Why does cooled water sometimes suddenly freeze?

Why does cooled water sometimes suddenly freeze?

It’s a beautiful effect it’s called super-saturation or super-cooling a liquid. What’s going on is that O°C is the temperature below which water is more stable as a solid than as a liquid. That doesn’t mean you can’t cool water below O°C. That’s because forming a very very small crystal takes quite a lot of energy and is quite unstable. So with water, it could be down to -3°C, you can supercool water to almost -20 °C, and if its below 0 every time a little tiny crystal starts to form it just gets knocked apart, so there are no small crystals there and nowhere the ice crystals can grow from, and the whole thing won't freeze. But if somewhere you get a crystal which is bigger than the certain critical size it’s much more stable for that crystal to grow and grow so you get a few small crystals spreading out and the whole things freezes very quickly, because its below zero. You also get that effect with sodium acetate. There are hand warmers based on this principle. You heat them it melts the liquid inside, you cool them down again, you flick a little clicky thing and that releases some crystals which causes the whole thing to crystalise. This releases lots of heat which warms your hand nicely.