Shouldn’t the ice cubes be more dense than the water because they’re the solid form of water?
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Yes but water doesn’t behave that way. The solid form is less dense than the liquid form. If it wasn’t we wouldn’t even be able to have this conversation because no life would be possible.
When water freezes into ice, it expands by about 10%. So it’s the same amount of stuff (mass), but in a larger volume. So it’s less dense.
Water is the only thing that gets bigger when it freezes, it’s also the only thing that freezes from the middle out which is why frozen lakes are most dangerous by the bank. I can’t say for certain these are true but I choose to believe
Water molecules have this odd “V” shape to them. When they’re in liquid form, they can mash together really closely. Picture all the ways two “V”s can be mashed together.
But when water freezes, all those “V” shapes connect up into something that looks more like cubes. Cubes can do things like stack, but they have a lot of open space in the middle of the cube. That means the same number of water molecules wind up taking up more space when they’re frozen than they do when they’re not.
The same mass but a bigger volume means ice is less dense, and floats.
Most materials get denser when they turn from liquid to solid. In that way they’re kind of like a pile of lego bricks that you stick together to make one cube. The resulting cube is more dense than the pile of bricks because there’s less space between them when they’re together.
Water is different. It’s more akin to a pile of [kinect pieces](https://assets.fishersci.com/TFS-Assets/CCG/product-images/89790-6-Ft-Ferris-Wheel-Model_300dpi.jpg-650.jpg) that you stick together to make a tower since it forms [water crystals](https://scx2.b-cdn.net/gfx/news/hires/2012/dhgerte.jpg) as it freezes. As a loose pile they can fit closer together, but as a crystal there is more empty space between them. Since it’s the same mass but more volume when frozen, ice is less dense than liquid water.
I could go on about why things less dense than water float, but that doesn’t seem to be the part you’re hung up on.
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Solids are *usually* denser than liquids but there are exceptions. Water is pretty much the only everyday example of this but there are others. Solid silicon, for instance, is less dense than liquid silicon but since silicon melts at 2,500F you’re unlikely to ever notice that.
Ice is less dense than liquid water because when water freezes, its molecules form crystals with an unusually open lattice structure, which ends up being less dense than the liquid state. Technically this is only true for one type of ice, which is known as “hexagonal ice” or “ice I”. There are other crystal shapes, like ice II and III, which are denser than water. However, these shapes don’t form under normal Earth conditions (usually requiring extremely high pressures and/or low temperatures).
If the ice is made of heavy water (D2O – deuterium oxide), it will sink. It is denser than H2O. Water is at its most dense about 4C.
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