Everyone knows that water freezes at 32 degrees (0 °C) – or does it? When water freezes, it needs a nucleus in order for the solid crystals to form and become ice. Water is typically full of particles and impurities which have no problem kicking off the crystallization process. However, purified water by definition doesn’t have those impurities. With nothing for the water molecules to latch onto, purified water can be supercooled as far as -40°C.
The energy generated from firm hit on the side of the bottle forces the supercooled water molecules to form a crystal in a process called nucleation. That nucleus ice crystal is all that’s needed to start a chain reaction of crystallization throughout the entire bottle. Shaking or jostling the bottle has the same effect, so be very careful and have a steady hand when removing the water from your freezer.
Pouring the water onto a ice cube forms a slushy ice. As the supercooled water hits the ice cube a crystallization spreads up the stream of the water as it gets poured onto the pile. The latent heat that is released during the freezing process stops it from freezing solid.