Evaporative
Cooling
Experiments
from Team Labs
Experiment Profile
| Connections: | Physical Science, Physics, Chemistry, Mathematics | |
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| Skills: | Graphing, Analyzing, Measuring, Inferring | |
| Math Concepts: | Difference, Sum of Difference, Absolute Value | |
| Duration: | 1 Class Period | |
| Team Size: | 2-3 students per group, or whole class demonstration | |
| Content Standards: | Science
Standard A (grades 9-12) Science Standard B (grades 9-12) Math Standard 1, 4, 5, 13 (grades 9-12) |
Summary
In this experiment, students will gain a better understanding of evaporation and evaporative cooling. Students will submerse a Standard or Extended Temperature Probe in three different solutions and then measure the drop in temperature as these solutions change from liquid to vapor.
Materials
- Host Computer
- ThinkStation SP16 Interface Kit
- ThinkStation Interface
- Power Supply
- Communications Cable
- Excelerator 2000 Software
- Standard or Extended Temperature Probe with API
- Small fan
- Rubbing alcohol (isopropyl alcohol)
- Water
- Mineral oil or vegetable oil
- Three (3) small beakers
Background
Molecules in a liquid state are in constant motion. When these molecules, at any given temperature, possess the sufficient kinetic energy (KE) to escape from the surface, and do so, we call this evaporation. The process of evaporation, however, is not an indefinite process. The molecules moving from a liquid to the empty space above, establish a vapor phase. As this vapor phase increases, some of the molecules that have escaped the liquid will actually return to the liquid state, the process we call condensation.
To understand evaporation more completely, the more rapidly moving molecules in a liquid (that obtain the necessary and largest KE in the liquid to "jump" or escape to the gas state) leave first. The remaining molecules in the liquid now possess a lower average KE which is reflected in a lower temperature of the remaining liquid.
There are three manners in which the evaporative rate can be increased:
- Increase the temperature of the liquid, thereby increasing the KE of the molecules.
- Increase the surface area of the liquid surface thereby exposing it to more air for the molecules to "jump" state.
- Increase the presence of air currents over the liquid, which will remove the molecules that have jumped state to the gas above and prevent them from returning to the liquid state as they cool via condensation.
We will investigate in this experiment the way in which evaporative rates are increased by increasing air currents over the liquid. We will also investigate why different types of liquids (water, mineral or vegetable oil and isopropyl alcohol) have and will show different evaporative cooling rates in our experimentation.
Procedure
Collecting Data with the Temperature Probe
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1. Prepare your liquid samples by pouring 50 ml of water into a beaker. Do the same for the mineral or vegetable oil, and for the isopropyl alcohol. Cover each beaker and do not move them for several minutes. This allows the samples to adjust to the ambient room temperature. |
 Tip of the Extended Temperature Probe (left) and tip of the Standard Temperature Probe (right) |
2. Connect the ThinkStation Interface to your computer. Attach a Temperature Probe (either Extended or Standard) to one of the analog jacks on the ThinkStation interface (outlined in black). |
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3. Next, launch Excelerator 2000 and click on the Connect&GOTM icon. Excelerator will automatically identify the temperature probe and create a graph of Temperature vs. Time. The software also sets a default sample rate and duration for the experiment, which you can change if needed. |
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4. To change the sample rate and duration of the experiment, click on the Edit Clock icon located on the right side of the Excelerator toolbar. |
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Set the sample rate to 10 samples per second and the duration to 30 seconds. |
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5. At this point, you are ready to record some data. Mark a distance of 10 cm from the fan, using tape or the like. Then, hold the temperature probe in the beaker of isopropyl alcohol, thoroughly coat the tip of the probe with the liquid and hold it at your marked distance from the fan. (Note: the fan should be left in the "on" position at a continuous speed. Try alternate speeds and see how it affects the data!) |
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Press the green GO button on the left side of the Excelerator toolbar. |
| 6. Excelerator will record the Temperature emitted from the probe for exactly 30 seconds and will display a graph similar to the one here. It may be necessary to click on Rescale in the Tools menu to see all of the data. Save the trial after completing. | |
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7. Repeat this procedure, for all three liquids, cleaning the probe between trials. Note the order in which the liquids were tested, as they will be recorded and indicated in the Excelerator Fast Graph as Trials 1, 2 and 3. |
Analysis of the Data
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1. Use your Fast Graph "Analysis" toolbar and choose to "Select" each trials’ data points by placing two selection lines on the data trend. Next, "Curve Fit" the selected data between those selection lines (again found using the Fast Graph’s "Analysis" toolbar). This will produce a slope formula of each trials’ data plot in the form of: y = mx + b You may choose to do this for each trial run after taking the data, or highlight which data set you want to analyze by checking it under the "View" menu in Excelerator.
From the Fast Graph, it is possible to observe the Evaporative Cooling rate for each trial as it relates to the slope of the trial’s data (done by a curve fit) as the "m" component in the formula: y = mx + b. You will notice that by doing this experiment and analysis procedure, the isopropyl alcohol has the steepest slope, followed by water and finally the mineral or vegetable oil sample. The explanation involved in understanding their differences is due to their intermolecular attraction within the differing liquids. |
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2. The measure of how strong molecules are held together in the liquid state is referred to as its Molar Heat of Vaporization (ΔH vap). It is represented by the energy required to vaporize 1 mole of a liquid. The Molar Heat of Vaporization is directly related to the strength of the intermolecular forces that exist in the liquid. If the intermolecular attraction is strong, the molecules in that liquid cannot easily escape into the vapor phase. We noted these differences by measuring the rate at which vaporization took place as these liquids were placed in front of a moving air current (the fan) to help initiate a faster rate of vaporization (or more commonly understood as, evaporation). |
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3. We can relate these findings to the Clausius-Clapeyron equation, which notes the quantitative relationship between the vapor pressure P of a liquid and the absolute temperature T, or the Molar Heat of Vaporization (ΔH vap) as related to temperature.
This equation has the form of the linear equation, y = mx + b, which we are able to create using the Excelerator software by analyzing a curve fit of our evaporative cooling trials. |
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| 4. We can finally sum up these findings by noting mineral or vegetable oil has the strongest intermolecular bonding (as noted in its extremely small evaporative rate), followed by water in the middle and finally isopropyl alcohol with the least. To look at it another way, the intermolecular forces of attraction in isopropyl alcohol are the weakest of the three samples. | ||||||
Conclusions
- Oil has the greatest intermolecular forces of attraction and the corresponding lowest evaporative rate of cooling.
- Alcohol has the least intermolecular forces of attraction and thus the highest evaporative rate of cooling.
- Water fell between the two samples in intermolecular forces of attraction and evaporative rate.
Extensions
Have your students measure and compare the evaporative cooling rates of 95% Isopropyl Alcohol to that of 70% Isopropyl Alcohol. Investigate other liquids as they relate to their "feel" or "texture" to see if they correlate with your expected intermolecular attraction by touch alone.
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About the author... Marc Mueller is the Secondary Curriculum Specialist at Team Labs. His background includes packaging and mechanical engineering, secondary science, technology and vocational instruction. His real-world experience is mirrored in his curricula as he has been focused on engineering, creating applied technology laboratories, and the creation of pre-engineering, computer technology and vocational coursework and activities throughout his career. |
If you have a great experiment idea, please send mail to the WebMaster. |
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