Experimental techniques and chemical analysis

Choosing apparatus to measure time, temperature, mass and volume

Every measurement in chemistry is only as good as the apparatus that produced it. You are expected to name the right piece of apparatus for measuring time, temperature, mass and volume, and to know how precisely each one can be read. Choosing a measuring cylinder when the question needs a burette loses marks as surely as a wrong number does.

Time is measured with a stop-watch or stop-clock, normally to the nearest 0.1 s. Temperature is measured with a thermometer, usually read to the nearest 0.5 °C or 1 °C, and for finer work you would use a digital thermometer or a temperature probe. Mass is measured with a balance: a two-decimal balance reads to 0.01 g and a three-decimal balance to 0.001 g, and you should use the same balance before and after a reaction because a balance measures mass, not weight.

Volume offers the widest choice, and the right answer depends on how accurate the volume must be. A measuring cylinder is quick and gives a rough volume, to the nearest 0.5 or 1 cm3. A volumetric pipette delivers one fixed, accurate volume such as 25.0 cm3. A burette delivers a variable volume and can be run in drop by drop, read to the nearest 0.1 cm3. A gas syringe measures the volume of a gas, usually to the nearest 1 cm3.

Every method has advantages and disadvantages, and a question may ask you to weigh them up. A burette is accurate and can be added drop by drop, but it must be rinsed and read carefully and it is slower than a measuring cylinder. A water bath holds a steady temperature but cannot heat much above 100 °C. Collecting a gas in a gas syringe gives an accurate, dry volume but the syringe is fragile, whereas collecting gas over water is cheap but only works for gases that are not very soluble.

• Stop-watch or stop-clock: measures time, read to 0.1 s.
• Thermometer: measures temperature, usually read to 0.5 °C or 1 °C.
• Balance: measures mass, to 0.01 g or 0.001 g depending on the balance.
• Burette: measures a variable volume of liquid to the nearest 0.1 cm3 and can be added drop by drop.
• Volumetric pipette: measures one fixed volume accurately, for example 25.0 cm3.
• Measuring cylinder: measures an approximate volume, to the nearest 0.5 or 1 cm3.
• Gas syringe: measures the volume of a gas, usually to the nearest 1 cm3.

Solvent, solute, solution, saturated solution, residue and filtrate

Before you can separate a mixture you have to describe it with the right words, because the mark scheme is written in them. A solvent is a substance that dissolves a solute. A solute is a substance that is dissolved in a solvent. A solution is a mixture of one or more solutes dissolved in a solvent. In salt solution, water is the solvent and sodium chloride is the solute.

A saturated solution is one that contains the maximum concentration of a solute that can dissolve in the solvent at a specified temperature. The temperature is part of the definition and cannot be left out: solubility usually increases as the solution gets warmer, so a solution that is saturated at 20 °C will dissolve more solute if you warm it. Add more solute to a saturated solution at the same temperature and it simply settles at the bottom instead of dissolving.

When you carry out a separation the two products have names. The residue is the substance that remains after evaporation, distillation, filtration or any similar process, such as the sand left in the filter paper or the solid left in the basin after evaporation. The filtrate is the liquid or solution that has passed through a filter, so in a sand and salt mixture the filtrate is the salt solution.

These words are not decoration. A question that asks you to describe filtration awards marks for 'the sand is the residue' and 'the salt solution is the filtrate', and does not award them for 'the solid stays behind and the liquid goes through', however correct that is.

• Solvent: the substance that does the dissolving, usually water.
• Solute: the substance that is dissolved.
• Solution: the mixture of one or more solutes dissolved in a solvent.
• Saturated solution: holds the maximum concentration of solute at a stated temperature.
• Residue: what is left behind after filtration, evaporation, distillation or a similar process.
• Filtrate: the liquid or solution that has passed through the filter.

Acid-base titrations: burette, pipette and indicator

A titration finds exactly how much of one solution reacts with a known volume of another. It is used to find the concentration of an acid or an alkali, or the volume of acid needed to neutralise a given volume of alkali. The whole method depends on two pieces of accurate glassware working together.

