Examination of boiling
Practical Activity for 14-16
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.
Half fill a beaker with water and then bring it gently to the boil. Watch the process carefully, observing the formation of bubbles.
- 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);
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