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Elements, Compounds and Mixtures - Separation Techniques (Filtration, Distillation, Chromatography)

Grade 7ICSE

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

🔑Concepts

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Filtration: A method used to separate an insoluble solid from a liquid by passing the mixture through a filtering medium (like filter paper). The solid left on the paper is called the residue, and the liquid that passes through is the filtrate.

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Evaporation: Used to recover a soluble solid from a liquid solvent by heating the mixture until the liquid turns into vapor, leaving the solid behind. Example: Obtaining NaClNaCl from a salt-water solution.

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Simple Distillation: A process used to separate a pure liquid from a solution or to separate two miscible liquids with a large difference in boiling points (more than 25∘C25^{\circ}C). It involves two steps: Evaporation followed by Condensation.

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Fractional Distillation: Used to separate two or more miscible liquids with boiling points close to each other. A fractionating column is used to provide better separation. Example: Separating Ethanol (boiling point ≈78∘C\approx 78^{\circ}C) and H2OH_{2}O (boiling point 100∘C100^{\circ}C).

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Paper Chromatography: A technique used to separate different components of a mixture (like dyes in ink) based on their different solubilities in a solvent and their different rates of migration across the chromatography paper.

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Separating Funnel: Used for separating two immiscible liquids (liquids that do not mix, like oil and H2OH_{2}O). The separation is based on the difference in their densities; the heavier liquid forms the lower layer.

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Magnetic Separation: Used when one component of a mixture is magnetic (e.g., Iron filings) and the other is non-magnetic (e.g., Sulfur powder).

📐Formulae

Distillation=Evaporation+Condensation\text{Distillation} = \text{Evaporation} + \text{Condensation}

Rf value=Distance travelled by the componentDistance travelled by the solventR_{f} \text{ value} = \frac{\text{Distance travelled by the component}}{\text{Distance travelled by the solvent}}

Concentration of Solution=Mass of SoluteMass of Solvent×100\text{Concentration of Solution} = \frac{\text{Mass of Solute}}{\text{Mass of Solvent}} \times 100

💡Examples

Problem 1:

A mixture contains sand, common salt (NaClNaCl), and water (H2OH_{2}O). Outline the steps to separate each component.

Solution:

  1. Filtration: Use filter paper to separate the insoluble sand (residue) from the salt solution (filtrate). 2. Evaporation/Distillation: Heat the filtrate. If only salt is needed, use evaporation. If both salt and pure H2OH_{2}O are needed, use distillation.

Explanation:

Sand is insoluble in H2OH_{2}O, so it can be filtered. NaClNaCl is soluble, so the H2OH_{2}O must be turned into vapor to leave the solid salt behind.

Problem 2:

How would you separate a mixture of Oil and Water (H2OH_{2}O)?

Solution:

Use a Separating Funnel. Pour the mixture into the funnel and allow it to stand. Water, being denser, forms the bottom layer. Open the stopcock to collect H2OH_{2}O and close it just as the oil reaches the tap.

Explanation:

Oil and H2OH_{2}O are immiscible liquids. The principle of separation is based on the difference in their densities.

Problem 3:

Explain how black ink can be separated into its constituent colors.

Solution:

Use Paper Chromatography. Place a spot of ink on chromatography paper and dip the edge in a solvent (like water or ethanol).

Explanation:

As the solvent moves up the paper, the different dyes in the ink travel at different speeds depending on their solubility, resulting in separate colored spots.