For anything to float, it has to be less dense than the medium it is in, and, it has to be able to displace an amount of fluid equal to it’s own weight.
Water is one of the few substances that actually loses density as it freezes. It reaches it’s maximum density at around 40 degrees F, but, as it freezes, it loses more and more density, until it is less dense than it’s liquid form.
If ice was heavier, or more dense than water, rivers, lakes and other waterways would freeze starting at the bottom. By the end of a really cold winter, they would be frozen solid, eliminating any possibility of the survival of the fish that live there throughout the year.
Water is made up of two hydrogen and one oxygen molecule, joined together with covalent bonds, or, the sharing of electrons between atoms. The water molecules are also attracted to each other by hydrogen bonds. As the water cools, the hydrogen bonds adjust to keep the negatively charged oxygen atoms apart.
While in it’s liquid form, water molecules are packed tightly together. Water as a liquid means that every molecule bonds with 3.4 other molecules. However, as water freezes it expands, the molecular structure changes, and, every molecule now bonds with four other molecules. This molecular change causes the ice to have a lattice look. By leaving spaces between the molecules it causes the ice to be lighter than the surrounding water, and so, the ice floats.
We can see how the molecular structure of water changes and expands when it freezes by putting a plastic or paper container of water in the freezer. When the water is frozen, it will expand and change the shape of the container. When water freezes, it has an increase of 9 percent in its volume.
The fact that ice has this lattice work structure is why thin ice on a lake will break easily if stepped on. It is only when the water freezes deeper into the water, and gives the ice more strength that it is safe to cross.
The lattice work in ice is also the reason why we can crunch ice cubes.
Pure water is less dense than water that has additives. So, if you add salt, sugar, or any other substance to water, it will tend to increase the density, and, cause the ice to sink.