Melting Point and Boiling Point

What is the Melting Point?

The melting point is usually defined as the point at which materials changes from a solid to a liquid.

The temperature at which solid changes its state to liquid at atmospheric pressure is called the melting point of that liquid. This is the point at which both liquid and solid phase exists at equilibrium. The melting point of the substance also varies with pressure and is specified at standard pressure.

The term ‘freezing point’ is used to denote the temperature at which a liquid is converted into a solid and can, therefore, be viewed as the opposite of the term ‘melting point’. However, substances can be cooled below their freezing points without the formation of a solid. Such liquids are known as supercooled liquids.

Melting Point and Boiling Point

Melting Point and Boiling Point

What is Boiling Point?

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure of the liquid’s environment. At this temperature, the liquid is converted into a vapour.

The boiling point of the liquid depends upon the pressure of the surrounding. When the liquid is at high pressure, it has a higher boiling point than the boiling point at normal atmospheric pressure. The boiling point of different liquids is different for a given pressure. In 1982, IUPAC defined the standard boiling point of a liquid as the temperature at which the liquid boils under a pressure of 1 bar.

The boiling point of a substance is dependent on the pressure of its surroundings. In mountainous terrains (where the altitude is high), the pressure of the atmosphere is relatively lower than the atmospheric pressure at sea level. This is the reason why food cooks at a slower pace in mountainous areas (the lower atmospheric pressure causes water to boil at temperatures below 100oC).

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Melting and Boiling Points of Some Elements

A tabular column listing the melting and boiling points of some important elements is provided below.

   Name of the substance          Boiling point(K)                   Melting point(K)
             Aluminium                2740                   932
              Copper                1460                   1359
              Gold                2933                    1336
            Hydrogen                 20.3                    13.8
            Mercury                 630                    234


1. What is the melting point?
Ans: The temperature at which a solid becomes a liquid due to enough heat. For a given substance, its solid form’s melting point is the same as its liquid form’s freezing point and depends on factors such as the substance’s purity and surrounding pressure.

2. What increases the melting point?
Ans: A compound’s melting point is determined by the force of attraction between molecules and symmetry. Higher melting points result from stronger intermolecular interactions. Ionic compounds usually have high melting points because the ion-ion interaction of the electrostatic forces is much stronger.

3. Are melting and freezing points the same?
Ans: To sum up, as matter transforms from solid to liquid (melting) or liquid to solid (freezing), its temperature is set at the same temperature as the melting/freezing point.

4. What determines the boiling point?
Ans: A molecule’s boiling point is based on its structure. Check for the difference in functional molecule groups when determining the boiling point of molecules of similar size. Ethers have a lower boiling point than alkanes because they have dispersion powers in London and interactions between the dipole and the dipole.

5. Why is the boiling point important?
Ans: Organic compounds ‘ boiling point can provide important information regarding their physical properties and structural characteristics. The boiling point helps to identify a compound and to characterize it. Higher-Pressure water has a higher boiling point than when the atmospheric pressure is lower.

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