Activity: Freezing Water

Talia with the icy waters.

On SCOPE's Antarctica episode, you saw Talia experiment with freezing sea water, with some surprising results. Here's what she did. Watch the clip.

What you need

• Four plastic cups
• Marker pen
• Salt
• Thermometer
• A freezer
  

What to do

1. Label the cups: 0, 1, 2 and 3.
2. Then pour 200 millilitres of tap water into each cup.
3. Leave the cup marked 0 as is.
4. Mix in one teaspoon of salt into the cup marked 1, two teaspoons into the cup marked 2, and add three teaspoons of salt into the cup marked 3.
5. Put the four cups into the freezer and wait.
6. Check the containers every 15 minutes and record the temperatures until each one is frozen – or almost frozen.
   

What's happening?

Everyone knows how to make ice: you freeze water. Ice is just the solid form of the chemical water, and it occurs at 0 °C. But most of the ice around is pure or fresh water ice.

Sea water is not pure water, it contains salt (or a mixture of salts, to be precise).

During the winter, the sea around Antarctica can also freeze, but because it contains salt, it takes much cooler temperatures to freeze. In fact around 20 million square kilometres of the Antarctic seas can freeze – that's almost three times the size of Australia!

In this experiment, different concentrations of salty water were tested to see how the salt affected the freezing of the water. It turns out that adding salt to water interferes with the formation of ice crystals. So the saltier the water, the lower the temperature it needs to reach before it will freeze.

And as your freezer may not be as cold as it is in Antarctica, the saltiest water may not freeze at all!

This principle is called freezing point depression, and it applies to any mixed substances. The same would be true for adding sugar to water, although the effect may not be as dramatic.

Salt is often used on snow-covered roads, as it lowers the freezing point, so helps reduce the amount of snow cars have to drive through.

And what about the fish that live in Antarctic seas? Why doesn't the water in their bodies freeze along with the seawater?

They have very high salt concentrations in their bodies plus there are glycoproteins in their blood which act like a type of antifreeze!