What happens when water evaporates?

Transcription

1 What happens when water evaporates? Introduction Water is an important part of the Earth s climate system. Recall water vapor is the doant greenhouse gas. It is relatively transparent to solar or shortwave radiation while it absorbs Earth or longwave radiation. Liquid water drops in the atmosphere, i.e., clouds, reflect solar radiation and absorb longwave radiation. Although water is a common substance that we need for everyday existence, it exhibits several unusual properties. Water is the only substance that exists as a solid, liquid and gas at normal temperatures and pressures existing at the Earth's surface. It has a high specific heat, i.e. it requires considerable energy to change its temperature. As water cools from 4 C to 0 C, its density decreases, i.e., it expands. And water has a high latent heat of vaporization, i.e.; it required considerable energy to change it from its liquid to its gaseous phase. This latent heat energy is released when water vapor condenses its liquid phase. The figure to the right shows the latent heats released or needed for water to change phases. Note that it requires more than seven times the energy to evaporate 1 g of water than is required to melt 1 g of ice. The evaporation rate from a water surface increases as the wind speed increases, the temperature of the water increases, the air temperature increases, and the air relative humidity decreases. Evaporation of Water Experiment This experiment requires water that is at room temperature. Use a thermometer to measure both the room temperature and the water temperature. Be sure that the thermometer is dry when you are measuring the air temperature. If the water is too cool, add some warm water to bring its temperature up to room temperature. Likewise if it is too warm, add some cold water. Part I: Now place about a 1/2-inch of water (enough to cover the reservoir bulb of a thermometer) in a shallow plastic plate. Place one thermometer in the water and another

2 thermometer on the table near the plate of water. Because the water began at room temperature, both thermometers should read the same temperature at the beginning of the experiment. Fan the plate of water to help circulate the air over the water. Record the temperature of the two thermometers every two utes in the table below. Time Initial 2 Air Water Make a plot (see last page of this handout) of the temperature (vertical axis) versus time (horizontal axis) for both thermometers. Do the two temperatures remain the same throughout the 14 utes of the experiments? If not, why not? It is expected that the room temperature would remain constant during the experiment. Did the room temperature remain constant during the experiment? If not, why not? Is evaporation a warg or cooling process? Does evaporation add or remove heat energy from the water?

3 What does this say as to why you may feel cool when you get out of a swimg pool on a breezy warm day? Part II: Empty the water from the plate and again place about a 1/2-inch of room-temperature water in the plastic plate. Again place one thermometer in the water and the other thermometer on the table near the plate of water. Remember, both thermometers should read the same temperature at the beginning of the experiment. Now seal the plate with the water and thermometer with clear plastic food wrap. Try to make the seal sufficient to limit the exchange of air between air trapped above the water in the plate and the room air. Stretch the plastic wrap tight enough that it does not sag and touch the water surface. Again fan the plate of water to help circulate the air over the plate and record the temperature of the two thermometers every two utes in the table below. Time Initial 2 Air Water Add the plot (see last page of this handout) of temperature versus time for this experiment to the graph you did for Part I. Describe what happens to the water temperature now. Explain what is happening? Why is the behavior of the water temperature changed by covering the plate?

4 If you were to leave the unsealed plate of water over night what would happen to the water? Would the amount of water increase, decrease or remain the same? If you were to leave the sealed plate of water over night what would happen to the water? Would the amount of water increase, decrease or remain the same?

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