to the pump should be just long enough to give space for the clip to be used, before detaching the tube from the pump.

A cork ring to support the flask in an upright position will help with the weighings.

Avoid using a flask with a smaller volume. The mass of the air is correspondingly more awkward to measure, and finding the density is more difficult.

Procedure

a Open the clip (making sure the rubber tube also opens). Weigh the flask, bung, clip, and tubes.

b Connect the flask to the vacuum pump to remove as much air as possible from the flask. Tighten the clip and close any inlet valve on the pump. Stop the pump and detach the tubing from the pump inlet.

c Weigh the flask again to find the mass of the air removed.

d Invert the flask over water in the trough, so that the rubber tube remains under the water surface. Carefully open the clip to allow water to fill the flask (and tubes). Then empty the water into a measuring cylinder to find its volume.

Teaching notes

1 Seeing the water refill the flask is impressive. It also helps those who wonder whether the volume is indeed 1 litre. Even to those for whom it is self-evident that the flask has a volume of about 1 litre.

2 Getting students to realize that the mass of a litre of air is about 1 g, so that the mass of a cubic metre is about 1 kg, is no mean achievement. (Should they find the mass of the litre of air is nearer 1.2 grams that is a pleasant bonus.) Generally, they will be surprised that air is 'so heavy!'

3 A useful extension is to estimate the volume of the room and hence the mass of the air it contains. This might lead to seeing why, living as we do at the bottom of an ocean of air, the pressure exerted by the atmosphere is so great. Comparing the density of air with that of water can lead to a discussion of the hazards of descending into the depths of the sea.

4 You could say:
Returning to the calculation of the speed of air molecules, we can move forward another step. Air has a density of about 1.2 g /litre, and water has a density of about 1 kg /litre. Air is therefore about 830 times less dense than water. The height of a column of water in a water barometer is about 10 m. If we had a uniform atmosphere of air, then it would have to have a height of 10 x 830 m; the height of the atmosphere would be 830 m or 8.3 km.

However, the atmosphere is not of uniform density as can be demonstrated in the 3-D kinetic theory model, which you may remember seeing, or we could look at it again. The story we are telling is an artificial, simplified one in order to arrive at an interesting guess. Desperate measures for desperate needs.

This experiment was safety-checked in August 2007