Topic 12: Experimental techniques and chemical analysis

Good chemistry depends on careful measurement and on knowing how to separate and identify substances. This topic covers the apparatus used to measure time, temperature, mass and volume, the methods used to purify mixtures, and the standard tests that reveal which ions and gases are present in a sample.

Measuring time, temperature, mass and volume

Choosing the right apparatus is the first step in any experiment. Time is measured with a stopwatch or clock in seconds. Temperature is measured with a thermometer in degrees Celsius. Mass is measured on an electronic balance in grams, and the balance should be tared (zeroed) before weighing. Volume of a liquid is measured with a measuring cylinder for rough amounts, but with a pipette or burette when greater precision is needed, such as in a titration. A pipette delivers a fixed accurate volume, while a burette delivers a variable volume that can be read to the nearest 0.05 cm cubed. Volumes of gas can be collected and measured in a gas syringe. Always read the bottom of the meniscus at eye level to avoid parallax error.

Criteria of purity: melting and boiling points

A pure substance has a sharp, fixed melting point and a sharp, fixed boiling point. Impurities change this behaviour in a predictable way: an impurity lowers the melting point and broadens it over a range of temperatures rather than a single value, and an impurity raises the boiling point. So a substance that melts sharply at exactly its expected temperature is pure, while one that melts over a range is impure. This is why melting point is a useful check of purity for solids. Purity matters in real contexts: medicines and foods must be pure because impurities can be harmful or alter the dose. Note that a 'pure' substance in chemistry means a single substance with nothing else mixed in, which is different from everyday uses of the word.

Filtration and crystallisation

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel; the solid stays behind as the residue and the liquid passing through is the filtrate. Crystallisation separates a soluble solid from its solution by removing the solvent so that crystals form. The solution is heated to evaporate some of the solvent until it becomes saturated (the point of crystallisation can be tested by dipping in a cold glass rod). It is then left to cool slowly, so pure crystals grow while soluble impurities stay in the remaining solution. The crystals are filtered off and dried. Crystallisation is preferred over simply evaporating to dryness when the solid would decompose on strong heating.

Distillation: simple and fractional

Simple distillation separates a liquid (the solvent) from a dissolved solid, for example obtaining pure water from salt water. The solution is heated, the water boils off as vapour, passes into a condenser where cooling water turns it back to liquid, and the pure distillate is collected. Fractional distillation separates two or more liquids that are miscible and have different boiling points, such as ethanol and water, or the components of crude oil. A fractionating column packed with glass beads is fitted above the flask. As the mixed vapour rises, the liquid with the lower boiling point reaches the top first and is collected, while the higher boiling point liquid condenses and runs back down. A thermometer at the top of the column shows when each fraction is distilling over.

Paper chromatography and Rf values

Chromatography separates and identifies the substances in a mixture, such as the dyes in an ink or coloured food. A spot of the mixture is placed on a baseline drawn in pencil near the bottom of chromatography paper. The paper is stood in a small depth of solvent, keeping the baseline above the solvent level. As the solvent rises up the paper, the different substances travel at different speeds and separate into spots. Each substance has a characteristic Rf value: Rf = distance moved by the substance divided by the distance moved by the solvent front. Rf values are always between 0 and 1 and have no units. Worked example: a spot moves 4.5 cm while the solvent front moves 9.0 cm, so Rf = 4.5 / 9.0 = 0.5. The same substance gives the same Rf in the same solvent, so values can be compared with known references.

Locating agents for colourless substances

Some substances, such as amino acids or sugars, are colourless and produce invisible spots on a chromatogram. A locating agent is a chemical that reacts with these colourless spots to make them visible so their positions can be seen and their Rf values measured. For example, ninhydrin is sprayed onto the paper and turns amino acid spots purple. Without a locating agent, the separated substances could not be located, so the chromatogram could not be interpreted. The Rf calculation is then carried out exactly as for coloured spots.

Identifying cations and the effect of alkalis

Metal cations can be identified in two main ways. A flame test identifies some metals by the colour given to a flame: lithium gives red, sodium gives yellow, potassium gives lilac, calcium gives orange-red, copper gives blue-green, and barium gives light green. For tests in solution, sodium hydroxide solution and aqueous ammonia are added. Adding a few drops of sodium hydroxide forms a coloured precipitate (an insoluble hydroxide): copper(II) gives blue, iron(II) gives green, and iron(III) gives red-brown. Aluminium, calcium and zinc all give white precipitates, but they can be told apart because the aluminium and zinc precipitates dissolve in excess sodium hydroxide while the calcium precipitate does not. Aqueous ammonia gives similar coloured precipitates, and the zinc precipitate dissolves in excess ammonia while the aluminium one does not. Ammonium ions are detected by warming with sodium hydroxide, which releases ammonia gas that turns damp red litmus blue.

Tests for anions and for gases

Common anions have characteristic tests. Carbonate: add dilute acid and bubble the gas through limewater, which turns milky if carbon dioxide is released. Chloride, bromide and iodide: add dilute nitric acid then silver nitrate solution, giving a white (chloride), cream (bromide) or yellow (iodide) precipitate. Sulfate: add dilute nitric acid then barium nitrate (or barium chloride) solution, giving a white precipitate. Nitrate: add sodium hydroxide and aluminium foil and warm; ammonia gas is produced, turning damp red litmus blue. Tests for gases: hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint; carbon dioxide turns limewater milky; ammonia turns damp red litmus blue; chlorine bleaches damp litmus paper, turning it white.