The burette holds the solution that is being added. It has a tap so the liquid can be run in drop by drop near the end, and it is read to the nearest 0.1 cm3 from the bottom of the meniscus with your eye level with the mark, to avoid a parallax error. The volumetric pipette measures a fixed, accurate volume of the other solution, such as 25.0 cm3, into a conical flask. A measuring cylinder is never good enough for either job.

The method runs in order: rinse and fill the burette with the acid, then record the initial reading. Pipette 25.0 cm3 of the alkali into a conical flask and add two or three drops of indicator. Run the acid in while swirling the flask until the indicator changes colour - this is the end-point. Record the final reading and subtract the initial reading from it to find the volume used. Repeat until you have at least two concordant results, within 0.1 cm3 of each other, then take the mean.

Choosing an indicator matters, because you want the colour change to happen as close as possible to the point where the acid and alkali have exactly neutralised each other. Methyl orange is red in acid and yellow in alkali, and phenolphthalein is colourless in acid and pink in alkali; both change sharply, so a single drop tips the colour over. Universal indicator would give a gradual rainbow and no clear end-point, so it is never used.

• Burette: holds the solution being added, delivers it drop by drop and is read to 0.1 cm3.
• Volumetric pipette: delivers a fixed, accurate volume of the other solution.
• Conical flask: holds the measured volume and is swirled during the titration.
• Suitable indicator: methyl orange or phenolphthalein, both of which change colour sharply.
• End-point: the point at which the indicator changes colour.
• Volume used = final burette reading - initial burette reading.

Paper chromatography and the Rf value

Chromatography separates the substances in a mixture by making them move at different speeds through a stationary material. In paper chromatography the paper is the stationary phase and the solvent soaking up the paper is the mobile phase. A substance that is very soluble in the solvent and only weakly held by the paper travels a long way up the paper, while a substance that is less soluble, or more strongly held, travels only a short way.

The method has details the examiner looks for. Draw a pencil baseline near the bottom of the chromatography paper, because pencil is insoluble and will not run. Put a small spot of the mixture on the baseline and let it dry. Stand the paper in a beaker of solvent so that the solvent level is below the baseline, and cover the beaker so the solvent does not evaporate. As the solvent rises it carries the substances with it. When the solvent front is near the top, remove the paper, mark the solvent front immediately in pencil, and dry the paper.

To interpret a chromatogram, remember that each spot is a separate substance. A pure substance gives a single spot, while an impure substance, or a mixture, gives more than one spot. To identify an unknown substance you run it on the same chromatogram as known reference substances and look for a spot at the same height, that is the same Rf, as one of the references. If the substances are colourless you cannot see the spots at all, so you spray the dried chromatogram with a locating agent, which reacts with the substances to produce coloured spots. You do not need to know a named locating agent, only what one is for.

The Rf value turns the position of a spot into a number: Rf = distance travelled by the substance / distance travelled by the solvent. Both distances are measured from the baseline, and because Rf is a ratio of two lengths it has no units and is always less than 1. Rf is a fixed value for a substance in a particular solvent, which is why it can be used to identify it. If a dye moves 3.6 cm and the solvent front moves 9.0 cm, Rf = 3.6 / 9.0 = 0.40.

Separation and purification: filtration, crystallisation and distillation

Filtration separates an insoluble solid from a liquid. Fold the filter paper into a cone, wet it so it sticks to the funnel, and pour the mixture in. The solid is trapped as the residue and the liquid passes through as the filtrate. Use filtration when the solid you want does not dissolve in the liquid, such as sand in water, or when you need to collect a precipitate.

Crystallisation recovers a dissolved solid from its solution. Heat the solution in an evaporating basin until it is concentrated and crystals just begin to form, then leave it to cool and crystallise slowly. Filter off the crystals and dry them. Do not heat to dryness: the solid spits, and any impurity left in the solution is baked into the product, so the crystals are less pure and the yield is poor.

