Melting Point Theory
Melting Point Determination
The melting point of a substance is the temperature at which crystalline substances change from a solid to a liquid state. During the melting process, all of the energy added to a substance is consumed as heat of fusion, and the temperature remains constant. A pure substance melts at a precisely defined temperature, characteristic of every crystalline substance. With a pure substance, a melting point is the quickest and most accessible method for an organic chemist to confirm the identity of a compound. Additionally, it can also be used as a way to assess the purity of a product by comparing measured melting points to known literature values. At the melting point, the solid and liquid phase exist in equilibrium. Thus the melting point depends on pressure and usually reported at standard pressure.
At UTSC, the equipment used to measure melting point is called the “Melting Point System MP50”. The apparatus consists of four test tube slots in which substances can be tested as well as a screen where individuals can adjust key criteria with respect to their substance.
First of all, you would take a sample of your solid out of your sample bag or from an unknown sample that has been given to you by your TA, and using the capillary tube, obtain a small quantity of the fine powder by gently rolling the tube through the substance.
If your product particles are too large to fit into the capillary tube, place it in a mortar and crush it into a fine powder.
Secondly, to lower the substance to the bottom of the tube, either tap the closed end of the capillary tube vigorously on the bench top, or drop the tube into the long clear plastic tube and allow it to bounce on the bench top. Ensure that there is approximately 3mm of sample within the tube.
Then, press the method icon on the MP50 machine as has been instructed by your TA.
Subsequently, insert the capillary tube into the sample holder of the machine. Press the start button, and wait for the melting point to appear on the screen. You can view your capillary and solid melting on the screen of the machine.
Record the melting point of your substance within your notebook and clear the screen by pressing the home button for the next student. After finding your melting point value, proceed to compare it with documented known melting point values. This will assist you in determining what compound you have. The machine will usually give you a range for the melting point, with a starting value when the solid starts to melt, and an end value for when all the solid has melted. Be sure to record these numbers as a range for reporting your melting point.
Once you have your melting point and determine what compound you have, you can continue to complete your analysis of the lab. Based on the melting point obtained, you can determine if any impurities exist in your sample. If your melting point is much lower and a wider range than the literature value, impurities are present in your sample. These can be due to experimental errors that occurred within your experiment. Ensure to note these observations in the discussion of your lab notebook.