Apparatus, techniques and safety: the twelve required practicals in outline

Measuring a volume and a mass

Four pieces of glassware measure a volume of solution: a 25 cm³ pipette, a 50 cm³ burette, a 250 cm³ volumetric flask and a measuring cylinder, and each has a different job. The pipette and the volumetric flask each measure one volume very exactly. The burette measures any volume up to 50 cm³, to ±0.05 cm³ a reading. The measuring cylinder is quick and rough, for volumes where a few per cent does not matter. Reading a cylinder carefully does not close the gap: its bore is wide, so a meniscus misjudged by half a millimetre is a far larger volume than the same slip in the narrow neck of a flask or the stem of a pipette.

Describe the drawing

Four pieces of glassware standing on one bench at the same scale. A volumetric pipette with a bulb in the middle, a single line on its upper stem and a pipette filler on top: 25.00 ± 0.06 cm³. A tall burette with a scale from 0.00 at the top to 50.00 just above its tap: ±0.05 cm³ per reading. A volumetric flask, a round bulb with a long narrow neck carrying a single line and its stopper beside it: 250.0 ± 0.3 cm³. A short, much wider measuring cylinder with a graduated scale: ±0.5 cm³.

A pipette is rinsed with distilled water and then with a little of the solution it will hold, so that water left inside cannot dilute it. A pipette filler draws the solution up past the line; nobody pipettes by mouth. Solution is let out until the bottom of the meniscus sits on the line at eye level, and the rest runs into the conical flask. When the flow stops, the tip is touched against the inside of the flask. The last drop stays in the tip, because the pipette is calibrated to deliver its volume with that drop left behind.

A burette is rinsed with the solution it will hold, clamped vertically and filled through a small funnel, which is then taken out so that drips from it cannot add volume mid-titration. A little solution is run through to fill the tip below the tap, because an air bubble there would be counted as titre when it escapes. Readings are taken at eye level from the bottom of the meniscus. Near the end point the solution goes in drop by drop, with the flask swirled after each drop and a white tile beneath it so the colour change is easy to see.

A standard solution is made in a volumetric flask. The weighed solid is dissolved in a beaker in less distilled water than the final volume, and the solution is poured into the flask through a funnel. The beaker, the glass rod and the funnel are rinsed into the flask so that no solute is left behind. Distilled water is added until the meniscus is near the line, then dropwise until its bottom sits on it. The flask is stoppered and inverted several times, because an unmixed solution is not one concentration throughout. The full method is required practical 1.

Standard solution

A solution whose concentration is known accurately, made by dissolving a known mass of solute and making it up to a known volume.

Solids are weighed by difference. A weighing bottle containing the solid is weighed, the solid is tipped into a beaker, and the empty bottle is weighed again. If the bottle with solid reads 18.46 g and afterwards 15.91 g, the mass transferred is 2.55 g, uncertain by ±0.02 g because it comes from two readings of ±0.01 g each. Grains left clinging to the bottle are in the second reading, so they are not counted as if they had reached the beaker.

Weighing by difference

Finding the mass of solid transferred by weighing its container before and after the transfer and subtracting.

Heating, reflux and distillation

How a mixture is heated depends on what is in it. A water bath holds a set temperature up to just below 100 °C, heats evenly and has no flame, so it is the first choice for flammable organic liquids and for any reaction that must stay at a fixed temperature, such as a rate measurement. An electric heating mantle or a sand bath reaches higher temperatures, also without a flame. A Bunsen burner is for heating a solid in a crucible or a test tube, or an aqueous mixture with nothing flammable nearby.

Organic reactions are slow and organic reactants are volatile. Ethanol boils at 78 °C, and heated with acidified potassium dichromate(VI) in an open flask, much of it evaporates before it has time to react. A condenser fitted upright in the neck of the flask stops the loss. Vapour rises into the condenser, is cooled by the water flowing round it, condenses and runs back into the flask. The mixture can then boil for an hour and keep all of its reactants. This is heating under reflux.

Describe the drawing

Heating under reflux. A round-bottomed flask, half full of liquid with anti-bumping granules, sits in a water bath on an electric hotplate, clamped at the neck. A condenser stands upright in the neck with its top open. Cooling water enters the jacket through the lower side tube and leaves through the upper one. Arrows show vapour rising up the inner tube, and drops of condensed liquid run back down into the flask.

Four details of the set-up matter. Water enters the condenser at the bottom and leaves at the top, so the outer jacket fills completely; fed from the top, the water would drain straight through and leave part of the condenser uncooled. The top of the condenser is open, because a sealed apparatus that is heated builds up pressure until it bursts. Anti-bumping granules in the flask give bubbles of vapour somewhere to form, so the liquid boils smoothly instead of in sudden violent surges. And the flask is heated with a water bath or an electric heater, because the vapours are flammable.

Simple distillation separates a liquid from a mixture by boiling it off and condensing it in another vessel. The vapour passes through a still head into a condenser that slopes down and away from the flask, and the liquid collects in a receiver. A thermometer with its bulb level with the side arm of the still head reads the temperature of the vapour that is leaving, which is the boiling point of whatever is distilling over. The fraction kept is the one that comes over within a degree or two of the product's boiling point.

Describe the drawing

Simple distillation. A round-bottomed flask with anti-bumping granules sits in an electric heating mantle. A still head on the flask carries a thermometer whose bulb is level with the side arm. The side arm leads into a condenser sloping down and away from the flask, with cooling water entering at its lower end and leaving at its upper end. An adapter drops the distillate into a receiving flask standing in an ice bath, with a gap at the neck so the apparatus is not sealed.

