What happens when you leave a bottle of pure water in the freezer for a few hours? Weirdly it doesn’t freeze until you bang the bottle. Why?

This is special bottled water. It’s not regular mineral water, which normally has other chemicals dissolved in it. Smart Water claims to be pure distilled water, or near enough.

After a few hours in the freezer, the water should be around -20C but it will still be a liquid. Why?

Regular water, at 0C will begin to freeze. To form the ice needs a “seed” – a speck of dust or another chemical impurity that provides a site for the first ice crystals to begin. This is called nucleation.

Because it’s so pure, it’s possible for the smart water to exist in a supercooled state. There is nothing in the water to kick start the nucleation process off.

If I then bang the bottle, give it a shock, the shockwave provides the kick start to begin the freezing process.

In theory, it should be possible to cool the water down to -48 degrees Celsius before a different process kicks in and freezing will happen anyway.

Try it for yourself. Get a bottle of Smart Water, put it in the freezer for a few hours, then give it a shock. The best bit is you can then let it thaw out fully, then try it again!