What is Melting Point?

Melting is the process by which a substance changes from a solid to a liquid due to the application of heat.

The temperature at which a solid substance melts into a liquid at a certain pressure is called the melting point of that substance.

Dependence of Melting Point:

The melting point of a solid depends on two factors—

The pressure exerted on the substance, and
The presence of impurities in the substance.

Effect of Pressure on Melting point:

The melting point of a substance depends on pressure.

For substances whose volume increases during melting, increasing pressure increases the melting point. For example – copper, naphthalene, gold, silver, wax etc.
For substances whose volume decreases during melting, increasing pressure decreases the melting point. For example, ice, cast iron, bismuth, antimony, brass etc.

Experiment

If two pieces of ice are pressed together for a while and released, a piece of ice turns into one piece of ice.

The pressure causes the melting point of the ice to drop below 0°C at the junction of the two pieces. But the temperature of ice is 0°C. Therefore, the temperature of the junction is higher than the melting point, so the ice at that point melts to form water. The temperature of the ice and meltwater around the junction drops below 0°C as the heat required for this melting is collected from the ice itself. The melting point rises again to 0°C when the pressure is removed.

As a result, as the water temperature at the junction is below 0°C, the water refreezes into ice and the ice pieces stick together.

Determination of Melting point and Freezing point of solids

A quantity of powdered naphthalene was taken in a test tube. The mouth of the test tube was closed with a perforated cork and a thermometer was inserted through the hole. The bulb of the thermometer should be immersed in powdered naphthalene. Now the test tube with the thermometer is placed in a beaker full of water and heated and the water in the beaker is continuously stirred with the help of a stirrer. Temperature readings were taken at half-minute intervals.

It can be seen that as the temperature increases a certain temperature naphthalene starts to melt and that temperature remains constant until all the naphthalene melts. That specific heat is the melting point of naphthalene. After all the naphthalene had melted, the burner was removed when the temperature rose another 10°C to 15°C. Allow the test tube to cool and read the thermometer again every half minute.

It was found that liquid naphthalene started to condense at a certain temperature as the temperature decreased and remained at that temperature until the entire liquid condenses. That specific heat is the freezing point of naphthalene.