Question

Which of the following techniques is used to obtain pure water from a salt solution?

• A. Sublimation
• B. Evaporation
• C. Distillation
• D. Fractional distillation

Answer 1

The correct option is C

Distillation

The explanation for correct option:
(C) Distillation

• Distillation is the process of separating a liquid from its solution by evaporating it and then condensing its vapor.
• The process is carried out in a specially designed apparatus called the distillation flask.
• A solution is heated in the distillation flask due to which the liquid component gets evaporated, leaving the solid component behind.
• The vapor of the liquid pass through a tube called the condenser.
• The condenser is kept cool so that vapor can cool down.
• Finally, the vapor are collected in a receiving flask. This way, we get pure liquid from its solution.
• The distilled water that is used by doctors, chemists and pharmaceutical industries is obtained by distillation.

The explanation for incorrect options:
(A) Sublimation

Principle: This process is used to separate a mixture in which one constituent sublimes.

• Sublimation is the process in which a solid directly changes to a gas without passing through the liquid state.
• A substance is said to be sublime if it passes from the solid state directly into the gaseous state without changing into a liquid.
• Substances such as ammonium chloride, camphor and naphthalene undergo sublimation.
  

(B) Evaporation

• The process by which a liquid is converted into its vapor state is called evaporation.
• The rate of evaporation increases with an increase in temperature.
• Evaporation is used to separate a solid component from a homogeneous mixture of solids and liquids.
• The process of evaporation involves heating the solution until the solvent evaporate and turns into gas leaving behind the solid residue.
• For example, we can separate salt from a mixture of salt and water using evaporation. We heat the solution to a boiling point to obtain salt from salt water.
• As the solution boils, the water escapes as vapor and after some time all the water evaporates leaving a layer of salt at the bottom of the beaker.
  

(D) Fractional distillation
Principle: This process works on the basis of difference in the boiling points of different miscible liquids.

• Miscible liquids can be separated by the process of thermometer fractional distillation.
• The separation of mixture of liquids by distillation is called fractional distillation.
• This is based on the principle that different liquids have different boiling points.
• A mixture of alcohol and water can be separated by fractional distillation.
• Alcohol has a lower boiling point than water and therefore evaporates first.
• The vapor of alcohol get condensed in the condenser and alcohol is collected in the flask.
• Water having higher boiling point is left behind in the distillation flask. This technique is used when the difference between boiling points of two liquids is 25°C or more.
• Petrol, diesel and kerosene are obtained from crude petroleum by the process of fractional distillation.
  

Hence option C is correct.