Filtration, crystallisation and sublimation

Chemistry · Experimental techniques in chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

These are three ways of getting a pure solid. Filtration separates an insoluble solid from a liquid. The solid left on the filter is the residue and the liquid that passes through is the filtrate.

Crystallisation purifies a solid that dissolves well in a hot solvent but only a little in the cold solvent. The steps are: make a hot saturated solution, filter it hot, cool it slowly, collect the crystals, then dry them. Impurities stay behind in the mother liquor.

Sublimation is the change of a solid directly into vapour on heating, without melting. A substance that sublimes can be separated from one that does not.

Rules to remember

Filter paper: folded so it sits well in the funnel; a fluted paper filters faster. The stem of the funnel should stay full of liquid and touch the side of the beaker
Gooch crucible: porcelain with a perforated bottom, covered with filter paper or an asbestos mat, used with suction. With asbestos it can filter liquids that attack paper, such as concentrated HCl and KMnO₄ solution
Sintered glass crucible: has a porous glass disc sealed into the bottom, so it needs no preparation
A good crystallisation solvent dissolves a lot of the substance when hot and very little when cold, does not react with it, dissolves the impurities well or not at all, and is cheap, safe and easy to remove
Coloured impurities are removed by boiling the solution with animal charcoal and filtering hot
Crystals are dried between filter papers, in an oven, or in a vacuum desiccator over anhydrous CaCl₂, silica gel or P₂O₅
Solids that sublime: iodine, ammonium chloride, naphthalene, benzoic acid

Common mistake

Students think the hot filtration removes the soluble impurities. It removes only the insoluble ones; soluble impurities are left in the mother liquor when the crystals form.

Worked example

Impure potassium nitrate contains some sand and a trace of a coloured impurity. Potassium nitrate is much more soluble in hot water than in cold water. How is a pure, dry sample obtained?

Dissolve the solid in the smallest amount of hot water. The potassium nitrate dissolves; the sand does not.
Boil with a little animal charcoal, which takes up the coloured impurity, and filter while hot. Sand and charcoal stay on the filter.
Cool the filtrate slowly. Potassium nitrate is much less soluble in cold water, so crystals form.
Filter off the crystals, wash with a little cold water and dry them in a desiccator.

Answer: Crystallisation from water, with animal charcoal and a hot filtration. Sublimation is no use here because potassium nitrate does not sublime.

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Chapter 2: Experimental Techniques in Chemistry

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