Purification and Separation Exam Questions

(a) Filtration [1]

(b) Evaporation or crystallisation [1]

(c) Fractional distillation (or simple distillation) [1]

(d) Separating funnel [1]

Examiner note: The choice depends on the physical properties of the components. Filtration separates an insoluble solid from a liquid. Evaporation/crystallisation recovers a dissolved solid. Distillation separates liquids with different boiling points. A separating funnel separates immiscible liquids by density. Choosing the wrong technique is a zero for that part.

• Heat the copper sulfate solution gently in an evaporating basin to evaporate some of the water [1]
• Test for saturation by dipping a glass rod into the solution — if crystals form on the rod when it cools, the solution is saturated [1]
• Remove from heat and allow the solution to cool slowly so that crystals form [1]
• Filter the crystals, wash with a small amount of cold distilled water, and dry between sheets of filter paper or in a warm oven [1]

Examiner note: This is crystallisation, not evaporation to dryness. Copper sulfate is hydrated (CuSO4.5H2O) and decomposes if heated too strongly — the blue crystals turn white as water of crystallisation is lost. The slow cooling is essential to allow large, regular crystals to form.

• Set up a simple distillation apparatus: a round-bottomed flask containing the salt water, connected to a condenser, with a collection beaker at the end [1]
• Heat the salt water in the flask until it boils at 100 degrees C [1]
• Water vapour rises and passes into the condenser, where it is cooled by cold water flowing through the outer jacket, and condenses back to liquid water [1]
• The pure water (distillate) is collected in the beaker; the salt remains in the flask [1]

Examiner note: The condenser is essential — without it, the water vapour escapes. Cold water flows through the condenser in the opposite direction to the vapour (counter-current) for maximum cooling. Monitor the thermometer to ensure you collect only at 100 degrees C, confirming the distillate is pure water.

• Filtration separates an insoluble solid from a liquid by passing the mixture through filter paper [1]
• The liquid passes through the small pores in the filter paper — this is called the filtrate [1]
• The solid particles are too large to pass through and remain on the filter paper — this is called the residue [1]

Examiner note: The filter paper has pores small enough to trap solid particles but large enough to allow liquid molecules and dissolved substances through. Filtration does NOT remove dissolved substances — if salt is dissolved in water, it passes through with the filtrate. Only insoluble solids are retained.

Simple distillation:

• Used to separate a liquid from a dissolved solid, or two liquids with very different boiling points (more than 25 degrees C apart) [1]
• The mixture is heated, the component with the lower boiling point evaporates first and is condensed and collected [1]

Fractional distillation:

• Used to separate two or more miscible liquids with similar boiling points [1]
• Uses a fractionating column packed with glass beads, which provides a large surface area for repeated evaporation and condensation, giving much better separation [1]

Examiner note: In fractional distillation, the fractionating column creates a temperature gradient. Vapour with a higher boiling point condenses on the beads and drips back down; vapour with a lower boiling point continues to rise and exits at the top. This repeated process gives a much purer separation than simple distillation.

(a) The sample is impure [1] because a pure substance melts at a sharp/fixed temperature, but this sample melts over a range (80-85 degrees C) rather than at exactly 83 degrees C [1]

(b) The student could purify it by recrystallisation: dissolve the sample in a minimum volume of hot solvent, filter while hot to remove insoluble impurities, then cool slowly to crystallise the pure product [1]

Examiner note: Impurities lower the melting point and broaden the melting range. A pure substance melts sharply at a single temperature. This is why melting point determination is used as a standard purity test. The closer the observed melting point is to the literature value and the narrower the range, the purer the sample.

• Draw a pencil line (baseline) near the bottom of a piece of chromatography paper [1]
• Place spots of the mixture and spots of known/reference dyes on the baseline [1]
• Stand the paper in a suitable solvent (e.g., water or ethanol) with the solvent level below the baseline so the spots are not submerged [1]
• Allow the solvent to rise up the paper by capillary action, carrying the dyes different distances depending on their solubility in the solvent and attraction to the paper. Remove and dry. Compare the spots from the mixture with the reference dye spots to identify each dye [1]

Examiner note: Use pencil for the baseline, not pen — ink would dissolve and run with the solvent. The solvent level must be below the baseline spots. Different dyes travel different distances because they have different relative solubilities in the mobile phase (solvent) versus the stationary phase (paper).

(a) Water forms the lower layer [1] because water is denser than oil [1]

(b) Open the tap/stopcock at the bottom of the separating funnel and allow the water to drain into a beaker. Close the tap as soon as all the water has drained through, before the oil reaches the tap [1]

Examiner note: The denser liquid always sinks to the bottom. The upper layer (oil) is then poured out from the top of the separating funnel. This technique only works for immiscible liquids — if the liquids are miscible (e.g., ethanol and water), use distillation instead.