SUPERCOOLING OF WATER

BACKGROUND

Solids, liquids and gases are the three main states of matter. Water is one of most useful examples of the liquid state. It has some of the most amazing properties; one of which is supercooling. We have learnt from times immemorial that water freezes to become a solid (ice) at 0 °C. However, pure water transforms into ice at a much lower temperature of around -40 °C. This phenomenon of pure water existing in the liquid state even below its equilibrium freezing temperature of 0 °C is called supercooling of water.

When water is in a supercooled state then essentially it is in metastable equilibrium and requires little perturbation to instantaneously change its state to the solid state (ice) with a very little persuasion.

An experiment is described below which highlights the above concept of the supercooled water and is great fun to perform.

EXPERIMENT

STEP 1: Converting pure water to supercooled water

Put a bottle of pure water in a freezer and cool it for 2 to 3 hours. The aim is to cool the water sufficiently but stop just short of ice formation. This will require a lot of hit and trial.

Hint: Put a bottle each of tap water and pure water in the freezer for sufficient time. If the tap water turns to ice and pure water does not then the pure water is in a supercooled state.  

STEP 2: Converting supercooled water to ice

Remove the water bottle carefully and bang it on a table. You will see ice crystals form immediately at the top which will flow down to the bottom of the bottle.

ALTERNATE STEP 2

Another method of achieving the same result is to add on an ice cube to act as seed in the container carrying supercooled water. This has the effect of the water converting into ice instantaneously around the small seed crystal. In a matter of seconds the complete conversion takes place.

A BIT OF SCIENCE

Reason for the supercooling of water is explained. At room temperature the molecules of water in the liquid state move around freely past each other. For  transforming it into a solid, what is required is lowering of temperature along with some sort of seed crystal so that the crystals begin to form. The process is called nucleation. In case of impure water, the impurities themselves act as sites for formation of crystals. Therefore, ice formation takes place at 0 °C. However, for pure water, there are no seeds or impurities and ice formation requires a lower temperature of -40 °C. What we are attempting through supercooling is to somehow line up the molecules of water so that crystallisation (ice formation) event starts in case of pure water once it is cooled in a freezer for 2 to 3 hours. By banging the cooled bottle on a table or adding an ice cube to it, we are getting the molecules to line up together so that a crystal forms. Once the event starts, it instantly forms sites for crystal formation for all the water contained in the bottle.

A video demonstrating the above is shown here