How to Demonstrate a Supersaturated Solution

This demonstration illustrates three concepts: the process of crystal growth from a solution, the properties of saturated solutions, and the idea that a seed crystal must be made of the same substance as the crystals that will grow.

MATERIALS

Saturated water solution of sodium acetate
Small samples of crystals of sodium chloride, sugar, or other substances
Overhead projector
Two or three petri dishes for use on the overhead projector stage
Paper towels for spillage

PRESENTATION

After explaining the concepts of saturation, supersaturation, crystallization, and seed crystals, pour the supersaturated solution into a petri dish on the overhead projector stage to a depth of no more than 1/4". Add one or more seed crystals of sodium acetate trihydrate to the saturated solution. Needle shaped crystals will start to grow slowly. The process will continue for 20-30 minutes. Plan on discussing other material while this is happening; come back to the overhead projector to check the progress of the crystallization every few minutes and comment on the process of crystal growth.

To illustrate the fact that the seed crystal must be made of the same material as the crystals to be formed, repeat the process of pouring more saturated solution into one or two other petri dishes on the overhead projector stage. Add a crystal of sodium chloride to one dish. Nothing happens. Repeat with a sugar crystal. Again, nothing happens. If you wish, repeat with some other substance. Nothing happens. Now add a crystal of sodium acetate trihydrate and cyrstallization will commence.

DISCUSSION

Sodium acetate trihydrate is unusual in the stability of its supersaturated solutions. Most such solutions will spontaneously crystallize from the slightest mechanical disturbance. This should be pointed out to your class in the overall discussion.

Supersaturated solutions of sodium acetate trihydrate are extremely sensitive to the presence of even microscopic crystals of the substance. It is not unusual to have solutions spontaneously crystallize in a place where sodium acetate trihydrate has been previously handled. Before repeating this demonstration it is good practice to wipe down all surfaces with a moist sponge to dissolve any microcrystals of sodium acetate trihydrate that may be lurking in the vicinity. Wash your hands between shows.

HAZARDS

Sodium acetate trihydrate and its solutions in water are neither toxic nor harmful.

REFERENCES

B. Shakhashiri, Chemical Demonstrations, 1, 27.