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Chemistry - Separation and Purification

Grade 9IGCSE

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

🔑Concepts

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A pure substance consists of only one substance and has a sharp, fixed melting point (M.P.M.P.) and boiling point (B.P.B.P.).

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Impurities decrease the melting point (M.P.M.P.) and increase the boiling point (B.P.B.P.) of a substance. They also cause the substance to melt or boil over a range of temperatures.

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Filtration is used to separate an insoluble solid from a liquid. The solid left on the filter paper is the residue, and the liquid passing through is the filtrate.

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Crystallization is used to obtain a pure solid sample from its solution. The solution is heated until the saturation point is reached (tested with a glass rod), then cooled to allow crystals to form.

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Simple Distillation is used to separate a volatile solvent from a non-volatile solute (e.g., obtaining pure water from sea water).

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Fractional Distillation is used to separate miscible liquids with different boiling points (e.g., ethanol at 78∘C78^{\circ}C and water at 100∘C100^{\circ}C). It utilizes a fractionating column packed with glass beads to provide a large surface area for condensation.

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Paper Chromatography separates mixtures of soluble substances, such as dyes or pigments. Separation depends on the different solubilities of the solutes in the solvent.

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Locating agents (like Ninhydrin) are used in chromatography to make colorless substances, such as amino acids, visible on the chromatogram.

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The RfR_f (Retention Factor) value is a constant for a specific substance under standard conditions and is used for identification.

📐Formulae

Rf=Distance moved by the substanceDistance moved by the solvent frontR_f = \frac{\text{Distance moved by the substance}}{\text{Distance moved by the solvent front}}

Purity (%)=Mass of pure substanceTotal mass of sample×100\text{Purity (\%)} = \frac{\text{Mass of pure substance}}{\text{Total mass of sample}} \times 100

💡Examples

Problem 1:

In a chromatography experiment, a red dye moved a distance of 4.5 cm4.5\text{ cm} from the baseline, while the solvent front moved 9.0 cm9.0\text{ cm}. Calculate the RfR_f value of the red dye.

Solution:

Rf=4.5 cm9.0 cm=0.5R_f = \frac{4.5\text{ cm}}{9.0\text{ cm}} = 0.5

Explanation:

The RfR_f value is calculated by dividing the distance traveled by the solute by the distance traveled by the solvent. It is always a value between 00 and 11 and has no units.

Problem 2:

A student has a mixture of ethanol (boiling point 78∘C78^{\circ}C) and water (boiling point 100∘C100^{\circ}C). Which method should be used to separate them, and which substance will be collected first?

Solution:

Method: Fractional Distillation. First substance collected: Ethanol.

Explanation:

Fractional distillation is used for miscible liquids with close boiling points. The substance with the lower boiling point (ethanol, 78∘C78^{\circ}C) evaporates first, passes through the fractionating column, and condenses into the receiver.

Problem 3:

A sample of water is suspected to be impure. It is observed to boil at 102∘C102^{\circ}C. Is the sample pure? Justify using the provided temperature.

Solution:

No, the sample is not pure. Pure water boils at exactly 100∘C100^{\circ}C at 1 atm1\text{ atm}.

Explanation:

The presence of impurities increases the boiling point of a liquid. Since the observed B.P.B.P. is 102∘C102^{\circ}C, which is higher than the standard 100∘C100^{\circ}C, the water contains dissolved impurities.