N.B. Read entire lab thoroughly before starting, especially the SAFETY and PROCEDURE sections.

N.B. Report any problems or breakage. You will not be penalized for problems, even if you helped to create them. We want only to fix problems, not to fix blame.

1.0 Objectives

Melt a sodium thiosulphate pentahydrate sample. Observe and explain the behaviour of the melt.

2.0 Materials

Sodium thiosulphate pentahydrate

Test tubes, small, #9, Test tube cleaning brush

Ground glass Stoppers for above test tubes, Forceps, Cotton balls

Spatula

Alcohol burner, Matches

3.0 Safety

1. Read the MSDS sheet for sodium thiosulphate pentahydrate before proceeding further.

2. Note position of nearest fire extinguisher.

3. Note position of nearest telephone. It is marked with emergency numbers.

4. All participants must wear eye protection ( goggles or face shields) at all times.

5. Keep fume hood glass between you and the work at all times.

6. Heat the test tube only when holding it with the test tube forceps.

7. Be careful of heated surfaces.

8. Be careful to avoid knocking over the alcohol burner. It is a potential fire hazard.

4.0 Procedure

1. Remove the metal cap and light the alcohol burner. You should get a reasonably steady flame about 3-5 cm high.

2. Take a clean, dry test tube and using the spatula fill the tube about 2 cm from the bottom with sodium thiosulphate. Hold the neck of the test tube over the neck of the open sodium thiosuulfate bottle during filling, in order to minimize spillage.

3. Grip the open, unstoppered test tube in the test tube forceps. Holding the tube at a slight angle, position the bottom of the tube over the alcohol flame (~1 cm), WITH THE OPEN MOUTH OF THE TUBE POINTING AWAY FROM YOU AND OTHERS, until all of the sodium thiosulphate is completely melted. Melting the sodium thiosulphate will take less than a minute. Do not boil it out of the test tube.

4. Place the hot tube in the rack and plug it with a cotton ball.

5. When the test tube is completely cool to the touch, carefully (i.e. without jiggling the tube) remove the cotton ball and discard it. Plug the tube with the ground glass stopper.

6. Hold tube firmly in your hand, securing the stopper gently but firmly with your thumb. Shake vigorously for a few seconds and observe. Check that the stopper is not stuck – loosen it a bit after you have finished shaking the tube.

NOTE: If you would like to characterize the crystallization of the thiosulphate quantitatively I have provided a thermometer which you can use to monitor the temperature of the sample.

I suggest you do the experiment as described above, then re-melt the sample, getting it good and hot without boiling it, then quickly place the test tube in the rack and insert the thermometer. Try recording the temperature of the sample at 5-10 second intervals. Comment on your observations and discuss the influence of the thermometer on the experiment.

Contrast the case of keeping the test tube (and thermometer) very still during the cooling process and the case of agitating the test tube and/or thermometer. Speculate on observed differences.

You might also like to try `seeding' the supercooled sodium thiosulphate with a sodium thiosulphate crystal. Do other `seeds' work as well? You might try small crystals of other materials, such as salt, or non-crystalline materials, such as fragments of plastic.

5.0 Clean-up

1. When finished, extinguish the alcohol burner by smothering the flame with the metal cap. Be sure to leave the metal cap on the burner to prevent evaporation of the alcohol fuel.

2. Clean test tubes and stoppers in warm water. If stopper is difficult to remove, soak in warm water briefly and pull it out carefully. Sodium thiosulphate will take only a few minutes to dissolve from the test tube. Place clean tubes upside down in rack to dry. Lay clean stoppers on rack to dry.