Simple distillation separates a solvent from a solution when you want the liquid rather than the dissolved solid. The solution is heated so the liquid with the lower boiling point evaporates, and the vapour is cooled in a condenser and collected as a pure liquid. It is used to get pure water from seawater or from a salt solution. Fractional distillation separates two or more liquids that are miscible and have different boiling points, using a fractionating column. As the vapour climbs the column it condenses and re-evaporates many times, so the vapour that reaches the top is almost pure. It is used for crude oil, for liquid air and for ethanol and water.

Choosing a method is a skill in itself. An insoluble solid from a liquid, use filtration; a dissolved solid from its solution, use crystallisation; a solvent from a solution, use simple distillation; two or more liquids, use fractional distillation; and if one substance dissolves in a solvent that the other does not, use a suitable solvent to dissolve one and then filter. Melting point and boiling point then tell you how pure the product is. A pure substance has a sharp, exact melting point and boiling point, whereas an impurity lowers the melting point and raises the boiling point and makes both change over a range of temperatures. A melting point of 130 to 134 °C suggests an impure sample, while a sharp 132 °C suggests a pure one.

Identifying cations: flame tests and aqueous sodium hydroxide

Flame tests identify metal cations by the colour they give in a flame. Clean a nichrome or platinum wire by dipping it in concentrated hydrochloric acid and holding it in a hot flame until no colour is seen, then dip the wire in the sample and hold it in the flame. The colours to learn are lithium crimson, sodium yellow, potassium lilac, calcium orange-red, barium green and copper(II) blue-green. The wire is cleaned between samples because a trace of sodium, which is everywhere, would swamp the colour of the next test.

Aqueous sodium hydroxide is the workhorse for identifying cations in solution, because most metal hydroxides are insoluble and form a coloured precipitate. Aluminium, calcium and zinc all give a white precipitate, so you must use the behaviour in excess to tell them apart: the aluminium and zinc precipitates dissolve in excess sodium hydroxide to give a colourless solution, but calcium hydroxide does not. Copper(II) gives a light blue precipitate, iron(II) a green precipitate, iron(III) a red-brown precipitate and chromium(III) a green precipitate.

Aqueous ammonia gives a second set of results that separates the cations that look alike. Aluminium hydroxide is insoluble in excess ammonia, but zinc hydroxide dissolves in excess ammonia to give a colourless solution, so excess ammonia tells aluminium from zinc. Copper(II) hydroxide dissolves in excess ammonia to give a deep blue solution, while the iron(II), iron(III) and chromium(III) precipitates do not dissolve in excess ammonia.

Ammonium ions are the odd one out. Adding sodium hydroxide gives no precipitate at all, but if you warm the mixture, ammonia gas is produced. Test the gas with damp red litmus paper, which turns blue. That colour change, together with the smell, is the test for ammonium ions.

Identifying anions and gases

Carbonate ions are tested by adding dilute acid: the mixture fizzes and the gas produced turns limewater milky. Sulfate ions are tested by first acidifying the sample with dilute nitric acid and then adding aqueous barium nitrate, which gives a white precipitate of barium sulfate. Sulfite ions are tested with acidified aqueous potassium manganate(VII), which is purple; a sulfite decolourises it.

Chloride, bromide and iodide ions are tested in the same way: acidify the sample with dilute nitric acid, then add aqueous silver nitrate. A white precipitate means chloride, a cream precipitate means bromide and a yellow precipitate means iodide. The acidification step is essential, because carbonate ions also give a white precipitate with silver nitrate and would make you think you had found a chloride.

Nitrate ions need a different approach. Add aqueous sodium hydroxide and a piece of aluminium foil and warm the mixture. The aluminium reduces the nitrate to ammonia, and the ammonia gas turns damp red litmus paper blue. Sodium hydroxide on its own would produce no change, so the aluminium foil is the detail that scores.

The gas tests are quick and you must know each one exactly. Ammonia turns damp red litmus paper blue. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper, turning it white. Hydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless.