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

Effect of a pump


This demonstration allows students to see air being removed by a pump.

Apparatus and materials

Vacuum pump

Round-bottomed flask, 1 litre

Bung with glass tube to fit (take care when inserting the glass tube into the bung)

Smoke filter (see Technical notes)

Length of pressure tubing, 1 m

Light source and appropriate power supply, OPTIONAL

Safety screen

Health & Safety and Technical notes

Wear eye protection. Use a safety screen to protect observers. A round-bottomed flask is likely to have fewer stresses caused in its manufacture and is therefore more likely to withstand the evacuation process. Care should be taken in looking for hairline cracks before connecting the apparatus together. It is a good idea to keep a particular flask for evacuating and not just one ‘off the shelf’. When the flask is evacuated, take care not to tap the flask against a hard surface as it may shatter.

Glass wool is used inside the smoke filter (as described above). This has been a standard practice for years. Glass wool is an irritant and it should be left inside the glass tube containing it. If you are making your own filter, use ordinary cotton wool instead, though not too much of it.

The vacuum pump should be a motor-driven rotary pump and not a hand pump. For connection to the vacuum pump, it is essential to use pressure tubing. The smoke filter is made with a 20 cm length of 3.5 cm diameter glass tubing containing glass wool. Bungs are provided at each end through which glass tubing connects on the one hand to the vacuum pump, on the other to the flask to be evacuated.



a Allow smoke from a smouldering spill into the one-litre round-bottomed flask so that it is clearly visible. 

Apparatus set-up

b Close the flask with the rubber bung, which is attached to the other apparatus as shown above.

c Switch on the pump so that the smoke can be seen being pumped out. To achieve this, adjust the needle valve on the pump so that the rate of pumping is as slow as possible. Otherwise, the operation is over too quickly. If the pump is fitted with a gas-ballast valve, this should be left open during this process.

d It is helpful to illuminate the flask brightly and to have a dark background.


Teaching notes

Most young students are unfamiliar with the action of a pump and even the idea of a vacuum is strange to many. To give them an idea of the pump’s effect and to let them see ‘air being taken out of the bottle’, some visible gas should be pumped out of a flask. Unfortunately, visible gases such as bromine are highly corrosive and should not be used with school pumps. However, smoky air can be pumped out of a clear flask without harming the pump if a smoke filter is inserted between flask and pump.


The experiment was safety-checked in July 2007


Related experiments