Collect six small test tubes (clean and dry) as well as clamps or tongs. Label the test tubes 1, 1a, 1b, 2, 2a, and 2b. The test tubes marked 1 and 2 should be Pyrex since they will be heated later in the experiment. Wipe off any fingerprints or other smudges from tubes 1 and 2, and then weigh them carefully to the nearest 0.0001g. Do not touch them with your fingers from now on. Test tubes 1a, 1b, 2a and 2b will not be weighed and can be touched as needed..

Each student in the lab section will be assigned certain volumes of the 0.50M Cu(NO3)2 and 0.50M KI. Tubes 1 and 2 will have identical mixtures so that if any error is made there is a duplicate. Add the assigned volume of Cu(NO3)2  to the assigned volume of KI in tubes 1 and 2. Be sure to record any observations of the reaction in your lab notebook

Holding the top of the test tube gently but firmly between the thumb and forefinger of one hand, tap the bottom of the test tube gently with the forefinger of your other hand to thoroughly mix the chemicals. Set the tubes in the test tube rack for five minutes to allow the reaction to go to completion. Remember not to touch test tubes 1 and 2 with your hands. Use a clamp, tongs or a Kimwipe.

Centrifuge the test tubes to collect the solid. (Your instructor will demonstrate the proper use of the centrifuge.) Place the two test tubes directly opposite each other. Make sure they are seated properly in the holders. Each centrifuge can accommodate more than 2 test tubes, and the experiment will go much faster if several people use the centrifuge at the same time. Make sure you can identify your test tubes, both by the labels and by the position in the centrifuge! Let the test tubes spin for 5 minutes. When turning off the centrifuge, wait until the rotor has completely stopped before attempting to remove your test tubes. 

**Note: Be aware of any unusually loud noises or “walking” of the centrifuge, which might indicate that the tubes are off balance. If this occurs turn it off immediately.**

Remove the test tubes, being careful not to touch them with your fingers. They should contain a clear brown-green colored liquid (supernatant) and a gray solid at the bottom (precipitate). Put the test tubes in the test tube rack.

The next step is to draw off the supernatant, leaving the precipitate in the test tube. It is not essential that every last drop of supernatant is removed from the test tube, but it is important that the precipitate be undisturbed, so follow these directions carefully. Obtain two disposable pipettes. Squeeze the air out of the pipette before putting it into the liquid, and then carefully draw up the supernatant.

Split the supernatant from test tube 1 between the test tubes labeled 1a and 1b. Discard the pipette in the sharps disposal box at the front of the room. Use the second pipette for test tube 2 and follow the same procedure.
        Test tubes 1a, 1b, 2a and 2b contain an excess of either Cu(NO3)2 or KI. The supernatants will be tested later so don’t throw them away. Test tubes 1 and 2 contain the precipitates. This precipitate is wet and must be dried before it can be weighed. Any unreacted reagents that may be trapped must also be removed.

Wash the precipitate by adding approximately 3 mL methanol to each test tube. It is important that the same amount be added to both test tubes. Gently shake the tubes so that the precipitate starts to cloud the solution. Remember not to touch the tubes with your fingers. Also, be careful not to spill any methanol on the outside of the test tubes because it will dissolve the marker.

Spin the test tubes in the centrifuge for 5 minutes. The supernatant should be clear once again.
Without disturbing the precipitate, remove the supernatant with a disposable pipette. Since this supernatant will not be tested, you can use the same pipette for both test tubes. Discard the liquid in the proper container.

To dry the precipitate, place test tubes 1 and 2 in a 100 mL beaker. Write your name on a label or a piece of paper and place it on the inside of the beaker. Put the beaker and test tubes into the drying oven for at least 20 minutes. If there is more time left in the period, leave them in longer, keeping in mind that the test tubes must be cooled before weighing.

**Note: Methanol is used to wash the precipitate rather than water because methanol is much more volatile. This means that it evaporates very quickly, making the precipitates dry much faster. Methanol is also flammable, so no flames in the lab today!**

Part II: Testing the Supernatants

While the test tubes are in the oven, test the supernatants. Add 1 mL of the Cu(NO3)2  solution to test tubes 1a and 2a. Record any observations.

Now add 1 mL of the KI solution to test tubes 1b and 2b and record observations.

Part III: Weighing the Precipitates

After the test tubes have been in the oven at least 20 minutes, remove them and place the test tubes in a desiccator to cool for at least 10 minutes. Remember not to touch the test tubes with your fingers (the beaker will be hot anyway).

Weigh each test tube to the nearest 0.0001g.

Calculate the mass of precipitate for each test tube. Give your instructor the results to post for the whole class. Make sure you have a copy of everyone’s results before leaving the lab.   

Be sure that precipitates, supernates, and methanol wash are all disposed of in the proper containers.