Examination of boiling

Class practical

This is a magnificent experiment, which at the outset may not appear very exciting.

Apparatus and Materials

For each student group

• Bunsen burner
• Pyrex beaker
• Tripod, gauze and heat-resistant mat
• Thermometer - 10°C to 110°C

Health & Safety and Technical Notes

Students must not sit down to watch this experiment: serious scalding has occurred when the beaker breaks or falls and the pupil has been unable to move away instantly.

Read our standard health & safety guidance

Procedure

Half fill a beaker with water and then bring it gently to the boil. Watch the process carefully, observing the formation of bubbles.

Teaching Notes

• This experiment could begin with a block of ice in the beaker which is allowed to melt.
• Students see small bubbles forming from dissolved air; but when boiling starts there is a quite different formation of water vapour (steam) in bubbles.
• Students are apt to have very careless views of the essential nature of boiling:
• a fixed (!) boiling point (it depends on atmospheric pressure);
• vapour pushing the outer air away (when in fact vapour molecules diffuse through air easily);
• a vague story of more copious evaporation with no clear reason for the constancy of boiling temperature.
• Ask: "what tells you water is boiling?"; and insist on the clear answer, "bubbles of water".
• Bubbles cannot form and grow in the liquid until the vapour pressure in them matches the outside atmospheric pressure. The liquid boils away as fast as heating provides the 'exit-taxi' of latent heat. Once the liquid is boiling, further heating simply equips more molecules with speed needed to evaporate into vapour bubbles. Therefore, the temperature stays constant at the boiling point.
• Thus, evaporation acts as a thermostat for a boiling liquid. (The energy needed to pay the 'exit-taxi' makes distilling an expensive business.)
• Let students carry out the experiment the first time without a thermometer in the water. A very able group could plot temeperature-time graphs.

This experiment was safety-tested in December 2004