Determination of Melting Points

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DETERMINATION OF MELTING POINTS
LAB PP 1
From Gannon University SIM and Auburn University SIM

INTRODUCTION
The melting point of a pure compound is an intensive property, like density and boiling point. Intensive properties are independent of the amount of substance present. The melting point of a compound is the temperature at which it changes from a solid to a liquid. Experimentally, melting point is actually recorded as the range of temperatures in which the first crystal starts to melt until the temperature at which the last crystal just disappears. Reasons for determining melting points: 1. The melting point indicates the level of purity of a sample. An impure compound melts over a wider range of temperatures, usually greater than 2 degrees. The melting point helps to identify unknown samples, narrowing the number of possibilities, because a pure solid melts reproducibly over a narrow range of temperatures. The melting point helps to characterize new compounds.

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In this lab, the identity of an unknown organic compound will be determined by comparing its experimental melting point to those of a variety of known compounds.

PURPOSE
The purpose of this experiment is to determine the melting points of various organic compounds and to use these to identify unknowns.

EQUIPMENT/MATERIALS
Mel-Temp apparatus Thermometer Capillary tubes Mortar and pestle (optional) Dropping tube Organic compounds, known Organic compound, unknown Hairdryer with cool setting Weighing boat

Westminster College SIM

PP1-1

Determination of Melting Points

SAFETY
• • • • Always wear safety goggles in the lab. The parts on the top of the Mel-Temp are HOT while it is turned on. Do not touch these parts or place your eye on the eyepiece. You will get burned! Capillary tubes break very easily, handle them with caution. Wash hands after performing experiment.

PROCEDURE
1. Obtain a capillary melting point tube and a known compound. The known compound may need to be ground into a fine powder with a mortar and pestle. Place a small amount of the finely ground known compound in a weighing boat. Push the open end of the capillary tube into the compound to load sample into tube. Load only 1-2 mm of sample into the tube. Larger samples will heat unevenly. Hold the closed end of the capillary tube over a dropping tube; the dropping tube should be held perpendicular to the table and on top of the table. Drop the capillary tube into the dropping tube; the capillary tube will bounce on the table packing the powder into the bottom. Remove the capillary tube from the dropping tube. Place the capillary melting point tube in the Mel-Temp apparatus chamber, with the closed end pointed down. Turn power switch ON. Set the power level to obtain the desired heating rate. Start with a setting of 40, and adjust if needed to control the rate of temperature increase. The sample should be observed continuously, so that the melting point of the sample is not missed. Heat slowly to acquire the most accurate results. Record the melting range, which begins when the sample first starts to melt and ends when the sample is completely melted. The known sample should melt within the range found in Table 1, on the following page. Turn off the Melt-Temp to allow it to cool to about 50ºC before trying your unknown. Use a hairdryer set on the cool setting to lower the temperature of the Mel-Temp faster.

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Westminster College SIM

PP1-2

Determination of Melting Points

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Prepare a sample of your unknown in the same way that you prepared the known sample and find its melting range.

Known Compound palmitic acid stearic acid Vanillin oxalic acid(dihydrate) benzoic acid acetylsalicylic acid

Melting point(ºC) 63-64 69-70 81-83 101-102 122 138-140

Table 1 Melting Points of Known Compounds

10.

Identify your unknown compound by comparing the experimental melting range of the unknown compound to...
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