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Water doesn’t always freeze when it’s supposed to. Learn about supercooling, and how to supercool a bottle of water at home -- and then turn it to ice instantly!

Hosted by: Michael Aranda
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At standard pressure, water freezes at zero degrees Celsius and boils at 100 degrees.   Standard science, right?   I mean, the freezing and boiling points of water are how we define the entire Celsius temperature scale.   Except that water doesn’t always freeze when it’s supposed to.   When the conditions are just right, you can put a bottle of water in the freezer for hours, and it won’t turn to ice...   … until you make it.    That’s because liquids are subject to a special, weird state known as supercooling, when the liquid is below its freezing point, but doesn’t solidify.   Liquids don't just suddenly become solid at a certain temperature. There's a process involved, and it has to do with how molecules line up.   Whether something is a solid, liquid, or gas at constant pressure mostly depends on its energy, which comes from heat.    In a gas, molecules are warmer, meaning they’re higher in energy. They bounce around a lot, colliding with each other and flying every which way.   But in a liquid, molecules don’t have as much energy. They’re still moving around, but they're slowed down enough that bonds form between them, keeping them close together, and loosely ordered.   Now, at even lower temperatures, where molecules have the least amount of energy, they form a solid. They’re still vibrating a bit, but at this point the bonds between them are so strong that they keep the molecules rigidly in place.   Now, let’s talk about the water in the bottle. To freeze, the water needs two things: It needs to have low energy - that is, it needs to be cold - and it needs to form the intermolecular bonds that are going to hold it together as a solid.    For this second part to happen, the water has to first form a nucleus, or central point, of what will eventually be solid ice. This is called a substance’s nucleation site.   It's where the water molecules first slot together in exactly the right way to form the structure of the ice crystal. This then allows other water molecules around it to form bonds, spreading out until they all make one solid piece of ice.   However, nucleation sites don't usually happen by themselves; something needs to cause them.    And, often, impurities in water act as nucleation sites. A tiny deposit of some mineral, for example, could be enough to nudge the molecules around it so they start to arrange themselves into a solid.   Or sometimes, a nucleus can form if the water is physically disturbed somehow -- say, shaken up a little. As the very cold water sloshes around, some of the molecules will slot in next to each other in just the right way, so the water is able to start the process of turning into ice.   So if you want to supercool water yourself, you have to keep both of those things -- the impurities and the movement -- from happening.    All you need is a freezer, a watch, and a bottle of standard drugstore-brand purified water. Don’t use mineral water, because that stuff is full of things that will act as nucleation sites.   First, just stick your bottle of water in the freezer. We do want it to be below freezing, but the longer we keep it in there, the more likely the molecules will find a nucleation site. So around two and a half hours should do the trick.    During that time, don’t open the freezer! The cooling needs to be gradual, and the change in temperature when you open the door disturbs the water enough that it might find a nucleation site. So just leave it alone.   If all goes well, when you take the out bottle out, you’ll have yourself some supercooled, below-freezing liquid water.   But, if you shake it up...   Supercooled liquids are highly unstable. When they’re below their freezing point, their molecules REALLY WANT to form a solid. Like, REALLY BAD. The laws of physics are commanding them, but they just can’t comply.   And that is why, with just a slight provocation, those supercooled molecules will snap into place. By slamming a bottle of supercooled water against the table, you create a nucleation site, and you can actually see the ice crystal form almost instantly.   Which is SUPER COOL! Get it? Supercool?   Thanks for watching this episode of SciShow! If you have an idea for an experiment you’d like to see, let us know in the comments below, on Facebook, on Twitter, or on Tumblr.   And of course, don’t forget to go to youtube.com/scishow and subscribe!