Supercooling - the energetics of freezing
Students melt sodium thiosulfate crystals, and these are then cooled to a state well below the melting point. The melt now exists in a metastable supercooled state. The supercooled liquid will freeze rapidly on the addition of a crystal of sodium thiosulfate or dust particles, or on stirring. Students add a crystal of the solid and this seeds the crystallization process. Temperature changes are observed throughout.
Read our standard health & safety guidance
Lesson organisation
This is best carried out individually or in pairs. The experiment takes between 20 and 30 minutes.
Apparatus and chemicals
Each group will need:
Test-tube (see note 1)
Test-tube holder
Stirring thermometer (-10 – 110°C) (see note 2)
Beaker (100 cm3)
Bunsen burner
Tripod and gauze
Cotton wool, small tuft to fit test-tube
Sodium thiosulfate-5-water (Low hazard), about 20 g
Technical notes
Sodium thiosulfate-5-water (Low hazard) Refer to CLEAPSS Hazcard 95C
1 The test-tube must be very clean.
2 A temperature sensor attached to a computer can be used in place of a thermometer. Thus a continuous plot can be drawn as the processes occur.
Procedure
a Half fill a very clean test-tube with crystals of sodium thiosulfate-5-water.
b Put a tuft of cotton wool in the top of the test-tube to exclude dust.
c Warm the test-tube gently in a beaker of hot water (about 50 oC) to melt the crystals.
d When all the crystals have melted, remove the cotton wool, put a thermometer in the melt and record the temperature. If the liquid does start to crystallise on inserting the thermometer, re-heat it in water to melt all the solid.
e Stand the test-tube in an empty beaker and leave it in a still place to cool.
f Observe the temperature at various intervals until the value is in the region of 30 – 40 °C. No crystallization should have occurred.
g Add a fresh crystal of sodium thiosulfate, observe the rapid crystallisation which occurs, and once again continue to monitor the temperature at regular intervals
h Wait until the temperature has fallen to about 25 – 30 °C.
Teaching notes
Once the experiment is over the thermometer can easily be removed by flushing with water, since sodium thiosulfate is water-soluble, or by re-melting the solid. Do not heat the test-tube directly over a Bunsen flame as at higher temperatures the thiosulfate decomposes and may form toxic products.
The temperature changes occurring show a steady fall as the liquid cools, and, once a crystal is added to the supercooled liquid, the temperature rapidly rises as solidification takes place, confirming this process is exothermic.
All solids exhibit supercooling to a greater or lesser extent, but sodium thiosulfate is particularly prone to exhibiting this metastable condition. With more time available, it is possible to cool down the melt to a value well below room temperature, but to achieve this involves waiting for more time to elapse, thus lengthening the experiment considerably.
An impressive teacher demonstration can be carried out using cold running water to cool the melt down rapidly to about 5 – 10 °C before a crystal is added to seed the crystallisation process. The test tube warms up extremely rapidly and becomes quite hot in the process.
Health & Safety checked, August 2008
Updated 29 Oct 2008Average rating: 
3 out of 5
Your reviews
I've done this experiment before in a chem class. The cotton or wool tuft can be used but is not necessary. I didn't use one and my experiment turned out just fine. Also unless you are measuring the melting point, there is no need to heat gently, just make sure that you have a generous amount of water in your beaker and that your tube is not touching the sides of your beaker. To do this you set your beaker on a ring stand and then place the test tube in a tube holder above the beaker on the same ring stand and as long as the tube is not touching the beaker, you can turn your burner up all the way and my sodium thiosulfate was about 90 oC when I started recording the temps. This experiment is great fun. It baffles me that a liquid can stay liquid well below the freezing point for a long period of time (I took temp readings over a period of 140 minutes using a computer that took 60 readings per minute) until you add a solid particle the the liquid.
Submitted by: Boris Radloff on 7 February 2010
How are we going to interpret the temperature readings during the time intervals recorded?
Submitted by: andrew deuda on 10 January 2010