You may have seen videos of what looks like an ordinary bottle of cold water hanging out and minding its own watery business until bam! Someone taps it against the table and the whole bottle turns instantly to ice. What is this dark wizardry?
Well let us be the first to tell you that it isn't magic but instead science, and some pretty simple science at that. And yes, you absolutely can try this one at home.
When any substance changes state — like liquid water changing to solid ice — the process involves nucleation. That's when there's an imperfection in the substance, like some dust in the water, that the new state can use as a kind of anchor — or nucleus — for the change from one state to another to take place. One molecule forms an ice crystal at the nucleus, which creates more icy surface area, which causes more nearby molecules to change to ice, which creates more icy surface area, which causes even more nearby molecules to change to ice ... you get the idea.
Usually, there are lots of nuclei present in water. So when you pop a bottle of water in the freezer, it starts freezing around those imperfections all throughout the liquid. This is called heterogeneous nucleation, because the water is not uniform. There's stuff in it, and the ice forms throughout the water over time.
But it is possible to create homogeneous nucleation. That happens when there are no impurities in the water because there are no nuclei for the ice to form around as the water gets colder and colder. This is the process that creates "instant" ice.
Super Cool Science in Your Freezer
Water famously freezes at 32 degrees Fahrenheit (0 degrees Celsius). But purified water, like the stuff you buy in bottles from the store, doesn't have anything but water molecules in it. So there's nothing to kick-start the usual heterogeneous nucleation process.
If you have bottles of purified water, you can pop them into the freezer and leave them undisturbed — no moving, no touching — for a couple of hours. They'll still be liquid because pure water with no nuclei in it freezes at -43.6 degrees Fahrenheit (-42 degrees Celsius). It's now a supercooled liquid, which does indeed sound super cool.
The exact timing it takes for the water to freeze will depend on the size of your water bottles and your freezer, but it will take about two-and-a-half to three hours to get the water to this supercooled state.
It's only after these few hours of preparation that the "instant" part of instant ice happens. But it is amazing. Carefully remove the bottles from the freezer. Then shake one or whack it on the table. Anything can act as a nucleus at this point — air bubbles, a slight dent in the bottle. Any little change will be enough to cause homogenous nucleation.
Once that disturbance is present, the uniform water molecules will become ice so quickly that it looks instant.