5.2: Forms of Water on Earth
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Water comes in three forms, or phases: solid, liquid and gas. Solid water is commonly known as ‘ice’. Ice forms when water cools to 0ºC. As stated above, when water becomes solid, it takes up a greater volume than when it was liquid. Most substances tend to take up a smaller volume, or become more compact when they change from liquid to solid. Solid water can expand up to 9% from its liquid volume. This can be a very powerful and destructive weathering force. Liquid water that gets into very small cracks in rocks or roads, then freezes, forcing the cracks to expand. Another property of solid water is that it floats in liquid water. When solid water expands, it becomes less dense than its liquid counterpart. This is why ice in a glass of water floats at the surface, and why icebergs form. Without this property of water, lakes would freeze from the bottom up in winter, making it impossible for fish and water plants to survive.
It takes 80 calories (a unit of energy) to melt one gram of ice. This energy is called ‘latent heat’ (recall this term from lecture 3). Liquid water also exhibits the unique quality of hydrogen bonding discussed above which makes it such an important molecule to all living things. To raise one gram of liquid water from 0ºC to 100ºC, 100 calories of energy must be added. To change one gram of liquid water at 100ºC to its gaseous phase, 540 calories of energy must be added. This is known as the latent heat of vaporization. When gaseous water changes to liquid water (such as when raindrops form from a cloud), 540 calories are released for each gram of water. This is known as the latent heat of condensation.
Does this mean that all water that is evaporated (a term which simply means ‘change from liquid to solid’) is boiled? No. It takes 540 calories to change water from liquid to vapor at 100ºC, but water can be changed from liquid to vapor at a lower temperature by simply adding more energy. For example, at 20ºC, it takes 585 calories to evaporate 1 gram of water. This latent heat of evaporation is the dominant cooling process in the earth’s energy budget. Water is constantly evaporating from the surface of the ocean, lakes and streams. Solid water (in glaciers) also changes from solid directly to gas. It takes approximately 680 calories to transform one gram of ice into water vapor. This is known as the latent heat of sublimation.