Example

Dress Rehearsal

In chemistry class, you are often asked to prove the concepts that you have learned within the chapter. Let’s say that you have been studying states of matter, including liquids and gases. Your teacher has asked you to demonstrate your understanding of these two states by conducting an experiment in which you follow a step-by-step process, take measurements, and draw conclusions from the result.

You will be comparing rates of evaporation. You learned in the chapter that two elements determine this: the volume of the liquid and the amount of available energy. Your research question: How do intermolecular forces affect the evaporation rates of liquids? You’re curious to know that. When you forget to turn on the bathroom ventilation fan, the mirror fogs up and it takes forever to clear. We won’t mention that the textured ceiling is beginning to peel.

Your teacher has set up your lab station with all the appropriate materials, and you put on your safety equipment. Protective glasses. Check. Protective gloves. Check. You note the dangers that could arise during the experiments as well, such as toxic and flammable chemicals, extreme temperatures, and fumes. You’re ready, steady.

Be sure to conduct these step-by-step procedures in the order given. You label five small plastic cups with the letter of each of the following liquids: distilled water (a), ethanol, (b) isopropyl alcohol (c), acetone (d), and household ammonia (e). You put all of these on a paper towel. With an eye dropper, carefully measure 1 ml. of each liquid and put into the cup. Ahem… we said CAREFULLY! Place each eye dropper on the paper towel directly in front of each respective cup.

Next, place a drop of the first liquid on a piece of wax paper. With your stopwatch, time how long it takes for that drop to evaporate. Record your time. Repeat the wax paper and timing procedure until you have tested all five liquids. Be sure to record the evaporation times as you move through the experiment. Your teacher has provided you with a warm sample of ethanol so that you can compare how heat affects the evaporation rate.

Afterwards, clean up your area, remove your protective gear, and wash your hands. You’re ready to analyze your data. Which liquids evaporated the fastest? The slowest? In which liquids are the attractive forces between the molecules dispersion forces? Would pure alcohol and pure ammonia evaporate more quickly or more slowly than the mixtures used in the experiment? What effect does heat have on the two ethanol samples?

In this experiment, your results should align to the concepts learned in the chapter: Water has a high surface tension, making it slower to evaporate. Chemicals mixed with water will evaporate more slowly than pure chemicals alone. As temperature rises, molecules disperse, thus speeding up the evaporation process.

Now for a real-world application: Your bathroom mirror is slow to evaporate because water molecules hang on to each other for dear life. When you turn on the fan, it evaporates more quickly as molecules are pulled apart by the air movement. Make sense?

Source:
Buthelezi, Thandi, et al. Chemistry: Matter and Change. New York: McGraw Hill, 2008.

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