Background:

Have you ever heard of a person that has a high fever getting a "sponge" bath? A common home remedy for bringing down a fever is to use a sponge to apply either water or rubbing alcohol to the skin of the person with the fever. The water or alcohol is normally at room temperature to start. The method takes advantage of basic physical science theory: evaporation follows from the distribution of molecular speeds in a liquid.

The faster molecules have enough energy to escape through the liquid surface tension despite the attractive forces of the other molecules. The molecules left behind redistribute the available energy in collisions among themselves. Since the most energetic molecules have escaped, the average energy of the system is less than before and the liquid is now at a lower temperature. In the case of the sick patient this has the effect of reducing the temperature of the body.

You may have already observed this cooling as a result of evaporation in everyday experiences. Here are some examples:

• Rubbing alcohol feels "cold" to the touch even if it is at room temperature. Rubbing alcohol is often used to "sponge bath" a patient who is suffering from a high fever.
• Acetone, the main component of many fingernail polish removers, also feels "cold" to the touch, even when it is at room temperature. (*Acetone is a toxic substance and should not be placed on the skin and it should only be used in a chemical fume hood)
• Your body uses water (a coating of sweat on your skin) to cool down the body when it is overheating.

Substances that evaporate quicker require less energy to be changed from the liquid to gas state. Therefore, evaporation rate can be an indicator of heat of vaporization. In general, the longer a substance takes to evaporate, the higher the heat of vaporization.

In this lab you will use a temperature sensor to compare the cooling and evaporation rates for water, isopropyl alcohol, and acetone. In this way the relative heats of vaporization for these substances can be compared.

How does the rate of cooling due to evaporation for liquids serve as a relative indicator for heat of vaporization?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: You must take all appropriate cautions when dealing with alcohol and acetone. Gloves, apron, eye protection.

Put 10 mL of acetone, isopropyl alcohol, and water into separate 10-mL graduated cylinders.

Step 4:

Data collection:

1. Place the temperature sensor into the cylinder with the acetone until the sensor touches the bottom of the cylinder. When everything is ready, start recording data. Leave the sensor in the liquid for 10 seconds.
2. After 10 seconds, remove the sensor from the liquid. Hold the sensor vertically. The liquid will evaporate and the evaporation will take about 2 minutes.
3. Continue collecting data until the liquid on the sensor appears to be completely evaporated and then stop recording data.
4. Rinse the temperature sensor.

Step 5:

Repeat step 4 twice more - once for alcohol, and once for water. Dispose of the liquids in the graduated cylinders as directed by your teacher.

Analyzing and Interpreting:

Set up your Graph display so it shows the data for the first liquid (acetone). Scale the axes to fit the data using the "Scale to Fit" button (). Use the "slope" feature in the graph to determine the rate at which the liquid cools. Use the smart tool in DataStudio. Click and drag the cursor from the beginning point to the minimum temperature:

Repeat the data gathering process for the other two liquids, alcohol and water.

For each of the three liquids, calculate the rate of cooling by calculating the slope:

1.	Which liquid is more efficient (alcohol, acetone, or water) at cooling down an object as a result of evaporation? Explain.
2.	Is there a relationship between the rate of evaporation and the cooling effect that a liquid exhibits?
3.	Use a reference book or the Internet to find out the heats of vaporization for acetone, alcohol and water. What is the relationship of heat of vaporization to the rate of evaporation of these solvents?

Forming Conclusions:

Based on the data you have collected, write a summary statement for the following question:

4.	How does the rate of cooling due to evaporation for liquids serve as a relative indicator for heat of vaporization?