The Supercooling of Water

supercooling - crystallization - freezing points

What it shows:
A liquid can be taken to a temperature below its freezing point if it is cooled slowly and there are no nucleation sites for crystallization to begin. In this demonstration you can create a flask of liquid water at below 0°C that, when 'seeded' by the introduction of a nucleation site (in this case dry ice) will be instantaneously frozen.

How it works:
This is pretty much described in Setting it Up.

Setting it up:
The only specialized piece of apparatus is the flask itself (figure 1). It is a 500ml filter flask that has been cleaned thoroughly with 10% hydrofluoric acid to remove any surface markings such as scratches. The housing for the temperature probe is a 3mm bore glass tube whose end has been blown into a bulb. This bulb is filled with mercury for good thermal contact. The outside surface of the bulb also needs to be smooth, so give it the HF treatment too. The side spout of the flask has attached a short length of rubber hose, then a valve that links up to a vacuum pump. The temperature probe is an Omega PRS-GP-100 12 inch handle RTD with Omega DP703 digital thermometer. It is best to support the probe using a clamp stand otherwise the flask will be top heavy and you’ll put a lot of strain on the glass tube.

The flask preparation is as follows:

1. Before every use, clean the flask and bulb with 10% HF and rinse thoroughly with distilled water.
2. Place 150ml of distilled water into the flask, Evacuate the flask and keep it under vacuum until the second cooling stage.
3. Place the flask into an acetone bath and cool the flask to around 1°C by adding dry ice to the acetone.
4. Transfer the flask to a salted ice-water bath. During this period of cooling, which can take up to an hour, ensure that the bath has a fresh supply of ice and salt, and remove excess water. The vacuum can be gradually released from the flask at this stage; this will result in an increase in temperature and the possible drawing in of impurities. The water should cool to around -1.5°C.

The setup for the actual demonstration is illustrated in figure 2. Carefully remove the bung and temperature probe and transfer the flask to a tripod. It should sit on a plate of glass and a piece of Polaroid material that are cut to fit the tripod platform. Mounted about 50cm above the flask is a light box fronted with Polaroid material, held in position with lab clamps and rods so it shines down onto the flask. The setup allows light to pass through the water which is now between two crossed Polaroids; this shows the crystal growth in beautiful color. Use the Canon CCD camera with 10mm lens to look up through the base of the flask. To initiate crystallization, use tweezers to drop a CO2 crystal into the water.

figure 1. Flask at time of cooling


figure 2. The demonstration setup

Comments:
Legend has it that there are some lakes in Switzerland that are supercooled, and that casting a stone into the water will instantly freeze the lake. After the hassle of supercooling pure water in an ultra-clean ultra-smooth flask, Mother Nature knows something we don’t. We are sure the preparation can be simplified, but at this time we’ve had no success with any elements of the cooling process changed. The increased size of the capillary tube to accommodate the Omega temperature probe has made things more difficult than earlier attempts which used thermocouple (iron/constantan) wire within a 1mm bore tube. Frustration is the name of the game, and ultimately, you’ll get more out of it than the audience. Rating *