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MELTING POINT AND BOILING POINT: ALL YOU NEED TO KNOW

A solid substance’s melting point can be defined as the temperature reaching which the substance changes its state from solid to liquid at a given atmospheric pressure. It changes with the change in pressure and is generally defined at Standard pressure.

 

MELTING POINT

A solid substance’s melting point can be defined as the temperature reaching which the substance changes its state from solid to liquid at a given atmospheric pressure. It changes with the change in pressure and is generally defined at Standard pressure. After reaching this point both the Liquid and Solid phase attain equilibrium. Another term is Freezing point which is practically the exact reverse of Melting point. It is the temperature at which the substance changes its state from liquid to solid state.

BOILING POINT

A Substance’s boiling point is the temperature reaching which the liquid’s vapor pressure comes at par with the liquid’s atmospheric pressure and the liquid changes its state from liquid state to vapor state. When there is high pressure the liquids boiling point is comparatively higher than what it would be at normal atmospheric pressure. For a given pressure, different kinds of liquid have different boiling points. Generally, with an increase in altitude the boiling points of liquid also changes, that is the major reason behind the increased amount of cooking time in hilly areas. In 1982 IUPAC, coined a term ‘Standard Boiling Point’ which means the temperature reaching which the liquid boil under the given pressure of 1 bar.

Both of these processes are reversible and quasi static and are dependent on polarity.  In spite the similarities there are certain differences between the two which are stated below.

Melting points depend on the convenience of arrangement of molecule into crystal. Neopentane, which is sphere shaped is easy because it is more convenient to arrange the molecules into a crystal but a few other substances like Pentane, which is line shaped and Isopentane, whose shape is non-specific is difficult.

On the other hand, boiling point depends on a completely different parameter, the surface area in which the molecules can interact with each other. Because of this characteristic the lesser branches hydrocarbon will have the higher its boiling point will be.

At melting point, the temperature is such that the solid and liquid attain an equilibrium whereas in boiling point the liquid and gas state attains an equilibrium.

Melting Point is a term specified for solids when a solid substance changes its state from solid to liquid. On the other hand, boiling point is specified for liquid substances which changes its state from liquid to gas.

These are some examples of the melting and boiling point in certain substances which will also help us differentiate the two:

 

Name of the substance  Boiling point

 

Melting point
Gold

 

2933 1336
Hydrogen 20.3 13.8
Mercury 630 234

 

 

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