Why do icebergs float in water? Why do pipes burst in the winter? How is it that life can exist in arctic seas? The answers lie in the thermal expansion of water.
Flora, Fauna & "Fysics" - The Grand Trio
Rudyard Kipling's short stories, novels, and poetry have established him as one of the premier authors of his generation. He was the first English writer to win the Nobel prize in literature, and remains the youngest ever to do so. As a child, one of my favorite characters from the "Jungle Book" was the story of Kotick, the white seal. Kotick had problems learning to swim at first, but later was able to glide easily around the icy waters in the Alaskan Bering Sea. Kipling wrote of the fish, walruses, sea lions, and killer whales that also inhabit the area. These creatures he wrote of so fondly owe their existence to a most peculiar property of water.
Sea Lion: Matthew Hull
Thermal Expansion of Water
Most solids and liquids expand with rising temperature. Water, the most common and most important liquid on Earth, has a limitation on this expansion. Ice water that is at a temperature of 0oC or 32oF will contract when it is heated. This contraction continues until after an increase beyond 4oC or 39.2oF. Then water that continues to be heated will finally begin to expand like other liquids until it reaches a gaseous state.
At 4oC water has its smallest volume and also its greatest density. Water that is below 0oC has a larger volume and a smaller density. Therefore the reason why ice floats in your cup of soda is because ice is less dense or weighs less than water. This is also the reason why your water pipes may burst in the winter if you are not careful - the water inside increases in volume and weight as it cools from 4 to 0oCelsius.
Why Should We Be Thankful for this Property?
If not for this peculiar property, water in deep bodies such as ponds and lakes and oceans would begin freezing from the bottom up, eventually becoming completely frozen as the ice would never melt. As it is, water freezes from the top down; water that is very cold turns to ice and floats, leaving its warmer, still liquid form below. Normally for deep water, the thin layer of ice that does form on the top behaves as an insulator in that it slows down the rate of heat leaving the water beneath. This behavior of water coupled with temporary winters means deep waters rarely become completely frozen. This why life can exist in water even in artic climes.