Examination of boiling
This is a magnificent experiment, which at the outset may not appear very exciting.
Apparatus and materials
For each student group
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.
Half fill a beaker with water and then bring it gently to the boil. Watch the process carefully, observing the formation of bubbles.
1 This experiment could begin with a block of ice in the beaker which is allowed to melt.
2 Students see small bubbles forming from dissolved air; but when boiling starts there is a quite different formation of water vapour (steam) in bubbles.
3 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".
4 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 enough motion energy 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.)
5 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-checked in December 2004