The Thermal Expansion of Water
Water, like most other substances, almost always increases in volume when its temperature increases. However, water between 0 degrees Celsius and 4 degrees Celsius behaves unusually.
Imagine you want to freeze a glass of liquid water which is originally at room temperature (about 20 degrees Celsius). As you cool the water, its volume will decrease because the water molecules slow down and move closer together. When the liquid water reaches 4 degrees Celsius, microscopic water crystals start to form. The water crystals have a highly-structured shape, and the molecules in a water crystal are actually farther apart than the molecules in liquid water. As a result, when water starts freezing, it actually expands. Thus, as water cools from 4 degrees Celsius to 0 degrees Celsius, it expands, which is the opposite of what water does under other circumstances. As the water cools below 0 degrees Celsius, it begins to contract once more.
The opposite occurs when you take frozen water (ice) at 0 degrees Celsius and increase its temperature. As frozen water starts melting, the highly-structured ice crystals begin collapsing, and the water molecules move closer together. So, as the temperature of water increases between 0 degrees Celsius and 4 degrees Celsius, the water contracts (the volume of the water decreases). When the temperature of the water has increased to 4 degrees Celsius, the water begins expanding as its temperature increases.
Imagine you want to freeze a glass of liquid water which is originally at room temperature (about 20 degrees Celsius). As you cool the water, its volume will decrease because the water molecules slow down and move closer together. When the liquid water reaches 4 degrees Celsius, microscopic water crystals start to form. The water crystals have a highly-structured shape, and the molecules in a water crystal are actually farther apart than the molecules in liquid water. As a result, when water starts freezing, it actually expands. Thus, as water cools from 4 degrees Celsius to 0 degrees Celsius, it expands, which is the opposite of what water does under other circumstances. As the water cools below 0 degrees Celsius, it begins to contract once more.
The opposite occurs when you take frozen water (ice) at 0 degrees Celsius and increase its temperature. As frozen water starts melting, the highly-structured ice crystals begin collapsing, and the water molecules move closer together. So, as the temperature of water increases between 0 degrees Celsius and 4 degrees Celsius, the water contracts (the volume of the water decreases). When the temperature of the water has increased to 4 degrees Celsius, the water begins expanding as its temperature increases.