O-Level Experimental Chemistry: worked solution

Question

From the separation techniques listed (filtration, fractional distillation, chromatography, crystallisation, magnetic attraction, simple distillation), choose the best method to: (a) recover pure water from salty water (b) detect illegal colourings in a food sample (c) take sand out of a sand-and-water mixture (d) separate ethanol from water (e) obtain copper(II) sulfate crystals from its solution

Choose the method from the physical difference between the substances: is the solid dissolved or not, do you want the liquid or the solid, and do the two liquids mix? (a) recover pure water from salty water - simple distillation. The salt is dissolved, so it would pass straight through filter paper. The water boils at 100 °C and leaves as vapour; the salt is non-volatile and stays behind in the flask. The vapour is cooled in a condenser and the pure water is collected as the distillate. (b) detect illegal colourings in a food sample - chromatography. The colourings are a mixture of dyes present in very small amounts. On the paper each dye travels a different distance, because each has a different solubility in the solvent and a different attraction to the paper. The separated spots can then be compared with spots from known, permitted dyes. (c) take sand out of a sand-and-water mixture - filtration. Sand is an insoluble solid, so its grains are too large to pass through the pores of the filter paper. The sand is left in the paper as the residue and the water runs through as the filtrate. (d) separate ethanol from water - fractional distillation. Both are liquids that mix completely, so filtering is useless, and their boiling points (ethanol 78 °C, water 100 °C) are fairly close. A fractionating column is needed: the vapour repeatedly condenses and re-evaporates on the way up, so the ethanol, which has the lower boiling point, distils over first. (e) obtain copper(II) sulfate crystals from its solution - crystallisation. Here the dissolved solid is the product wanted, not the liquid, so the water must go rather than be collected. Heat the solution to evaporate some of the water until it is saturated (a drop on a cold glass rod forms crystals), then leave it to cool slowly. The salt is less soluble in cold water, so crystals form; filter them off and dry them between filter papers.