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

Change of volume on vaporization

Demonstration

Measuring the volume of vapour produced when a small drop of hydrocarbon solvent is vaporized.

Apparatus and materials

Measuring cylinder, Pyrex, 100 cm3 capacity

Bung to fit cylinder carrying two short glass tubes

Tall-form beaker, 2000 cm3

Plastic tubing

Hypodermic syringe

Petroleum ether, 60-80°C boiling fraction

Rubber cap

Beaker, 400cm3

Electric kettle to provide hot water

Health & Safety and Technical notes


Petrol as used in cars may NOT be brought into a school laboratory because it contains benzene. Petroleum ether can be regarded as 'petrol without the hazardous carcinogen'. It is still highly flammable and must not be used in the same room as naked flames.

Here is a way to fill the tubes with water. Fill the measuring cylinder and the two rubber tubes on either side of the stopper with hot water (at a comfortable temperature to work with) before inserting the bung loosely into the measuring cylinder. The water will siphon over until there is about a depth of 2 cm in the small beaker. The measuring cylinder is topped up with water and the bung is secured. The measuring cylinder is then put into the large beaker and boiling water is poured around it.

 

Procedure


Apparatus set-up

a Fill the measuring cylinder and the flexible tube leading through the bung with water, as if a siphon were being made. The second glass tube through the bung is closed with a rubber cap on the inside of the bung. 

b Immerse the whole cylinder in hot water (at about 90°C) in the tall-form beaker. 
 
c Inject about 0.1 cm3 of "petrol" into the cylinder through the rubber cap tube. It will evaporate and occupy some 80 cm3
 
As the water cools, the vapour will condense again and the expelled water will return to the cylinder.


Teaching notes


The volume expansion when "petrol" turns into "petrol vapour" is an increase of about 800 times. When water turns into water vapour it is a volume change of about 1600 times. 

This experiment was safety-checked in March 2006