Frozen water, of course, is ice and as we know, a cup of liquid water forms slightly more than a cup of frozen ice. The reason for this lies down below the range of human vision, in the miniature realm of minuscule molecules. It seems that water molecules come in odd shapes. When they lock together to form solid ice there are mini spaces in the latticework.* Zillions of these spaces cause the frozen water to expand.
Water, as we know, is a compound of the elements hydrogen and oxygen. Every water molecule is a package of one atom of oxygen and two atoms of hydrogen arranged just so. Usually these identical water molecules cling together in pairs. Their behavior depends largely on energy which comes from heat. At normal earth temperatures, groups of water molecules have enough energy to string together and slither around like follow the leader.
In this liquid state, it is. possible for the slitherin:; molecules to crowd quite closely together. However, as a substance loses its heat energy, its molecules slow down and tend to crowd closer together. As more p4rticles of matter are packed into a smaller space, the cooling substance contracts and becomes denser. This happens also to cooling water right up until its temperature drops to 4 degrees C.
Other cooling liquids continue to contract and become more dense until they freeze to the solid state. But water refuses to obey this general rule. At 4 C. degrees it contracts no more. At this point the cool, slow moving molecules are beginning to arrange themselves in a solid structure. And the particular shape of this icy structure depends on the shape of the individual, identical water molecules.
It is possible to build a solid wall from cube shaped bricks. The sides of the bricks are all alike and the structure can fit together with no spaces between. But suppose, for example, each brick had a smallish bump on this side and perhaps a smallish bump on that side.
When you tried to fit them together, there would be assorted spaces throughout the solid structure. If you could build such a solid wall, certainly it would take up more space than one built from neat cube¬ shaped bricks.
This is a greatly enlarged example of what happens when liquid water freezes and expands to form solid ice. When the cooling molecules move slowly enough, they begin to settle themselves in an intricate lattice design. But because the water molecule has an odd shape, the particles of matter cannot be fitted tightly together, like the cube shaped bricks in that solid wall.
Ice is a delicate latticework of water molecules riddled with miniature spaces. There are more spaces in the frozen solid than there were in the liquid water. Hence, the ice expands.
When ice expands, it becomes less dense and therefore lighter than liquid water. This explains why it floats. Imagine~what the world would be like if frozen water contracted, as most frozen substances do. Ice would sink to the bottom of streams and seas where the sun's warmth could not melt it.