Melting Point Lab Essay

658 Words May 21st, 2012 3 Pages
Melting Point Determination and Thermometer Calibration Lab

Background: The melting point of a substance, a solid in this case, signifies the temperature at which the first crystal starts to melt until the temperature at which the last crystal disappears is reached. That being said, melting points are expressed in ranges, am organic compound will have a sharp range depending on the purity of the compound. Impurity of an organic substance will register an abnormal melting point based on its expected propensity observed, this will cause the temperature range to widen. Using this hypothesis, mixed melting point technique could determine if the unknown compound added to the mixture causes a temperature depression, if not, the unknown is
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The mixture melting points were harder to be accurate with because there is no definite way to measure exactly the ratios needed of both compounds. Also, not mixing the compounds well enough and not getting an even mixture of the compounds in the capillary tubes may alter the results to be different from the lit values. The graph of the melting point ranges of the mixtures of urea/cinnamic acid (4:1, 2:1, 1:1, 1:2,1:4 ratios) enables us to find the Eutectic Point. The Eutectic Point is the lowest temperature at which a substance will melt. As for the unknowns, the two unknown numbers were #2 and #53. We found unknown #2 to be benzoic acid and unknown #53 to be salicylic acid. The melting points we measured matched these to lit values with an accuracy of +/- 1°C. The difficulty in finding the identity of the unknowns is being extremely careful only to raise the temperature by 1-2°C per minute. This is extremely important for finding all melting points because if this is not done the temperature will increase too rapidly and wrong melting points will result. This lab enables us to experimentally find the melting points of certain compounds and mixtures of compounds. If the temperature is raised too quickly, an innacurate melting will be taken. When compounds are mixed, this fluctuates the melting point and creates a symmetrical phase diagram as the one done in this experiment with urea and trans-cinnamic acid. We learned how to use the thiele tube

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