Just Keep Cool—How Evaporation Affects Heating and Cooling
AbstractWhen we get hot, we sweat. The physiological role of sweat is to cool us down. When the water evaporates, it removes energy from our bodies. This sort of evaporative cooling can also be used to cool homes, using what are referred to as swamp coolers. Evaporative cooling is also a potential source of energy waste in the kitchen because it increases the time it takes to heat water. In this chemistry science fair project, you will study how a variety of things cool down, whether for better or worse, using the process of evaporation.
David Whyte, PhD, Science Buddies
The objective of this chemistry science fair project is to investigate several aspects of evaporative cooling.
Evaporation is the process by which molecules in a liquid escape into the gas phase. In any liquid, such as a glass of water at room temperature, the molecules in the liquid are moving. They bump into each other as they meander about the liquid. The speed with which they move depends on the temperature—in hotter liquids, the molecules move faster. The average speed depends on temperature, but around this average speed, there will be some molecules moving faster (more energetically), and some moving slower. When the more-energetic molecules are near the liquid's surface, they can escape into the gas above. As more and more of the most energetic molecules evaporate into the gas, the average energy of the molecules left behind decreases, so the liquid cools.
The rate of cooling caused by evaporation depends on the rate at which molecules can escape from the liquid. You might have noticed that when you pour rubbing alcohol on your skin, it cools your skin more than when you pour water on it. This reflects the greater volatility, or tendency to evaporate, of the rubbing alcohol.
The Experimental Procedure for this science fair project has three sections. They might seem unconnected at first, but each is related by the underlying concept of evaporative cooling. In the first section of the Experimental Procedure, you will compare evaporative cooling caused by water, rubbing alcohol, and cooking oil. The cooling effect studied in this part of the procedure is the basis for swamp coolers. In these cooling devices, outside air is blown over a wet surface and then into the home. You are familiar with the principle if you have ever had wet clothes on with a breeze blowing—the evaporation of water cools you off, just as it cools the wet surface in the swamp cooler. Thermal energy in the hot air is "extracted" and used to convert some of the liquid water into water vapor. Because energy is used to evaporate water, the air is cooled after passing over the wet surface. The cool air is then circulated around the interior of the building.
In the second section of the procedure, you will look at the temperature change that occurs when water (sweat) evaporates off of skin. Sweating is a physiological response that uses evaporative cooling as a mechanism to remove excess heat.
In the third section, you will look at evaporative cooling in the kitchen. When heat is applied to water to make it boil, some of the energy can be lost to evaporative cooling. You will investigate how evaporative cooling affects boiling time by comparing how long it takes to boil a pot of water both with and without a lid.
Terms and Concepts
- Evaporative cooling
- Swamp cooler
- Why are some liquids more volatile than others?
- Swamp coolers are most often used in areas that are hot and dry. Would a swamp cooler work in hot, muggy conditions?
- Dogs do not sweat, but is their cooling mechanism similar to that of humans?
Materials and Equipment
- Measuring cup
- Rubbing alcohol
- Cooking oil, such as olive oil
- Plastic plates, disposable (4)
- Paper towels (12)
- Clear tape
- Ballpoint pen
- Infrared thermometer; available online, from websites like Amazon.com
- Small fan; if you do not have a small fan, you will need an extra plate.
- Pots to boil water, identical, 2-qt. size or larger, with lids (2); you can use one pot repeatedly if you do not have identical pots.
- A helper
- Lab notebook
- Graph paper
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Evaporative Cooling in Buildings
Fill a measuring cup with tap water and allow it to come to room temperature.
- The rubbing alcohol and the oil should also be at room temperature.
- This step is just to ensure that the liquids are at the same temperature at the start of the experiment.
Place four disposable plastic plates, with the up sides down, on a work surface.
- Use a waterproof surface (such as tile or laminate) since you will be using alcohol that could damage wood finish.
- Fold each paper towel in half twice, so that each has four layers.
Place a folded paper towel on top of each plate.
- The plates keep the towels from being in contact with the work surface, which would affect their temperature. You could also use StyrofoamTM or other insulating material.
- Tape the edges of the paper towels to the plates.
Label the paper towels 1–4.
