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chemist. If you want to use the melting point Experiment #2 Melting Points to identify a solid compound that you have isolated in the lab, you will need to compare PURPOSE its To gain skill in lab procedure and measuring techniques and to learn about the properties of organic compounds. melting point with that of the true compound. Melting points are listed in various sources of scientific data, as referenced on the Chem Info page or in the physical data OBJECTIVES tables on this organic chem site from the 1. Determine several melting points (mp's). University of Colorado: 2. To see how mixing different compounds changes their melting points. (http://orgchem.colorado.edu/cheminfo/chem 3. To determine the effectiveness of using melting points in determining compound purity Table of Physical Constants and Hazards of Chem Info info.html) Some Organic and Inorganic Compounds (http://orgchem.colorado.edu/hndbksupport/ MATERIALS physdata.html) • Mel-Temp, chemicals Time:~2 hour How to set up the Mel-Temp Apparatus. BACKGROUND http://orgchem.colorado.edu/hndbksupport/ meltingpt/mtset.html. Melting points of pure compounds are Melting Points easily also recorded in handbooks, such as the measured physical property of solid-state Handbook of Chemistry and Physics (CRC) or organic the Merck Index. Melting points compounds are useful an in product characterization and purity determination. Measurements of the melting point of Pure, crystalline solids have a characteristic a solid will provide you with information about melting point, which is expressed as the the purity of the substance. Pure, crystalline temperature range over which the solid solids melt over a very narrow range (melting range) of temperatures, whereas mixtures melts to become a liquid. The transition between the solid and the liquid is so sharp melt over a broad temperature range. for small samples of a pure substance that Mixtures also tend to melt at temperatures melting points can be measured to ±0.1o C. below the melting points of the pure solids. Most solid substances prepared in the Typically it is no more than ±1o C. organic laboratory are initially impure. These Since melting points are relatively impurities easy and inexpensive to determine, they substance. are handy identification tools to the organic affect In a the melting sample that point of a contains a mixture of two compounds, each component 1 usually lowers the melting point of the other, giving an observed melting point Note: range that is lower and broader than the a) the temperature at which the powder first melting point of either component. We Mel-Temp b) the temperature at which the sample is apparatus for measuring the melting point completely liquid. (This is the melting of range, which we call a melting point. our will use compounds. the collapses or shows some liquid, and The powdered compound is pushed into a small glass tube Always report a melting range) with a closed end called a capillary tube Remember: and ♦ packed as demontrated by the instructor. the sample should be heated through the melting range at a rate of 1 o C or less per minute. Procedure ♦ Pay close attention and tell your partner when the melting begins and ends. I. Determine the melting point of two pure compounds and a mixture. Melting points are best determined using a finely ♦ Turn off the apparatus and let it cool. ♦ Capillaries can not be reused. divided powder. Grind the sample using a Always put used capillaries in mortar and pestle to ensure homogeneity. the glass waste container! Fill and pack to a height of about 2-3 mm one capillary tube each with: II. Determine the melting point of a solid unknown. Your unknown is one of the 1. benzophenone, following 2. napthalene compounds: cinnamic acid, acetanilide or benzoic acid. Samples of all of 3. a 1:1 mixture of benzophenone AND these compounds are available in the lab. napthalene. From Put the 1-2 tubes into the Mel-Temp the unknown, found melting decide which point of of the your listed apparatus closed end down. Make sure that compounds it might possibly be. Identify your you unknown based on the best match. can see the samples through the magnifying glass. Set the voltage to zero List any important observations you and turn on the Mel-temp. Turn the voltage make while performing the experiment. For to 45 and observe both the samples and example, temperature reading as you heat. compound when it melts and any other visible (Never set the voltage to more than 70) changes occurring 2 describe the appearance of a CH 102 Experiment #2 Melting Points Name_________________________ Questions 1. How fast do you heat the sample in the Mel-temp when determining a melting point? 2. If you heat too fast, will your observed melting point be higher or lower than the true value? Explain. 3. Will the melting point of a mixture of two compounds be higher or lower than the melting point of the pure compounds? 3