Purification

Add water and heat the mixture. After adding more water, cool the solution slowly.

The video shows the bottom of a graduated test tube filled with approximately 0.75 mL of milky white substance that appears to fog up and/or adhere readily to the sides of the tube. Also in the frame is a small beaker filled with clear liquid labeled “DI water.” A small quantity of clear liquid is added to the test tube from above, bringing the volume of the contents of the test tube up to the 1.5 mL marking. A boiling stick is inserted and used to agitate the contents of the test tube, which induces some bubbling as the white substance in the bottom of the test tube dissolves to yield a mostly clear mixture. Next we see another graduated test tube labeled “W” clamped in a sand bath with a boiling stick. Also in the sand bath is a metal temperature probe. The test tube with the clear mixture from the previous scene appears and is inserted into another clamp, with its bottom end immersed in the sand. Motion is visible in this test tube, but it is unclear whether this is due to bubbling or heat. Now the screen goes black except for a caption that reads “5 minutes later.” When the video returns, we see that the solution in the graduated test tube is boiling rapidly, producing a thin layer of white foam near the 1.0 mL mark of the test tube. A pipette is inserted into the “W” test tube and used to transfer clear liquid into the other test tube until its total volume is just above 3.0 mL. Bubbles rise to the top of this now 3 mL volume, but eventually settle. Next we see this test tube clamped in midair outside of the sand bath. A thick layer of white foam or solids has formed at the top of the test tube. In the next frame, we see that this layer has extended down towards the bottom of the tube considerably, with flakey white crystals suspended in the clear solution below the opaque layer. As time passes, the top layer extends farther down the tube and the clear solution in the bottom layer becomes more and more clouded with suspended solids. Crystals also appear to be growing around the boiling stick. Next we see the test tube immersed in a plastic tub filled with ice. In the final frame, it has been removed from the ice again, although its appearance is roughly unchanged.

You will now remove the solvent using the square-tip pipette method. Where should you place the pipette?

At the bottom of the test tube.

In the top layer of solution only.

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