My brother asks: If substances contract when they get colder (Charles' law), why is ice less dense than water?
My (far more observant than me) brother noticed something interesting in the first video I posted about Charles' law. In the video I talk about how cooling something down will make it take up less volume because the particles take up less space. But that's not what ice does, it's well known to expand by about 9% when it forms. So what's going on?
The answer is that Charles' law applies to what's called an "ideal gas", that is, a gas in which each particle flies about without interacting with anything else. As you cool them down the particles start squashing and we get less volume etc. etc. But here's the thing: water isn't an "ideal" gas because water molecules interact.
Water molecules act a bit like magnets, boomerang shaped magnets. When they've got lots of energy they roll and tumble over each other in close proximity; they have enough movement energy to make the attractions between them too small. But if we cool the water down, the molecules move around less and the attractions between them start to become the dominant effect. This means the water molecules snap together into a crystal-grid shape, with gaps in between each molecule. This structure takes up more space than the liquid water.
Water is one of the few substances on Earth which don't obey Charles' law. Plutonium is another, as is silicon dioxide (the main ingredient in sand). These substances do get smaller as you cool their gases down, then even smaller as they condense, but then they get bigger again in the solid form. Keep cooling things down and the ice rearranges once more to make a smaller type of crystal, and then smaller and smaller. So water does eventually follow the same rules as every other substance, but it takes a bit of a detour around zero degrees.