Methods of separation and purification

(a) Using a suitable solvent

• A solvent is chosen that dissolves one substance in a mixture but not the other
• Example: A mixture of salt and sand → water dissolves salt but not sand → filtration separates the dissolved salt solution from sand
• Explanation: Different solubilities in the solvent allow selective separation

(b) Filtration

• Used to separate an insoluble solid from a liquid
• The mixture is poured through filter paper in a funnel
• The liquid (filtrate) passes through, while the insoluble solid (residue) remains on the paper
• Example: Separating sand from water

(c) Crystallisation

• Used to obtain a pure solid from a solution
• The solution is heated to evaporate part of the solvent, making it more concentrated
• Then the hot solution is cooled so crystals form
• Crystals are collected by filtration and dried
• Example: Producing pure copper(II) sulfate crystals from a solution

(d) Simple distillation

• Used to separate a liquid from a solution
• The solution is heated until the liquid boils
• The vapor rises, passes into a condenser, and condenses back into pure liquid, which is collected
• The dissolved solid remains in the flask
• Example: Obtaining pure water from salt water

(e) Fractional distillation

• Used to separate a mixture of liquids with different boiling points
• The mixture is heated, and the liquid with the lowest boiling point evaporates first
• The vapors pass through a fractionating column, which helps separate components
• The vapors condense in the condenser and are collected
• By gradually raising the temperature, different fractions are collected in order of increasing boiling points
• Example: Separating ethanol from water, or separating crude oil into fractions like petrol and diesel

Suggest suitable separation and purification techniques, given information about the substances involved

To choose the correct separation technique, we must consider the physical properties of the substances involved (such as solubility, boiling point, or state). Below are common scenarios with the suitable method:

• Insoluble solid + liquid: Use filtration

Example: Sand and water

• Soluble solid + liquid: Use crystallisation (to obtain the solid) or simple distillation (to obtain the solvent)

Example: Salt solution → crystallisation gives pure salt; distillation gives pure water

• Two miscible liquids with different boiling points: Use fractional distillation

Example: Ethanol and water

• Two immiscible liquids (do not mix, form separate layers): Use a separating funnel

Example: Oil and water

• Soluble coloured substances: Use paper chromatography

Example: Dyes in inks

• Gas collection: Use gas syringe or an upturned measuring cylinder depending on solubility in water

Example: Oxygen (insoluble) collected over water; Carbon dioxide (more soluble) better with gas syringe