SUPERSATURATION
Name ________________________________ Period _____ Date ________________
SUPERSATURATION
Perhaps you’ve made rock candy by placing a string in a sugar solution and letting the sugar crystallize on the string. But did you know this candy-making technique will work only with a particular kind of sugar solution? A saturated solution is one where no more solute can be dissolved. Under certain conditions, a solution may contain more solute than is normally contained in a saturated solution at the same temperature. This type of solution is unstable and is called supersaturated. The addition of a single crystal of solute often causes the excess solute to crystallize. You must use a supersaturated solution when making rock candy. The addition of the string disturbs the unstable solution and begins the crystallization.
The solubility of most substances decreases as temperature decreases. This fact sometimes leads to the formation of supersaturated solutions. As the solution cools, the excess solute may or may not crystallize out. If excess solute remains in solution, the solution becomes supersaturated.
In this experiment, you will make a supersaturated solution and observe the effect of adding a seed crystal to it.
Purpose: To prepare a supersaturated solution and see the effect of seeding the supersaturated solution.
Materials: goggles Bunsen burner ice
matches test tube rack and holder seed crystals
250 mL beaker test tube w/ sodium thiosulfate
Safety: Wear goggles throughout the entire lab. When using a Bunsen burner, remember to tie back long hair and avoid loose clothing. Do not reach across the flame and never leave a flame unattended. When heating a test tube, never point the mouth of the tube at yourself or anyone else.
Procedure: 1. Get a test tube filled with sodium thiosulfate.
2. **Hold the test tube with the test tube holder and heat it in the burner flame. Make sure to heat the bottom and sides of the test tube evenly. Never heat only the bottom of the tube. Melt the sodium thiosulfate completely! Record your observations.
3. Place the test tube in the test-tube rack.
4. **Add one crystal of sodium thiosulfate to the warmed solution and gently agitate (shake) it. Record your observations on the next page.
5. Place the test tube and its contents into a beaker of ice water to cool. Be careful not to disturb the test tube during the cooling process. If crystals begin to form as it is cooling, reheat the tube to dissolve the crystals and cool the tube again.
6. **When the solution is cold, gently remove the tube from the ice water bath and put it in the test tube rack. Add one small crystal of sodium thiosulfate. Record your observations.
7. **Touch the bottom of the test tube to the palm of your hand. Record your observations.
8. Clean up according to your teacher’s instructions.
Observations:
#2
#4
#6
#7
Analysis:
1. How did you supersaturate the solution?
2. Why did the solution get warm after the second crystal was added?
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