The melting point of a solid is that constant temperature at which the solid and liquid phases of the substance are in equilibrium with each other.

This is an important physical constant used for establishing the identity and determining the purity of a solid substance. A sharp melting point is usually indicative of high purity of a substance. The presence of impurities lower down the melting point. Thus, an impure solid substance has a lower melting point than that of its pure form.

Experiment: To determine the melting point of a solid substance having low melting point.

How To Perform Experiment

1. Take a glass capillary tube of about 5-6 cm in length and seal one end of it. This is done by heating the tip of tube with the side of the flame of the Bunsen burner. The end of the capillary tube melts and seals itself. It is indicated by the formation of a small glass bead at the tip.

2. Take a small quantity of a solid (about 50 mg) in watch glass, powder it by crushing it with the help of a spatula.

3. Fill the powdered substance in the capillary tube up to about 1 cm height. This is done by pushing the open-end of the capillary tube into the heap of powdered substance kept in a watch glass. Now drop the capillary tube with the sealed end down through a glass-tube (25-30 cm length) kept vertically on a plane surface so that the powder reaches the closed end of the capillary.

4. Take a beaker of 100 mL capacity. Fill it nearly half with paraffin oil. Place this beaker (paraffin bath) over a wire gauze kept on a tripod stand.

5. Take a thermometer and dip it in the paraffin bath so as to wet its lower end. Bring the capillary tube near to the wet end and spread a thin film of paraffin oil on one side of capillary tube. Bring the wet side of the capillary tube in contact with the thermometer. The capillary tube sticks to the thermometer due to the surface tension of paraffin. Now hang it on a clamp such that its bulb dips completely in the liquid paraffin but the open end of capillary is above it.

6. Heat the paraffin bath slowly with a burner and keep stirring the oil so as to maintain a uniform temperature.

7. Continue heating till the substance melts. Note down the temperature (t1) at which the substance begins to melt. Observe carefully whether the substance melts at a fixed temperature or it melts over a range of temperature. A melting point is said to be sharp when the compound melts at a fixed temperature.

8. Remove the burner and allow the temperature to come down. Note the temperature (t2) at which the solid just reappears.

9. The average of the readings is the correct melting point.

Precautions

  1. The substance whose melting point is to be determined should be completely dry.
  2. The thermometer and the capillary should not touch the bottom or the sides of the beaker.
  3. The heating of bath should be very slow with the help of a low flame so that the change in the state of the substance (solid to liquid) is clear.
  4. The bath should be stirred gently to maintain a uniform temperature.

Substance and Melting Point

Compound M.P. (°C)
Nephathalene 80
Citric Acid 100
Oxalic Acid 101
Benzoic Acid 121
Urea 132