Question

Gases can be obtained from air by the process of

• A. Fractional distillation
• B. Condensation
• C. Crystallization
• D. Evaporation

Answer 1

The correct option is A

Fractional distillation

The explanation for the correct option:

(a) Fractional distillation:

1. When the two miscible liquids having a boiling point difference of less than $25°\mathrm{C}$ are to be separated then fractional distillation is used.
2. It is the separation of a mixture into its component or fractions.
3. This process is based on differences in the boiling point of the components in the mixture.
4. The fractionating column is used which is a tube packed with glass beads.
5. These beads provide a surface for vapours to cool and condense.
6. By fractional distillation of liquid air, we may produce several gases from the air.
7. Air is compressed to form liquid air called liquefied air.
8. Liquefied air is allowed to warm up slowly in the column, where gases are separated at different heights depending upon their boiling points.
9. Nitrogen (boiling point $-196°\mathrm{C}$) will be collected first as it has the lowest boing point then argon (boiling point $-186°\mathrm{C}$) and finally oxygen (boiling point $-183°\mathrm{C}$) will be collected.

The explanation for incorrect options:

(b) Condensation:

1. Condensation is the transformation of water vapour into liquid.
2. It is the inverse of evaporation, in which liquid water turns into a vapour.
3. Condensation occurs in one of two ways: the air is either chilled to its dew point or gets so saturated with water vapour that it cannot retain any more water.

(c) Crystallization:

1. Crystallization, also known as crystallisation, is the formation of a solid in which the atoms or molecules are strongly arranged into a structure known as a crystal.
2. Crystallization is the process by which the atoms/molecules of a substance organize themselves in a well-defined three-dimensional lattice, reducing the total energy of the system.
3. It is used to separate potash alum from its impure sample.

(d) Evaporation:

1. The process of separating any substance from its solution by removing water is called evaporation.
2. It's easy to see when rain puddles "disappear" on a hot day or wet garments dry in the sun.
3. It is used to separate common salt from seawater.

Hence, option (a) is the correct option.