Experiment 31: Supersaturation

Background

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 method will work only with a particular kind of sugar solution? 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 a 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 the excess solute remains in the 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.

Goals

Equipment

Safety goggles

1 100-mL beaker

1 medium test tube

1 10-mL graduated cylinder

1 test-tube rack

1 gas burner

1 test-tube holder

8 centigram balances/class

Materials

Sodium sulfate decahydrate, Na2SO4·10H2O

Ice

Distilled water

 

 

Safety

Procedure

Copy Table 31.1 into your laboratory notebook. As you perform the experiment, record your observations in this table.

  1. Place 5 g of Na2SO4·10H2O in a clean medium-sized test tube. Add 10 mL of distilled water.
  2. Hold the test tube in a test tube holder and heat it in a burner flame, agitating the mixture gently until all of the solid has dissolved. Caution: When heating a test tube, never point the mouth of the tube at yourself or anyone else. Make sure to warm the bottom and sides of the tube evenly. Never heat only the bottom of the tube. Place the test tube in a test-tube rack. Add one more crystal of Na2SO4·10H2O to the warmed solution and gently agitate it. Record your observations in Table 31.1.

    3.  

  3. Place the test tube and contents in 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 redissolve the crystals, and cool the tube again.
  4. 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 Na2SO4·10H2O. Describe what you see. Touch the bottom of the test tube to the palm of your hand. Record your observations in Table 31.1.

    5.  

  5. Follow your teacher's instructions for the proper disposal of the materials.

Data Record

Table 31.1 Observations

Step

Observations

1

 

2

 

3

 

4

 

Conclusions

  1. Why is it necessary to heat the mixture in step 2 of the procedure?
  2. Is the solution unsaturated, saturated, or supersaturated at the end of step 2? Explain.

    3.  

  3. Is the solution unsaturated, saturated, supersaturated at the end of step 3? Give evidence for your answer.
  4. At the end of step 4, when crystallization is complete, is the solution unsaturated, saturated, or supersaturated? Explain.

    5.  

  5. Describe a simple test to determine whether a solution is unsaturated, saturated, or supersaturated. Explain how to interpret the test.

Extensions

  1. Devise an experiment to discover whether the rate of cooling of a supersaturated solution affects its stability.
  2. The concepts explored in this experiment are used in growing crystals. Find instructions for several crystal-growing labs. Ask your teacher which of these you can do in your school laboratory and which you can do at home. After you have successfully grown some crystals, design a crystal-growing lab for others in your class.