Separation Techniques for the ESAT

A mixture consists of two or more substances that are not chemically joined together. Because the substances retain their individual chemical properties, they can be separated through physical means. Unlike chemical reactions, physical separation processes do not involve the breaking or forming of chemical bonds and no new substances are created during the process.

Separating Insoluble Solids from Liquids

Filtration is the standard technique used to separate an insoluble solid from a liquid or a solution. An insoluble solid is one that does not dissolve in the liquid, such as sand in water or a solid precipitate formed in a chemical reaction.

1. The mixture is poured into a funnel containing filter paper.
2. The paper acts as a sieve: it has microscopic pores that allow small liquid molecules and dissolved ions to pass through, but trap larger solid particles.
3. The solid remaining on the filter paper is known as the residue.
4. The liquid that passes through into the collection vessel is called the filtrate.

Separating Soluble Solids from Liquids

When a solid is dissolved in a solvent to form a solution (such as salt in water), it cannot be filtered out. Instead, evaporation or crystallisation must be used. These methods rely on the solvent having a significantly lower boiling point than the solute.

Evaporation involves heating the solution in an evaporating basin until the solvent has boiled away entirely, leaving the solid behind. This is suitable if the solid is stable under high heat.

Crystallisation is used to produce high-purity crystals. The solution is heated gently to evaporate some of the solvent until it becomes saturated. A saturated solution is one in which no more solute can dissolve at that specific temperature. When the heat is removed and the solution cools, the solubility of the solute decreases, and it begins to form crystals. These crystals can then be separated by filtration, washed with a small amount of cold solvent, and dried.

Separating Immiscible Liquids

Immiscible liquids are those that do not mix and instead form distinct layers, such as oil and water. They are separated based on their different densities using a separating funnel.

1. The mixture is poured into the funnel and allowed to stand until the layers settle.
2. The more dense liquid forms the bottom layer, while the less dense liquid floats on top.
3. The tap at the bottom is opened to allow the lower layer to flow into a beaker.
4. The tap is closed precisely when the interface between the two liquids reaches the bottom of the funnel.

Separating Miscible Liquids

Miscible liquids mix completely to form a single phase, such as ethanol and water. They are separated using distillation, which relies on differences in boiling points.

Simple Distillation is used to separate a solvent from a solution or two miscible liquids with widely different boiling points (typically a difference of more than $50^{\circ}\mathrm{C}$). The mixture is heated in a flask; the component with the lower boiling point evaporates, passes into a condenser cooled by water, and turns back into a liquid (the distillate) for collection.

Fractional Distillation is required when boiling points are closer together. A fractionating column is used to create a temperature gradient: it is hottest at the bottom and coolest at the top. Vapours rise and condense repeatedly on the surfaces within the column (such as glass beads). The component with the lower boiling point reaches the top first and passes into the condenser, while the component with the higher boiling point condenses earlier in the column and drips back into the flask.

Worked Example: Separating a mixture of sand and salt

To separate a solid mixture of sand and sodium chloride ($\mathrm{NaCl}$):

1. Dissolve: Add water to the mixture and stir. The salt dissolves, but the sand remains insoluble.
2. Filter: Pour the mixture through filter paper. The sand is collected as the residue.
3. Crystallise: Heat the filtrate (the salt solution) in an evaporating dish until it is saturated, then allow it to cool so that $\mathrm{NaCl}$ crystals form.