Deep freeze

Follow these instructions to create super cooled water and learn why oceans don't freeze solid.

• 4 December 2006 | Updated 14 October 2011

• What you need
• Activity A
• Activity B
• What's happening

How do you make super-cool water? You can try putting it in the freezer but you’ll probably find the water will very quickly turn into ice. But there is a way to cool water down below its freezing point of 0 ºC without it freezing solid. Check it out!

What you need

To do this activity you will need the following:

• a freezer
• a spoon
• water
• salt
• ice
• two plastic glasses
• a marker pen
• an empty tin.

Activity A

1. Fill two plastic cups with the same amount of water.
2. Add four big spoonfuls of salt to one of the cups and mark the outside of the cup with ‘S’ using marker pen.
3. Place both cups in the freezer (make sure they’re balanced and won’t fall over!).
4. Check the cups every hour for four hours then leave them overnight and check the next morning. What’s the different between the two cups?

Activity B

1. Fill the tin can about three-quarters full of ice, then fill the tin can with water so the water just covers the ice.
2. Dry the outside of the can with a paper towel.
3. Sprinkle four big spoonfuls of salt over the top of the ice and give it a quick stir with the spoon.
4. Watch what happens to the outside of the can over the next ten minutes. Try adding more ice and salt.

What's happening

Water normally freezes (changes from a liquid to a solid) at around 0 ºC. However, adding salt to the water lowers the temperature at which water freezes. In other words, the salt makes it more difficult for the water to freeze solid so the water can be colder than 0 ºC and still not actually be frozen. Salty water does eventually freeze but the water has to be a lot colder than ordinary water.

The first activity proves this. The cup with plain water in it should freeze solid in about four hours, but the cup with the water and salt may not freeze solid at all. Instead, some bits of ice will form but they can only do this by getting rid of the salt. Eventually, you are left with solid ice and some very salty water at the bottom of the cup. When this happens in the ocean around Antarctica, the very salty water sinks to the bottom of the ocean and is called Antarctic Bottom Water.

In the second activity, adding salt to the can of ice and water lowers the temperature of the water to below 0 ºC. When water vapour (tiny bits of water in the air) in the air touches the cold sides of the metal, the water vapour freezes into a thin layer of ice or frost on the outside of the can.

For more hands-on activities, join CSIRO's Double Helix Science Club.