Determination of Melting Points
Introduction:
The melting point of a compound is the temperature at which it changes from a solid to a liquid. This is a physical property often used to identify compounds or to check the purity of the compound.
It is difficult, though, to find a melting point. Usually, chemists can only obtain a melting range of a 2 - 3oC accuracy. This is usually sufficient for most uses of the melting point.
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 / capillary tubes dropping tubethermometer / solid organic compounds microspatula
weigh dish or weighing paper
Safety:
- Always wear safety glasses in the lab.
- Avoid skin, eye, and inhalation contact with all chemicals. Handle all chemicals with care.
- Review provided Safety Data Sheets (SDS’) for all chemicals.
- Never insert a room temperature thermometer into a hot Melt-Temp. It may shatter.
- Do not heat the Melt-Temp beyond the upper temperature of the thermometer.
- Do not leave the Mel-Temp on if it is not being used, and do not leave it on when uattended.
- 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 are glass. They break very easily, handle them with caution.
Procedure:
1.Obtain a capillary melting point tube and a known compound.
2.Using the microspatula, place a small amount of the compound on a clean weigh dish or piece of weighing paper. Push the open end of the tube into the compound. Some of the sample will now be in the top of the tube. Use no more than 2 mm of sample in the capillary tube. A larger sample may lead to non-uniform heating and poor results.
3. Hold the closed end of the capillary tube over a dropping tube; the dropping tube should be held perpendicular to the table and a couple of inches above the table surface. Drop the capillary tube into the dropping tube; the capillary tube will bounce on the table packing the powder into the bottom.
4.Place the capillary melting point tube in the Mel-temp apparatus chamber. Start with a setting of “2” to “2 ½”; the temperature should slowly rise. 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. Always allow the Mel-temp apparatus to cool sufficiently before performing another melting point determination.
5.Obtain an unknown sample and determine its melting range. Identify the unknown by comparing the data on the knowns the class has obtained.
6.Time permitting, prepare a mixture of equal proportions of the two known substances and determine the melting point of the mixture.
Data Table:
Known Compounds:
Compound / Melting Range (oC)______- ______oC
______- ______oC
______- ______oC
______- ______oC
______- ______oC
Unknown:
compound / Unknown # ______/ melting range / - oCidentity of compound
(OPTIONAL) Determine the melting point of a mixture.
Components of the mixture are ______and ______.
Melting points of the components are______oC and______oC.
Melting point of the mixture is______oC.
Questions:
1. Define the "melting point" of a substance.
2. What is the purpose of determining melting points?
3. Why is this method not used for finding the melting points of inorganic compounds?
4. Why could the rate of heating influence the melting point?
Determination of Melting Points
Teacher Notes
Lab Time: 35-40 minutes
Preparations:
Time:10 minutes
Make several different organic compounds for the student to sample, but make sure that they are safe to the students and safe together for a mixture's melting point.
Make several mortar and pestles available for mixing compounds.
Hand out several unknowns for the students to test and distribute a list of possible compounds and their melting points for identification.
Answers to Questions:
1. Define the "melting point" of a substance.
The temperature at which a solid changes to a liquid.
2. What is the purpose of determining melting points?
A pure compound will have a definite melting point, therefore, the identity of a
substance can be determined or the purity of a substance can be determined.
3. Why is this method not used for finding the melting points of inorganic compounds?
Inorganic compounds have extremely high melting points.
4. Why could the rate of heating influence the melting point?
If the rate of heating was too high, the substance may melt before the thermometer
has a chance to register the actual temperature. The melting point will appear to
be lower than the actual value.
Compound / Melting pointpalmitic acid / 63 oC
stearic acid / 68 / 69.5 oC
p-nitrobenzaldehyde / 106 / 107 oC
vanillin / 81 / 83 oC
ethyl-p-aminobenzoate / 88 / 90 oC
Considerations:
The opportunity for students to use the Mel - temp apparatus is very valuable. Most college chemistry labs do not even have this convenience. It is good to show students the ease of a Mel - temp so they can appreciate their use later when they are stranded using a Theile tube.
Updated Mar 2017