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States of Matter and Separation - Separation Techniques (Filtration, Crystallization, Distillation, Chromatography)

Grade 9IB

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

🔑Concepts

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Filtration: A technique used to separate an insoluble solid from a liquid. The solid remaining on the filter paper is called the residue, and the liquid that passes through is the filtrate.

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Crystallization: This process is used to obtain a pure solid sample from a solution. The solution is heated to evaporate some solvent, reaching a saturated state, and then cooled to allow crystals (e.g., CuSO4⋅5H2OCuSO_{4} \cdot 5H_{2}O) to form.

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Simple Distillation: Used to separate a liquid from a solution or a mixture of liquids with significantly different boiling points (difference >25∘C> 25^{\circ}C). It involves evaporation followed by condensation.

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Fractional Distillation: Used for separating mixtures of liquids with closer boiling points (e.g., ethanol and water, or crude oil components). A fractionating column filled with glass beads provides a large surface area for repeated evaporation and condensation.

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Paper Chromatography: A technique to separate mixtures of soluble substances, such as dyes or pigments. It depends on the relative solubility of the solutes in the mobile phase (solvent) and their affinity for the stationary phase (paper).

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Retention Factor (RfR_f): A ratio used to identify substances in chromatography. Since the solvent front always travels further than or equal to the solute, the RfR_f value is always ≤1\le 1.

📐Formulae

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

Boiling Point of H2O=100∘C (at 1 atm)\text{Boiling Point of } H_{2}O = 100^{\circ}C \text{ (at } 1 \text{ atm)}

ρ=mV\rho = \frac{m}{V}

💡Examples

Problem 1:

In a chromatography experiment, a sample of red dye moved a distance of 3.2 cm3.2\text{ cm} from the baseline. The solvent front moved a total distance of 8.0 cm8.0\text{ cm} from the baseline. Calculate the RfR_f value for the red dye.

Solution:

Rf=3.2 cm8.0 cm=0.4R_f = \frac{3.2\text{ cm}}{8.0\text{ cm}} = 0.4

Explanation:

The RfR_f value is calculated by dividing the distance moved by the solute by the distance moved by the solvent front. It is a dimensionless quantity.

Problem 2:

A mixture contains ethanol (boiling point 78∘C78^{\circ}C) and water (boiling point 100∘C100^{\circ}C). Which separation technique is most appropriate to recover both pure ethanol and pure water?

Solution:

Fractional Distillation.

Explanation:

Because the boiling points are relatively close (22∘C22^{\circ}C difference), simple distillation would not be efficient. Fractional distillation uses a fractionating column to ensure the ethanol vapors are separated from the water vapors effectively.

Problem 3:

A student is given a mixture of sand and salt. Describe the sequence of steps to separate these two substances.

Solution:

  1. Dissolution: Add water to dissolve the salt (NaClNaCl).
  2. Filtration: Filter the mixture; sand remains as residue, salt solution is the filtrate.
  3. Crystallization: Heat the filtrate to evaporate water and leave behind pure salt crystals.

Explanation:

This utilizes the difference in solubility. Sand is insoluble in water, whereas salt is highly soluble.