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

Change of volume: petrol to petrol vapour


This experiment works more easily than that with water and is less expensive.

Apparatus and materials

Change of volume kit (as shown in the diagram) with long needle (8 to 10 cm)

Beaker, 400 ml

Wire stirrer to fit inside the tall beaker and go round the measuring cylinder

Plastic tube to carry over to beaker

Plastic tube to extend to the bottom of the measuring cylinder

Petroleum ether, 40 - 60 boiling fraction

Health & Safety and Technical notes

Apparatus set-up

Ordinary petrol must not be used as it contains benzene (carcinogenic) proportions. Petroleum ether can be regarded as 'pure petrol'. 
The security of the syringe with the long needle is an issue to be watched.

The change of volume kit should include:
100ml measuring jar 
rubber stopper with two tubes inserted 
rubber cap 
hypodermic syringe and needle 
syringe and needle 
If the measuring cylinder has a pouring lip, the rubber stopper, with its two glass tubes, must extend below the lip to prevent leakage there. 



a Place the measuring cylinder (without its stopper) in the tall beaker. 

b Put about 200 ml of water into the small beaker. 
c Fill the tall beaker and the measuring cylinder with boiling water from an electric kettle. 
d Close the cylinder with its bung and ensure the outlet plastic tube is in the water in the small beaker. 
e Push the needle through the cap on the short tube in the bung and inject 0.1 ml of petroleum ether. As the petroleum ether turns to vapour, the displaced hot water runs over into the small beaker.  

Teaching notes

To reverse the change, take the measuring cylinder out of the large beaker, ensuring the end of the plastic tube stays under the water in the small beaker. You have to wait for the temperature to fall to about 60°C. 

The expected volume change is 1 to 800. 
This experiment was safety-checked in August 2006


Related guidance

Avogadro's number and the mass of an air molecule