C12.40654 · Section C12: Experimental techniques and chemical analysis

Separation and purification

Key learnings

Five methods, five property differences
Five methods, one property difference each. Suitable solvent: dissolves the wanted substance selectively, leaving the rest. Filtration: an insoluble solid is caught as the residue, the liquid through is the filtrate. Crystallisation: a dissolved solid recovered by saturating, then cooling. Simple distillation: a liquid is evaporated then condensed back. Fractional distillation: miscible liquids of different boiling points, split by a column.
Crystallisation: saturate, cool, filter, dry
Crystallisation runs in a fixed order. (1) Heat the solution to evaporate solvent until it is saturated -- test by dipping a cold glass rod in and checking crystals form on it. (2) Cool it: solubility falls, so crystals of the pure solid come out of solution. (3) Filter: the crystals are the residue, the solution left (the mother liquor) is the filtrate. (4) Dry them between filter papers, or in a warm -- not hot -- place, so they do not melt or decompose.
Melting point: sharp point vs melting range
Statement 3 asks you to identify a substance and assess its purity. A pure substance has one sharp, fixed melting point matching the data-book value, so comparing a measured value with the book identifies an unknown. A mixture melts over a range, beginning below the pure value, and that range widens as the sample gets less pure. Boiling point data identifies a liquid the same way, but when a question asks about purity, answer on the melting point.

Common mistakes

Drawn from real examiner reports.

Purity: melting point, not boiling point

A vague "you can tell from its melting point" earns nothing, and answering with boiling-point evidence when a melting point was asked for is the specific error examiners reported. Two elements are needed: a pure substance has one sharp, fixed melting point; an impure substance melts over a range. Purity is judged from the melting point, not the boiling point.

Digest C12: pure = a sharp fixed melting point, a mixture melts over a range, not boiling point (Jun 2023 P42 Q2c)

Evaporation is not distillation

Both boil a liquid off, but answer different questions. Evaporation heats a solution in an open basin until the solvent has gone; that vapour escapes and is lost, so only the dissolved solid is recovered. To obtain the pure liquid -- pure water from salt solution, say -- the vapour must be condensed and collected, the extra step that makes it distillation.

Digest C12: evaporation alone does not yield a pure liquid from a solution (Jun 2022 P21 Q14)

Fractional distillation is not by density

The separating property is boiling point, not density -- density plays no part. Heated, the liquid with the lowest boiling point evaporates first and rises up the fractionating column (large surface area, cooler toward the top), while higher-boiling vapours condense and trickle back down. Omitting the column, or crediting density, are both reported errors.

Digest C11 (fractional distillation, applied here to any miscible mixture): separation is by boiling point, not "by density" (Jun 2022 P43 Q2aii; Jun 2023 P43 Q8b)

Filtration is not crystallisation

Filtration separates an insoluble solid from a liquid with filter paper -- the solid caught is the residue, the liquid through the filtrate (often swapped). Crystallisation recovers a soluble solid from solution by evaporating solvent then cooling. Filtration can never remove a dissolved solid from its solvent -- the particles pass through the paper.

Simple vs fractional distillation

Simple distillation takes ONE liquid out of a mixture whose other parts boil far higher. Fractional distillation separates TWO OR MORE miscible liquids from each other when their boiling points are close, using a fractionating column for the repeated condensation and re-evaporation a single pass cannot give. Simple distillation for close boiling points loses the mark.

Evaporating to dryness ruins crystals

In crystallisation, heat only until the solution is saturated, never until it is dry: evaporating to dryness overheats the solid, can decompose it, and leaves a powder rather than clean crystals. The order matters too -- saturate, then cool, then filter. Filtering before cooling removes the solution while the solid is still all dissolved, so no crystals are collected.

Exam tips

Start from what you want to keep
Insoluble solid in a liquid: filtration. Want the dissolved SOLID: crystallisation. Want the LIQUID pure: simple distillation. Miscible liquids to split: fractional distillation. Wanted substance soluble where others are not: a suitable solvent, then filter.
Name the technique AND the property
A suggest-and-explain question wants the technique AND the property it exploits. Naming filtration without saying the solid is insoluble, or crystallisation without saying solubility falls on cooling, only half answers it. One clause each: the method, then why it works here.
Distillation diagram: four fixed marks
The thermometer bulb sits level with the side-arm, not dipped in the liquid. Condenser water enters at the bottom, nearest the flask, and leaves at the top. Anti-bumping granules go in the flask. The collecting vessel is left open -- a sealed one traps vapour and pressure.
A range is a subtraction, with units
A melting or boiling range is upper reading minus lower reading. Quote it with the unit $^\circ\text{C}$ , and never answer with a single temperature when the range was asked for. Any range at all -- even one starting exactly at the literature value -- is evidence of impurity.

Revision notes