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

Weighing a sample of air - a rough estimate

Demonstration

Showing that the mass of air in a bottle is small.

Apparatus and materials

Round-bottomed flask (1 litre) or glass bottle

Bung and tube to fit flask or bottle

Vacuum pump

Balance, lever-arm or top-pan

Measuring cylinder, 1 litre

Length of pressure tubing, 1 m

Screw clip

Health & Safety and Technical notes


Wear eye protection when evacuating glass containers or handling them. Protect observers by using a safety screen. Use a flask that can withstand the pressure difference. Check it has no specks or marks.

1 Instead of the special nature of the Pyrex flask, you may prefer to use a more familiar glass bottle.

2 For connection to the vacuum pump, it will be necessary to use pressure tubing.


Procedure


a The one litre flask must have a well-fitting rubber bung with a glass tube through it, to which is attached a rubber tube with a screw clip. Weigh the flask.

b Attach the rubber tubing to the pump and evacuate the flask. Reweigh the flask. (The volume can be found by filling the flask with water and pouring into the measuring cylinder.)

 

Teaching notes


Apparatus set-up1 It is a useful order of magnitude to remember that 1 cm3 of water has a mass of 1 gram and a litre of air has a mass of just over 1 gram (1.2 gram/litre). Students should be able to come up with their own ideas for how to make the measurements.

2 A one litre round-bottomed Pyrex flask weighs approximately 350 g. The change in mass when the air is removed is of the order of 1.2 g. On the single-pan lever-arm balance this amount will scarcely be appreciated. The object of this experiment is to show how small the mass difference is, in fact too small for the balance to measure.

3 The suggestion of weighing a balloon empty, then blowing it up full of air and weighing it again may arise. This is a fallacy quoted by Galileo. A bladder full of air gives the same reading on the balance as it does when squashed flat, because the buoyancy of the surrounding air just compensates for the weight of the air inside in the first case. A rubber balloon does weigh a little more when inflated, because the air inside is at slighter greater pressure, but that is not sufficient for use here. The volume of the container must not change much between our two weighings because it is ignored.

4 The lever-arm balance will probably not register a change in mass when the flask is evacuated but a top pan electronic balance will. This is a useful lesson to learn - that of choosing the best instrument for the job.

This experiment was safety-checked in July 2007

 

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