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Mixtures and their Separation - Fractional Distillation-advanced

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Fractional distillation is a process used to separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K25\text{ K} (or 25∘C25^{\circ}\text{C}).

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The process uses a fractionating column fitted between the distillation flask and the condenser. This column is usually packed with glass beads.

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The glass beads in the fractionating column provide a large surface area for the hot vapours to cool and condense repeatedly, enhancing the efficiency of separation.

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During the process, the component with the lower boiling point distils over first and is collected in the receiver, while the component with the higher boiling point remains in the flask.

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A major industrial application is the separation of different gases from compressed and cooled liquid air (e.g., Nitrogen, Argon, and Oxygen).

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Fractional distillation is the primary method used in petroleum refineries to separate crude oil into useful fractions like petrol, diesel, kerosene, and paraffin wax.

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Miscibility is a prerequisite; the liquids must be completely soluble in each other to form a single phase before separation.

📐Formulae

T(K)=T(∘C)+273.15T(K) = T(^{\circ}\text{C}) + 273.15

ΔT=∣BP1−BP2∣<25 K\Delta T = |BP_1 - BP_2| < 25\text{ K}

Boiling Point of N2=−196∘C\text{Boiling Point of } N_2 = -196^{\circ}\text{C}

Boiling Point of Ar=−186∘C\text{Boiling Point of } Ar = -186^{\circ}\text{C}

Boiling Point of O2=−183∘C\text{Boiling Point of } O_2 = -183^{\circ}\text{C}

💡Examples

Problem 1:

A mixture contains liquid AA (boiling point 350 K350\text{ K}) and liquid BB (boiling point 365 K365\text{ K}). Suggest a suitable method to separate them and calculate the difference in their boiling points.

Solution:

The difference in boiling points is ΔT=365 K−350 K=15 KΔ T = 365\text{ K} - 350\text{ K} = 15\text{ K}. Since 15 K<25 K15\text{ K} < 25\text{ K}, simple distillation cannot be used effectively. Therefore, the suitable method is Fractional Distillation.

Explanation:

Fractional distillation is employed when the boiling point difference is small (less than 25 K25\text{ K}). The fractionating column allows for multiple evaporation-condensation cycles, ensuring the liquid with the lower boiling point (350 K350\text{ K}) is separated first.

Problem 2:

Arrange the following gases in the increasing order of their boiling points as they are separated from liquid air: Oxygen (−183∘C-183^{\circ}\text{C}), Magnesium (not a gas in air), Nitrogen (−196∘C-196^{\circ}\text{C}), and Argon (−186∘C-186^{\circ}\text{C}). Which gas is collected first when liquid air is warmed?

Solution:

The boiling points in ∘C^{\circ}\text{C} are:

  1. Nitrogen: −196∘C-196^{\circ}\text{C}
  2. Argon: −186∘C-186^{\circ}\text{C}
  3. Oxygen: −183∘C-183^{\circ}\text{C}

In increasing order of temperature: −196∘C<−186∘C<−183∘C-196^{\circ}\text{C} < -186^{\circ}\text{C} < -183^{\circ}\text{C}. Nitrogen has the lowest boiling point.

Explanation:

In fractional distillation of liquid air, the gas with the lowest boiling point distils out first as the temperature rises. Therefore, Nitrogen (−196∘C-196^{\circ}\text{C}) is collected first, followed by Argon, and then Oxygen.