CAUTION: Use appropriate safety guidelines in performing demonstrations.

Demonstration 1: Which Will Evaporate First?

Purpose

To observe the rate of evaporation of various volatile liquids and to discuss the results in terms of intermolecular forces and vapor pressure.

Materials

Samples of the following volatile liquids:

Methanol Ethanol

1-Propanol

2-Propanol

1-Butanol

Acetone

Water

Watch glass for each liquid

Cotton ball for each liquid

Pair of disposable gloves

Very clean chalkboard

Safety and Disposal

All of these liquids except water are flammable and their vapor-air mixtures are explosive. Do not have any flames or sparks of any kind in the room during this demonstration. The vapors of 1-butanol and 2-propanol are irritating to the eyes and respiratory system. Do not over use. Any remaining liquid in the watch glasses may be washed down the drain with a 20-fold excess volume of water. The moistened cotton balls should be placed into a container of water, squeezed out, and thrown into a waste can. The water in the container can then be flushed down the drain.

Procedure

1. Write the names of the selected volatile liquids on the clean chalkboard. Have the students help you to write under the names the formulas and structures of the liquids.

2. Start with methanol and ethanol. Have the students predict which liquid will evaporate first. Use plastic gloves. Place a cotton ball in the watch glass containing these two liquids, squeeze off excess, and make a swipe down the chalkboard with each liquid under its name. Measure the time necessary for each to evaporate.

3. Check the studentsÕ predictions and discuss their reasoning.

4. Repeat with 1-propanol and 2-propanol; with 1-butanol and acetone; with methanol and water. Be prepared to repeat with pairs of liquids from different sets to support or disprove suggested hypotheses.

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