In the next step, the paper towels will be treated as follows:
- 1: no liquid
- 2: water
- 3: rubbing alcohol
- 4: oil
- In the next step, the paper towels will be treated as follows:
- Start the stopwatch.
Take the temperature of the paper towels with the infrared thermometer.
- Take three readings of each paper towel.
- Keep the direction and distance between the thermometer and each plate the same.
- Record the temperatures and times in a data table in your lab notebook.
- Pour water on paper towel #2, just enough to wet it.
- Pour rubbing alcohol on paper towel #3, just enough to wet it.
- Pour oil on paper towel #4, just enough to wet it.
- Take the temperature of each paper towel, and record the temperature and time in your lab notebook.
- Repeat the temperature readings three more times, at 2-minute intervals.
- Which paper towel has the lowest temperature? What was the largest temperature difference between two paper towels that you noted? Record all observations in your lab notebook.
- Repeat steps 1-14 two more times, with fresh paper towels, but you can rinse and reuse the plates. Average the results in your final report.
- Repeat steps 1-15 three more times, only for these trials, with the fan gently blowing over the paper towels. If you do not have a fan, use a paper plate as a fan. Your helper can fan as you take and record the temperature at 2-minute intervals. Did the fan change the results? Why?
Evaporative Cooling on Skin
In this section, you will look at the cooling effect of evaporation on human skin.
- Mark a small spot on your arm with a ballpoint pen.
Measure the temperature of the skin on your forearm near the pen mark.
- As in the section before, take two more readings and average them.
- Pour some room-temperature water on your arm.
- Take the temperature of your skin near the mark. Record all data in your lab notebook.
- Take a temperature reading every minute until your arm dries.
- Repeat steps 1-5 two more times.
- Now repeat steps 1-6 of this section three times, this time using the fan or helper with the paper plate to blow air on your arm. Average all the results.
- Graph your results.
- What temperature change did you see?
- Repeat steps 1-9 of this section using rubbing alcohol. What is the difference in the temperatures between water and alcohol?
Evaporative Cooling When Boiling WaterYou have looked at two beneficial aspects of evaporative cooling—one used to cool buildings and one used to cool people. In this section, you will look at a situation where evaporative cooling is a source of energy waste and thus, something to avoid.
- Add 2 qt. (8 cups) of tap water to each pot.
- Check the starting temperature of the water with the infrared thermometer. Record the time and temperatures in your lab notebook.
- Start two burners on your stovetop. They should be set to the same setting.
- Cover one of the pots with a lid, but not the other.
- Put the pots on the burners.
Use the infrared thermometer to record the temperature of the water every 3 minutes.
- Measure the temperature of the open pot by pointing the thermometer at the surface of the water.
- Measure the temperature of the covered pot by briefly removing the lid and pointing the thermometer at the surface of the water.
- Determine how long it takes for the water in each pot to come to a boil.
- Stop taking the temperature of a pot when it is at a full boil. Use your judgment as to when this point is reached.
- Repeat two more times and average your results.
- Graph your results. Put "Covered" and "Uncovered" on the x-axis, and "Time to boil" on the y-axis. What was the time-to-boil difference between the two pots, in minutes and in percent change?
Ask an Expert
- Try other liquids for the first section, such as sugar or salt solutions, nail polish remover, etc.
- If you have access to a sensitive scale, weigh the paper towel with the alcohol during the course of the procedure. Find a relationship between weight change and temperature. For example, "on average, 1 gram (g) of alcohol was evaporated every minute to keep the paper towel 3 degrees cooler than room temperature."
- Demonstrate how a swamp cooler works in a model house made out of cardboard. Look up some design ideas online. Remember, the air coming in from the fan needs an open window or door to escape out of. This is a consideration for real houses with swamp coolers.
- Do some research on the energy used by your stove and calculate how much energy you used to boil the water on your stovetop with and without the lid. You might look at the gas meter to estimate how much gas is used. Make some rough guesses to come up with an estimate of how much energy could be saved nationally if everyone used a lid on a pot of water set to boil.
- Devise a method for reliably measuring small changes in temperature due to evaporation of low-volatility liquids, such as oil.
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