Earth Sciences and Ecology Question #1350

Laura, a 15 year old female from Toronto asks on March 24, 2003,

Why does lake water change temperature at a slower rate than the ground?

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The answer

Kyle Kirkwood answered on March 30, 2003

Water has considerable capacity to hold heat because of its polar nature. A water molecule has an unequal distribution of electrical charge over its surface, the top of the Oxygen is slightly negative, and the Hydrogens at the other end are slightly positive. This means that when two water molecules come near each other there is good chance the Hydrogen side of one molecule will form a weak bond with the Oxygen of the other molecule. This is called a Hydrogen bond (H-bond). These bonds have a "battery-like" capacity to store energy--or heat. In order to boil water you need to break the H-bonds that loosely connect the water molecules together, this takes extra energy, thus water seems to absorb heat with little change in its temperature. The heat is going into breaking H-bonds. The reverse is also true, as water cools it forms more and more H-bonds, which as they form actually give off a little heat, thus slowing the cooling process.

The ground on the other hand does not have the same system to absorb heat or radiate heat energy, so the ground heats up and cools down more quickly.

By the way, this feature of water--its polar nature and its tendency to form Hydrogen bonds--also the explains water tension, bubbles, why ice floats and many of the other wonderful fascinating qualities of water.

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