In order for water to become ice, it needs a nucleus in order for solid crystals to form. Usually, water is loaded with particles and impurities that enables ice to form. But purified water isn't. Because of this, purified water can reach an even colder temperature before becoming solid.
If you throw an unopened bottle of purified water into the freezer for a little less than three hours, the bottle will be chilled well below the temperature at which regular water freezes.
When you pour this super-cooled water onto a piece of ice, it provides the water with nuclei, causing it to freeze instantly.
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