Crystals out of "thin air"

There is a special fascination with crystals. Sharp edges give them distinctive characteristic shapes and many have flat surfaces that reflect light, giving these crystals an intriguing sparkle. Some people even claim that crystals have "magical" powers. One recent advertisement for a pendant asserts that it contains crystals that "help pace your natural energy field and guard against EMF radiation from computers".1

In one unusual type of crystal formation, crystals seem to come out of the air, out of "thin air". This process can be illustrated clearly with a common household chemical, as described in detail below. This experiment can serve as an introduction to the concept of molecules, especially when combined with parallel experiments utilizing the condensation of liquids and the perception of odors, as illustrated.

The material involved is sold under several names for several different purposes: mothballs, moth crystals, moth cakes, toilet bowl deodorizer, and air freshener. All these list as active ingredient the same chemical by its technical name, para-dichlorobenzene. The odor of this chemical is very familiar to most people because of its common usage in closets and bathrooms. In this experiment mothballs will be used, but other samples of this material can be substituted. Mothballs are designed to kill moths and carpet beetles.

Despite common usage, mothballs contain stern warnings: Keep out of reach of children; Hazard to humans and domestic animals; Causes eye, skin, and respiratory tract irritation; Harmful if swallowed; Avoid contact with skin, eyes, or clothing.2 Some of these admonitions seem surprising, considering the recommended application in a closet or on a carpet.

For safety reasons mothballs should only be handled in a sealed jar in the classroom; this allows observation without exposure. The container should be labeled clearly to avoid accidental opening.

Experiment 1: Crystals of Mothballs

Purchase mothballs in a grocery store or drug store. Make certain that the active ingredient listed on the label is paradichlorobenzene. Also obtain canning jars;3 the containers must be made of glass.4

Add one mothball to a glass canning jar and seal. Invert the jar and label clearly: "Do not open: para-Dichlorobenzene". To speed the formation of crystals, place an ice cube on top of the inverted jar. Replace the ice cube when it has melted. Leave the...