The evaporation of water for cooling purposes is called evaporative cooling. An important example of this type of cooling is the removal of body heat by humans through sweating. When your body needs to cool, perspiration is released to the surface of your skin where it evaporates. The evaporation of the water in the perspiration causes your skin to cool.
Breezes feel particularly cooling when you have perspiration on your skin. This is because the increased movement of air over your body evaporates more water from your skin than still air does. Water on your skin evaporates more slowly when the humidity is high. This is because the humid air already contains much water vapor. Humid air absorbs less water as vapor than dry air.
Electrical power plants that burn fossil fuels or use nuclear energy to generate electricity use huge water cooling towers for cooling purposes. The water to be cooled is pumped to the top of the tower and allowed to drip down through the tower. As the water moves down the tower, air from the bottom of the tower moves up through the tower, evaporating some of the falling water. The heat lost by the evaporating water cools the remaining water that is collected in a basin under the tower. One pound of water that evaporates in a tower can lower the temperature of 100 pounds (45 kilograms) of other water by nearly 50°C (100°F).
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Materials
- Unglazed clay flower pot
- Plastic bowl
- Unglazed clay saucer
- Water
- Modeling clay
- Thermometer (optional)
Procedure
Make sure to use an unglazed clay flowerpot for this experiment. Most unglazed clay flowerpots have an orange-red color and rough surfaces. The bottom of the unglazed clay saucer should have a diameter larger than the top diameter of the flowerpot. (Flowerpots and saucers can be purchased at most hardware and plant stores.)
Most clay flowerpots have drainage holes. Check to see if your clay flowerpot has a drainage hole in the bottom. If the one you are using has a hole, seal it with a piece of modeling clay. Add some water to the pot to make sure your seal is watertight.
Place the clay flowerpot and plastic bowl outside on a hot, sunny day and in a spot where they will get plenty of sunshine. Fill both the clay flowerpot and the plastic bowl nearly full with water. Place the clay saucer on top of the flowerpot and fill it with water.
Every hour for three hours, place your hands around the outside of the plastic bowl of water. Then place your hands around the outside of the clay pot filled with water. Next dip your hand into the plastic bowl of water. Remove the clay saucer from the flowerpot and dip your same hand into the pot of water. If you have a thermometer, place it in the plastic bowl of water. Read the temperature on the thermometer after it has been in the water thirty seconds. Next place the thermometer in the clay pot of water. After thirty seconds read the temperature of the water in the clay pot.
Observations
Does the outside of the clay flowerpot appear wet and feel wet after it has been outside several hours?
Does the outside of the plastic bowl feel warmer or cooler than the outside of the clay flowerpot after both containers have been in the sun several hours? Does the water in the clay flowerpot feel warmer or cooler than the water in the plastic bowl?
If you are using a thermometer, what is the temperature of the water in the plastic bowl? What is the temperature of the water in the clay pot?
Discussion
You should find that soon after you add the water to the unglazed clay flowerpot, the outside of the clay pot looks and feels wet. The outside of the clay flowerpot should feel cooler than the outside of the plastic bowl. Also, the water inside the flowerpot should feel much cooler than the water inside the plastic bowl. In fact, if you measured the temperatures of the water in both containers you may have found that the water in the clay pot was cooler by as much as 18°F (10°C). The temperature difference will depend on the air temperature, the humidity, and wind. The higher the humidity and the less wind there is, the smaller the temperature difference between the two containers will be.
The outside of the clay flowerpot becomes wet because water inside the flowerpot moves through the wall of the clay pot. When water reaches the outside surface of the clay pot, it can evaporate. When the water evaporates, it changes from a liquid to a gas or vapor.
Heat energy is required for water to evaporate. This heat energy comes from the clay pot and the water inside the clay pot. When the water evaporates and removes heat from both the pot and water, the temperature of both the pot and water decreases.
Water evaporates from the plastic bowl as well. However, since water cannot move through the plastic, water evaporates only from its surface in the plastic bowl. More heat is removed from the clay pot than the plastic bowl because water is evaporating from a larger area. This is why the outside of the clay pot and the water inside it should feel cooler than the outside of the plastic bowl and the water inside it.
Other Things to Try
Repeat this experiment and record the temperatures of the water in the two containers with a thermometer every hour for five or six hours. Is the temperature of the water in the clay pot always lower than the temperature of the water in the plastic bowl?
If you are ever near the bottom of a waterfall, notice that the air temperature around the waterfall is cooler than air away from the waterfall. Can you explain why?
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Substances such as pepper and sugar do not react with the ice the way salt does. Salt (sodium chloride) is dissolved by the usually thin layer of water on the surface of the ice. The sodium chloride bonds with the water molecules by getting in between the strong hydrogen bonds. This lowers the freezing point of the water because it is no longer pure ice. Add more salt, and the freezing point can go as low as -9F, which could cause frostbite if held long enough. What this means is that for the water to form as ice the temperature needs to be we’ll say -9F in order to freeze again. That is why it seems like the ice is melting with the presence of salt.
I wanted to know whether ice melts faster with salt, sugar, pepper or water on its own. I’ve been searching for the answers on the internet but I am not sure if I’ve got the right answers.
The question that I am looking for is, “Does ice melt faster when water is freezed with salt, water is freezed with pepper, water is freezed with sugar or water is freezed on its own?” Research shows me that salt melts the fastest, is there any scientific reasoning behind this?
Thank you!