Lakes freeze from top down We all know that ice cubes float in a glass of water. Why? Virtually every substance contracts when it solidifies—the solid is denser than the liquid. If this happened to water, ice cubes would sink to the bottom of a glass, and ice sheets would sink to the bottom of a lake. Fortunately, this doesn’t happen. Liquid water expands by 9% when it freezes into solid ice at 0 °C, from a liquid density of 1000 kg/m3 to a solid density of 917 kg/m3. Consequently, in the winter when the water in a lake freezes, the solid ice stays at the top, forming an ice sheet. Snow covering the icy surface forms a protective blanket that insulates the ice and water below and helps to keep the lake from completely freezing into a solid chunk of ice. Fish and lake plants below the ice survive during the winter. The expansion of water when it freezes has another important environmental benefit: the so-called freeze-thaw effect on sedimentary rocks. Water is absorbed into cracks in these rocks and then freezes in cold weather. The solid ice expands and cracks the rock—like a wood cutter splitting logs. This continual process of liquid water absorption, freezing, and cracking releases mineral and nitrogen deposits into the soil and can eventually break the rock down into soil.
A swimming pool at 0 °C has a very large chunk of ice floating in it—like an iceberg in the ocean. When the ice melts, what happens to the level of the water at the edge of the pool?
(a) It rises. (b) It stays the same.
(c) It drops. (d) It depends on the size of the chunk.
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