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Supercooled Water

A thermometer sitting in a clear plastic cup, which itself is sitting in a bed of ice cubes

Water can be cooler than ice and still remain a liquid? That's super cool!

Australian Curriculum links

  • Science > Chemical Sciences > Year 3 > ACSSU046
  • Science > Chemical Sciences > Year 5 > ACSSU077

You'll need

  • 20 mL of bottled water
  • clean, dry plastic cup with a smooth surface
  • flat-bottomed bowl
  • ice cubes
  • 4 tablespoons of salt
  • clean, dry thermometer.

Try this

  1. Pour the water into the cup and put the cup into the bowl.
  2. Fill the bowl with ice cubes. Make sure that no ice falls into the cup of water and that the level of the ice cubes in the bowl is higher than the level of the water in the cup.
  3. Sprinkle two tablespoons of salt evenly over the ice cubes. Avoid getting salt in the cup.
  4. Place the thermometer into the cup of water.
  5. Leave the cup of water to cool. For the remainder of the experiment, be careful not to bump the cup of water whilst it cools.
  6. After 10 minutes, carefully add more ice to the bowl as per step 3.
  7. Repeat step 4.
  8. Leave the cup of water to cool for another 10 minutes or until the temperature of the water is below 0°C.
  9. Once the water is below 0°C, carefully remove the thermometer.
  10. Drop a piece of ice into the cup of water and observe what happens to the cup of water.

Further investigation

Try different ways of making the water freeze, such as hitting the cup or dropping different objects into the water.

What happens if you try to pour the supercooled water onto an ice cube?

Try supercooling different types of liquids, such as soft drink or rainwater. Which liquids are easier to supercool?

What's happening?

By cooling the water slowly, the water remains liquid below its freezing point. This is known as 'supercooling'. When the ice is dropped into the water, the water suddenly freezes and turns into ice. This happens because the ice cube allows the supercooled water to grow new ice crystals.

The freezing point of water is 0°C. This means that water will normally change from a liquid to a solid as it is cooled to 0°C. However, if the water is pure enough, and if it is cooled slowly enough, the water can stay a liquid even when it is colder than 0°C. Supercooled water does not turn into ice as it cools because it is too pure for ice crystals to form. Ice crystals require something on which to grow in order to form. Ice crystals can grow on specks of dust, impurities in the water or ice itself.

In this activity, the bottled water is pure; it doesn't contain dust or impurities. Because the cup's surface is smooth, it doesn't contain any bumps on which crystals can form. Therefore, the water can be supercooled. However, when you add an ice cube, the ice cube allows new ice crystals to grow and causes the water to turn almost instantly into ice.

Real world links

Some clouds are made of droplets of supercooled water. For example, altocumulus clouds are mostly made of droplets of water at about -10°C. These supercooled water droplets can cause problems for airplanes. If the water droplets hit an airplane they can suddenly freeze and create a coating of ice on the outside of an airplane. The ice can affect the way the air moves around the outside of the airplane and cause instruments such as speed indicators to be incorrect.