Objectives: 1. To learn the theory and procedure of melting points.

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1 Melting Points Objectives: 1. To learn the theory and procedure of melting points. Discussion: Crystalline solids exist in an ordered network of atoms, ions, or molecules, depending on the element or compound. These particles are held together by attractive forces which must be overcome before the crystal loses its structure and the substance melts. This is done by heating the substance. At first, the temperature of the solid increases and no change in structure occurs. Then the temperature remains constant as the particles vibrate with enough energy to break away. The entire crystal structure does not collapse at one time, and the melting point is the temperature at which the crystalline structure (solid) and the free particles (liquid) coexist and are in a state of equilibrium. After all the solid is melted, the temperature of the liquid will again rise. In theory, this practice could be reversed: you could measure the freezing point of the liquid by cooling it, and the melting and freezing points should give the same result. However, in practice, supercooling often occurs*, making this process more difficult. If a sample of a pure crystalline compound is placed in a test tube with a thermometer and melted and then frozen the results might be as follows: The melting point (which is the same temperature as the freezing point) may be read directly from the flat part of the curve. Because the chemist often has only small amounts of compound with which to work, using the test tube/thermometer method is impractical. Chemists place a small amount of crystalline solid inside a small capillary tube and then leave it in a heating device while the temperature is increasing. They watch the solid carefully until it melts, and record the temperature. The problem with this method is that the amount of solid is so small *when this happens, the temp. of the liquid will drop below its normal freezing point before a solid appears (crystals have difficulty forming due to lack of nucleation sites). 1/5

2 that it sometimes melts very quickly and the melting point is missed. Also, the chemist assumes that the temperature of the device is the same as the temperature inside the capillary tube. Since glass and the air between the crystals are both poor heat conductors, this may be a poor assumption. By increasing the temperature very slowly, and recording a melting point range, these problems can be minimized, and a reasonable degree of accuracy can be attained. As you might realize, the melting point of a pure substance depends on the forces between the particles in the crystal lattice: the stronger the forces, the higher the melting point. Ionic substances generally have high melting points (positive and negative ions comprise the lattice), and covalent substances have lower ones (molecules comprise the lattice). Much can be learned about the structure of a substance by considering its melting point, along with other physical and chemical properties. Melting points also help to identify a substance. A pure substance will always have the same melting point, no matter how it was prepared. (Of course, it is possible for two different compounds to have the same melting point). An impure substance will melt differently than a pure substance in two ways: 1) It will melt at a different (usually lower) temperature than the pure sample, and 2) It will (usually) melt over a broad temperature range rather than a range of 1-3 degrees that is expected for a pure substance. General Procedure for Melting Point Determinations: Safety. Assume all substances are irritants and don t breathe the dust. 1. Obtain a melting point capillary. Place a small amount of the sample to be tested on a piece of paper and invert the capillary over it to force a small amount of sample into the open end of the capillary. With the open end up, tap the sides of the capillary to loosen the crystals, and then drop the capillary and crystals down inside a long piece of glass tubing until the crystals fall to the bottom of the capillary. Repeat until you have a 1-3 mm well-packed sample in the capillary. Prepare a second capillary in the same manner. 2. Use the MELT-TEMP apparatus to determine the melting point range of your sample (separate directions for use) as follows. Place one capillary in the apparatus and increase the temperature rather rapidly (10 degrees per minute or so) and note the approximate melting point. Immediately let the apparatus cool. When you are sure that the apparatus is has cooled below the melting point of your unknown (it should solidify, but sometimes doesn t until it disturbed), remove the first capillary and insert the second one. Now increase the temperature slowly (2-3 degrees per minute) in the approximate range, and carefully record the melting point range from the first appearance of liquid in the tube to the sample s complete liquefaction. Temperature control is vital here. 3. Your unknown is one of the compounds listed below. Compare melting points (and appearances where possible) with the compounds on the list to determine a preliminary identification of your unknown. 2/5

3 4. On a watch glass, make a 50:50 mixture (estimate amounts) of your unknown and a sample of the compound on the list you believe is identical to your unknown. Be sure to crush any large crystals and mix the two compounds thoroughly. (Otherwise, the small sample used for a melting point may not be a true mixture.) 5. Take the melting point of the mixture. Use the result to confirm or revise your preliminary identification. If you were wrong, repeat the mixing and melting point determination steps with another known until you have identified your unknown. Possible Melting Point Unknowns Melting Point, C Compound Resorcinol Acetanilide dl-mandelic Acid Benzoic Acid Urea trans-cinnamic Acid Benzoin Anthranilic Acid Cholesterol Adipic Acid Citric Acid Salicylic Acid 3/5

4 Name Section # Unknown Melting Points 1. Melting point range of unknown (fast) 2. Melting point range of unknown (slow) 3. Preliminary identification of unknown Evidence for this identification: 4. Melting point range of a 50:50 mixture of unknown and sample of compound in preliminary identification Evaluation of preliminary identification: 5. (If preliminary identification was incorrect) 4/5

5 If needed: 6. (If identification on step 5 was incorrect) 7. (If identification on step 6 was incorrect) 5/5