So this isn't something that e spend a lot of time thinking about but the fact that ice floats on liquid water is really weird and really important; and not just because it lets your ice tea make that nice tinkling sound but because most aquatic organisms depend on it.

If ice didn't float then rivers and lakes would freeze from the bottom up instead of freezing on their surfaces, and all the living things in them would be exposed to cold from both the ice below and the frigid air above. Most living things wouldn't be able to survive a single winter of this.

But fortunately ice does float, so it creates an insulating layer between the cold air and the water allowing organisms to survive and swim ll winter long.

Okay but why does ice do that? 

First we need to know why anything floats or sinks and the answer to that is simply density. If a material is more dense than the fluid that surrounds it it will sink; if it's less dense it will float. So in short the answer is that ice floats because frozen water is less dense than liquid water.

But how is that possible? Isn't it all just water? an aren't slid like definitionally more dense than liquids?

Well not definitionally, usually yes but water molecules are special in the way that they are shaped and how their electrons are distributed around them. So we all know that water is composed of two hydrogen atoms and an oxygen atom but you might not know that oxygen attracts electrons more strongly than hydrogen does. The result is that the electrons in a water molecule hang out closer to the oxygen than the hydrogen's; and this give the O side of the molecule a slightly negative charge while the H₂ side has a slightly positive charge. This makes water a molar molecule and a lot of waters coolest and most useful properties comes from this polarity

So when water is in it's liquid phase it's molecules, like in all liquids, randomly organized but as the water cools the molecules get closer together and at a certain point, starting around 4^oC they get close enough together that the positive and negative areas of those molecules start attracting each other and they snap into a rigid structure. Because water molecules are bent at an angle when they line up like this the structure they form leaves little holes between them which makes the structure less dense.

Those tiny holes are the entire reason solid water floats on top of its liquid form. So remember that next time you skate on a frozen pond or enjoy a nice ice beverage.

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