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

Filling balloons


Apparatus and materials

Balloons, 3, small and preferably of different colours

Aspirator, 10 litre

Rubber bungs, 3, (diameters of ends to take balloon necks stretched over them)

Rubber bungs fitted with glass tubes, 2, to fit aspirator

CO2 cylinder

Rubber tubing for connection to water tap

Health & Safety and Technical noes

If gas cylinders are used, care must be taken with handling to comply with the Manual Handling Regulations (see CLEAPSS Lab. Handbook section 9.9). Staff also need instruction in the use of regulators and (for hydrogen) the needle valve. If gases are generated chemically, see the relevant Hazcards.

Fill the balloons immediately before display. Hydrogen, in particular, will diffuse quite quickly through the rubber of the balloon. It is helpful to work the balloon before inflating it. This is done by blowing some air into it and then stretching the rubber between the fingers so that all parts of the balloon are equally stretched. This softens the rubber and ensures the balloon blows up more easily.



apparatus for filling balloons 1Air 
Connect the balloon to the top of the aspirator.

Fill the aspirator with water by connecting the lower end to a water tap. The water displaces the air which fills the balloon.

When the balloon is full, remove it without letting the air out and insert a rubber bung into its neck – or tie it.

Carbon dioxide and natural gas or hydrogen  
The procedure for filling the balloons is the same as for air, but first the aspirator must be filled with carbon dioxide, or with the natural gas or hydrogen.

This time the aspirator is filled with water before its top is connected to the gas supply, that is carbon dioxide from the CO2 cylinder, natural gas from a laboratory gas tap, or hydrogen (from apparatus borrowed from the Chemistry Department).

The water is drained out whilst letting in the gas at the top.

Then quickly fit the balloon to the top of the aspirator in place of the gas supply. Connect the side tube to a water supply and gently force the gas out into the balloon. 

Gas in and water out.
Hydrogen is lighter than natural gas so should be used if available.  

Gas out and water in

Teaching notes

The following balloon masses are typical: 

Flat balloon and bung 14.5 g 
Balloon full of air and bung 14.8 g (see note below) 
Balloon full of CO2 and bung 19.3 g 
Balloon full of natural gas and bung 10.5 g 
Balloon full of hydrogen and bung 5.0 g 
The balloon full of air weighs slightly more than the empty balloon even though Archimedes' Principle would appear to predict no extra weight. This is because the balloon compresses the air inside it to a slightly greater density than the air outside. (It might be as well to neglect this small difference rather than dwell on it in discussion.)


Related guidance

Crystals and atomic models for beginners


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