The Identification of an Unknown Substance from its Melting Point

MATERIALS: Capillary tubes, melting-point apparatus, known solids, and unknown solid

BE SURE TO WEAR PROTECTIVE GOGGLES.

1.         The melting point is a physical property which is often of great use in characterizing an unknown pure substance.  In this assignment, you will measure the melting point of a solid and, using the concept of melting point lowering (a colligative property of solutions), identify your unknown solid.

2.         EXPERIMENT:

For this experiment your unknown will be one of the following compounds: benzoic acid, 2-naphthol, or trans-stilbene.  Prepare three capillaries, each filled with about 0.25 inch (less than 1 cm) of powdered sample, firmly packed in the capillary tube.  Capillary tube A will contain ONLY your unknown substance (it will be one of the three substances listed above and in the table below).  Capillary tube B will contain a MIXTURE of your unknown substance plus a small amount (about 20 % (or less) of the total sample size) of one of the substances listed above.  REMEMBER TO RECORD THE NAME OF THE SUBSTANCE YOU USED TO PREPARE THE MIXTURE.  Capillary tube C will contain a MIXTURE of your unknown substance and a small amount of one of the other substances listed above.  It is important that you record which substance(s) you have put into each capillary and that you DO NOT MIX UP CAPILLARIES.

To prepare each sample, if it is not already powdered, place the sample on a clean, dry watch glass and use the bottom of a clean, dry test tube to grind it into a powder.  Transfer the sample into the capillary tube by "pecking" at it with the open end of the capillary.  When a small amount is in the open end of the capillary, pack it into the bottom by dropping the capillary through a 2-3-foot piece of glass tubing onto the counter.  Keep doing this until the required amount is present.  You should drop the capillary and sample through the glass tubing several times to make sure that the sample is firmly packed into the bottom of the capillary.  The two mixtures should each be prepared by adding 4 parts of your unknown to one part of the substance you choose (remember to record which compound you are mixing with your unknown) and mix them by grinding them togther, thoroughly using clean watch glass and the bottom of a clean test tube.  
            The melting point apparatus should have warmed (or cooled,if it had prevoiusly been used by another student) to about 110 °C – the digital readout will indicate the temperature of the heating block.  Do not push the SET button.  Put your capillary tubes into each of the three sample wells (openings), when the INSERT SAMPLE indicator is lit.  Put your pure unknown in the LEFT sample well, MIXTURE B in the center, and MIXTURE C in the right sample well.  Observe your samples through the magnified window.  If any of your mixtures melts (see below) ask the instructor for advice.  After 30 seconds, press the START button.  The temperature will begin increasing at the rate of 1 °C/min. 

The various stages you may observe as the sample melts:

            1.  the surface of the sample becomes shiny.

            2.  the sample “slumps”.

3.  droplets of liquid appear in the sample (THIS IS THE MELTING POINT)  Record this temperature as the melting point (temperature).  To record or store a temperature – push the BLUE button below the sample.  There are 3 BLUE buttons.  The leftmost BLUE button is for the sample in the left sample well.  The center BLUE button for the center sample, etc.  Do not push any BLUE button more than 4 times or you will lose data by overwriting your first data points.

4.  all of the solid disappears  (THIS IS THE FREEZING POINT)  Record this temperature as the freezing point, by pushing the BLUE button.  You may mark up to 4 different temperatures for each sample by pressing the blue button immediately below each sample.  REMEMBER: Do not push any BLUE button more than 4 times.

Once the temperature reaches 128 °C, the instrument will automatically stop heating and will begin to cool back to 110 °C.  You may manually end the heating cycle, when all of the   samples have melted completely, by pushing the STOP button.  Do not push any other buttons or you may lose data.

After the cooling cycle has begun, you may retrieve your data by pushing the BLUE button for each sample.  The first time you push the BLUE button, the temperature display will indicate the instrument’s estimation of the melting point, which you should record in your data table.  The second time you push the BLUE button, the temperature display will indicate the instrument’s estimation of the freezing point, which you should record in your data table.  The next 4 times you push the BLUE button, the temperature display will show the temperature for each of the 4 times you pushed the BLUE button while the sample was being heated.  Pushing the same button again will just repeat the cycle.  Record only the melting and freezing points you estimated for the sample. 

1

Remove your capillaries and place them in the waste capillary container at the stockroom window.  All of your left over samples should go into the same waste jar. 

The difference between the freezing and melting points is the melting range.  (NOTE:  The freezing point is ALWAYS measured while heating; NEVER while cooling.)  If the range for your unknown sample is more than three degrees, consult your instructor.  The range for capillaries B and C may be greater than three degrees.

3.                   Calculations; 

            Calculate the melting range for each trial.  Range = freezing point – melting point

            Use data numbers for your calculations.

4.         DISCUSSION: 

If a substance is absolutely pure and if the heating rate is slow enough, the temperature at which melting is first noted (the "melting point”) and the temperature at which the last it of solid melts (the "freezing point") are identical; the melting range is zero.  When impurities are present, however, there will be a lowering of both temperatures (but more of a change in the melting point) so that the melting range corresponds very roughly to the proportion of impurity present.  In fact, the melting range is a commonly used means to rate the purity of a sample.  Commercially available samples rarely have less than a one-to-two degree melting range. 

            i.  On the basis of the observed melting range, what comment do you have about the purity of the sample?

            ii.  Suppose you had to identify your unknown, not from a list of ten substances, but from a list of tens of thousands of substances as in the Chemical Rubber Company's Handbook of Chemistry and Physics:  Use the online reference at http://hbcpnetbase.com .  Your user name is lbccedu and the password is SLUUDSec .

                     a.  Count how many substances have the same melting point, within experimental error — plus or minus one degree Celsius —, as your unknown.  Note that this is NOT the melting range.  For the purpose of this question, the "melting point" will be the temperature you recorded in your result table for your unknown sample (rounded to the nearest whole number). 

                     b.  Give the systemic (IUPAC) name of three of the substances counted above.  Choose compounds which melt at the same temperature and ± 1° from your rounded temperature (i.e. one compound melting at each of the temperatures).  As part of your answer, include the identifying (CAS) number for the compound, the formula, the actual melting point listed in the table for that compound, and the boiling temperature.  Do not use your unknown as one of the substances to answer this question.