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A lab experiment to determine the melting point of an unknown compound using a mel-temp melting point apparatus and a capillary tube. The melting point is a crucial physical property for substance identification and purity assessment. The theory behind melting point determination, the technique for conducting the experiment, and the procedure for recording data.
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Identify an unknown compound by its melting point. MATERIALS
Pulverize a few DRY crystals of your solid using the rounded end of a test tube. Obtain a glass capillary tube that is open at one end but sealed at the other end. Jam the open end of the capillary into your sample so that some solid enters the tube. Invert the capillary and gently let the tube "bounce" repeatedly on the bench top until most of the solid vibrates down to the closed end. Your sample should be about 1 cm high (the height of your fingernail). If you have trouble filling the tube, your sample either needs more grinding or it is too wet. Make sure the Mel-Temp apparatus has cooled to near room temperature if it has been used recently. Insert your capillary tube into a slot that will hold the tube next to the heating block. Look through the eyepiece to see the solid sample. Make sure the rheostat is set to "zero" and switch "ON" the on-off switch for the heating block. If you know the expected melting point of your sample, set the power level to about "5" until the temperature reaches about 15 °C below the anticipated melting point. Then adjust the power so as to slowly approach your melting point. The rate of temp increase in the vicinity of the melting point must be small, about 2 °C per minute. A graph will be made available that indicates the approximate maximum temp of the apparatus for any given power setting. This may help you find the proper power setting, but be advised that each apparatus varies in its performance and these guidelines may not be accurate. As the temperature is increasing, observe the crystals through the eyepiece, but be careful not to accidentally touch the hot part of the apparatus. Record both the temp at which melting begins and at which the last crystal disappears. Report a range such as 88- 90 °C. Turn off the apparatus, reset the power to "zero", careful remove your pipet, and discard it in the nearby waste container. If you do not know the melting point of a compound (as in THIS experiment), first take a crude melting point by heating rapidly. Then cool the apparatus to about 20 °C below the crude melting point, and then take a more careful measurement on a second sample of compound. PROCEDURE All information should be recorded directly into your lab notebook, but if this is your first lab meeting, you will be allowed to transfer the data to your notebook at a later time. Obtain a sample from the instructor and immediately record the unknown code letter. Prepare the sample as described and take a crude melting point (record this value). Then determine an accurate melting point. Use the Table of Melting Point Data given to determine the identity of your unknown and record your conclusion. If you feel you cannot decide between two different compounds, discuss your choices and explain what you could do to make a final decision between the two choices.