How to Measure the Melting Point – a Step by Step Guide
Ever wonder how to determine the melting point of something? No? Well, here is an easy way
to do it. The melting point of a pure substance is defined as the temperature at which the solid and liquid phases are at equilibrium at 1 atmosphere of pressure. At a temperature slightly lower than its melting point, the substance is a solid. At a temperature slightly higher than its melting point the substance is a liquid.
Melting points are used to identify a substance and assess their purity. The melting point of
a pure substance is a unique property of that substance. For example, pure aspirin (acetyl salicylic acid) has
a melting point of 135°C. All pure aspirin will have the same melting point, regardless of how it was made.
1. Place a small amount of the substance you want to check in the bottom of a capillary tube.
Tap the tube to allow the substance to settle.
2. Half fill a beaker with one of the following liquids, depending on the melting point range you anticipate. The
temperatures stated here are the flash points of the liquids, and should not be exceeded: mineral oil (200°C);
glycerol (160°C); polyethylene glycols (various).
3. Attach a thermometer to the capillary tube containing your substance. Place these in the beaker.
4. Place the beaker on a heat source with stirring capabilities (otherwise the liquid bath will not heat evenly).
You can manually stir the liquid with a metal spatula.
5. Gradually increase the heat of your liquid bath. Once you are 15-20°C from the anticipated melting point,
slow down to no more than 5°C per minute. This is because you want the temperature of the thermometer to reflect
the CURRENT bath temperature. If the temperature increases too quickly, the thermometer will be showing you the
temperature a few minutes ago. VERY IMPORTANT!
6. Observe your substance in the capillary tube. Note the temperature when the solid just starts to liquefy. Monitor
until the substance is completely liquefied. This is your melting point range.
7. If the range is lower than the known value, the substance has impurities.
8. Another way to check the identity and/or purity, is to mix your substance with an identical substance you know
to be pure. When these are combined, the melting point should not change. If it changes, you know that the substance
you have is not pure, or it is a different substance all together.