Water. The most abundant compound on Earth, covering over 70% of the Earth’s surface. The only substance on Earth naturally occurring in all 3 states of matter; solid, liquid, and gas. Essential for human life. And an anomaly. Expanding when frozen solid, but also when boiled. Yet water does not need to be such a mystery.
Almost all liquids will expand when they are heated. Ice-cold water however does just the opposite. Water at the temperature of melting ice contracts when it is heated until it reaches 4⁰ C. Then, the water starts to expand, and the expansion continues until it reaches its boiling point, 100⁰ C. Why does it do this? We know that most objects expand when heated, because of increased molecular motion. This causes molecules to ‘jiggle’ faster, and they tend to move farther apart. Water is no different. Most other liquids contract when frozen, but water is an exception. This has to do with the odd crystal structure of ice. The crystals of most solids are structured so that the solid state occupies less volume than the liquid state. Ice, however, has open-structured crystals. When water freezes, it forms a sort of crystalline lattice, and because of water’s unique angular shape, the crystals that result have take up more volume than in its liquid form. After ice is heated to 4⁰ C, the crystal’s structure collapses causing the volume of the water to decrease. However the perpetual jiggling apart caused by heating will eventually overcome the decrease in volume, and the volume will increase.
This can, in turn be used to explain other phenomenon. For example, have you ever wondered why a lake freezes from the top to the bottom? Why doesn’t it all freeze at once? Does hot water not rise? Well, liquid water has a density maximum at about 4⁰ C. Therefore, as water cools at the top of the lake, the 4-degree water falls to the bottom, displacing slightly colder but less dense water. Therefore the lowest levels of water in a large lake never reach freezing and such bodies of water freeze from the top down, with it first freezing at the surface, then lower, and lower, and lower. In this way oceans will never completely freeze, as it would take forever to get the whole ocean to 4⁰ C, and the huge concentration of salt makes the freezing point less than 0⁰ C.
Water is not as confusing as it seems. With an inquisitive mind and a good amount of curiosity, you can uncover many more so called ‘secrets’ of water.