Super cool

Super cool

Can you get water below 0 degrees Celsius without freezing it?

Water freezes at 0 degrees Celsius, right? Not always! Here's how you can get water super cool and freeze it with a BANG!

What you need:

  • 1 bucket
  • Ice - at least 1 bag
  • Thermometer - helpful but not essential
  • 750g salt
  • 500ml plastic bottles of mineral water - at least 3 bottles

How to:

Take a look at this video from Bang Goes the Theory, which shows you how to make water super cool:

  1. Place the bottles of mineral water in the bucket and pour the ice around them.
  2. Add some tap water to make a slushy ice mix - make sure the tops of the water bottles stick out so you can move them without freezing your fingers.
  3. Add 750g salt to the slushy ice mix.
  4. If you're using a thermometer, place it gently in the bucket.
  5. Leave the bottles for around 45 minutes. Turn the bottles gently every so often, to ensure the water super cools evenly.
  6. If you're using a thermometer, after 45 minutes it should read -6 to -8 degrees Celsius.
  7. Gently remove one of the bottles. If it's frozen, put it to one side and carefully remove another bottle.
  8. If the water is still liquid, you can freeze it with a bang! Swing the bottle gently and bang it against a wall.

What did you see? The water should freeze rapidly.

What if it doesn't work?

If the water in your bottles is already frozen, it could mean that you left them in the bucket for too long, or the bucket was knocked whilst you were waiting. Alternatively, there may be impurities in your bottled water - like scratches in the plastic. You can't help this! Science is all about trial and error.

How does it work?

A super cool liquid is one that is liquid at a temperature lower than its freezing point. Salt lowers the freezing point of water, so the slushy ice mix in the bucket is below 0 degrees Celsius.

For water to become ice, all the molecules must align in a regular form. The ice crystals need a nucleation site to form - a starting point. This could be scratches on the bottle surface, dust or other ice crystals. Without a nucleation site, it's very hard for ice crystals to form. This is why the mineral water in your experiment doesn't freeze - there is no nucleation site to allow ice crystals to form.

Why does ice form when the bottle is banged?

One theory is that banging the bottle creates tiny air bubbles that can act as nucleation sites. This makes the water freeze very rapidly.

For more information, see the Bang Goes the Theory hands on website.

Go to Planet Science Weather Labs for more fun weather activities.