The two set-ups give different products from the same reagents. Under reflux, ethanol is oxidised first to ethanal, and the ethanal stays in the flask and is oxidised further to ethanoic acid. In a distillation apparatus, with the oxidising agent added slowly to the warm alcohol, the ethanal, which boils at 21 °C, leaves the flask as soon as it forms and is collected before it can be oxidised again. Distilling a product from a reaction is required practical 5.

Heating under reflux

Heating a reaction mixture with a vertical condenser fitted, so that vapours condense and return to the flask and no volatile substance is lost.

Distillation

Separating a liquid from a mixture by boiling it and condensing the vapour in a separate vessel.

Separating and purifying a solid

Filtration separates a solid from a liquid, and which part is kept decides the method. Under gravity, with a fluted filter paper in a funnel, the liquid runs through slowly; the folds give it more paper to pass through, which speeds it up a little. This is the method when the filtrate, the liquid, is wanted. Under reduced pressure, a Büchner funnel with a flat filter paper sits in a side-arm flask connected to a vacuum pump or water pump. The pressure below the paper falls, air pressure pushes the liquid through quickly, and the solid left on the paper is pulled almost dry. This is the method when the solid is wanted.

A solid made in a reaction is rarely pure. Recrystallisation uses the fact that most solids dissolve far better in a hot solvent than in a cold one. The impure solid is dissolved in the minimum volume of hot solvent, so that the solution will be saturated once it cools. An insoluble impurity is removed by filtering the hot solution through a warmed funnel. As the filtrate cools, the product crystallises. Soluble impurities, present in far smaller amounts, do not reach saturation and stay dissolved.

The crystals are collected by filtration under reduced pressure, washed with a little cold solvent to rinse off the solution clinging to them, and left to dry. Too much solvent at the start leaves more of the product dissolved when the solution is cold, and the yield falls. Washing with warm solvent would redissolve some of the crystals.

A pure solid melts sharply, within a degree or so, at the temperature listed for it in a data book. Impurities lower the melting point and spread it over a range of several degrees. Measuring the melting point is the purity test that ends required practical 10.

A liquid product is purified differently. It is shaken with a solution that removes an impurity, sodium hydrogencarbonate for an acid, in a separating funnel, with the stopper held and the tap opened often to release gas. The two layers settle, the lower one is run off through the tap, and the organic layer, upper or lower according to its density, is dried with an anhydrous salt such as magnesium sulfate, which takes up water, until the liquid is clear. A final distillation collects the product over a narrow range around its boiling point, which is the purity test for a liquid.

Filtrate

The liquid that passes through the filter paper during filtration.

Recrystallisation

Purifying a solid by dissolving it in the minimum volume of hot solvent and letting the solution cool, so that the product crystallises and soluble impurities stay in solution.

Working safely

A hazard is something that could cause harm. A risk is the chance that harm will actually happen in the way the substance is being used, together with how serious it would be. Concentrated sulfuric acid is a hazard wherever it is. The risk is high when it is poured from a large bottle at face height and low when 2 cm³ is dispensed from a small bottle standing in a tray. A risk assessment lists the hazards, then the control measures that lower each risk.

A fume cupboard draws air away from the user and out of the building. It is used for anything that gives off a toxic or corrosive gas or vapour: bromine, concentrated hydrochloric and nitric acids, sulfur dioxide, and reactions that release them. Its glass screen is kept as low as the work allows, which also puts a barrier in front of anything that spits.

Concentrated acids burn skin and eyes, and concentrated sulfuric acid releases a great deal of heat as it is diluted. Acid is added slowly to water, never water to acid: a little water dropped onto the concentrated acid boils on contact and spatters acid out of the container. Goggles rather than safety spectacles, and gloves, are worn when dispensing it.

Liquid bromine is dense, red-brown and very volatile. Its vapour is toxic and damages the lungs, and the liquid causes serious burns. It is handled only in a fume cupboard, with gloves, and most school work uses bromine water instead, a dilute solution that gives the same test for an alkene with far less risk.

Sodium reacts vigorously with water, giving hydrogen, which can ignite, and sodium hydroxide, which is corrosive. It is stored under oil to keep air and moisture away, cut on a dry tile, handled with tongs, and used in pieces a few millimetres across behind a safety screen. Leftover scraps are disposed of by adding them to ethanol, which reacts more gently, never by washing them down a sink.

Organic solvents such as ethanol, propanone and cyclohexane are flammable, and their vapours spread along a bench to a flame some distance away. No naked flame is used anywhere near them: they are heated in a water bath or with an electric heater, and kept in small volumes with the stopper on. Many of their vapours are harmful, so larger volumes are handled in a fume cupboard.

These are general precautions. Before any practical, the CLEAPSS Hazcard for each substance and the school's own risk assessment are checked, and where they differ from these general precautions, they are the ones followed.

Hazard

A substance, object or activity with the potential to cause harm.

Risk

The likelihood that harm from a hazard will occur in the way an activity is carried out, together with how severe that harm would be.

The set practicals, the apparatus list and CPAC

Every board sets a list of practicals that the written papers can ask about: the apparatus, the method, the measurements and the errors. AQA's list is twelve required practicals, the first six in the AS course and the other six A level only; OCR's is twelve PAGs and Edexcel's sixteen core practicals. Each sits with the chemistry it measures. Separately, the practical endorsement is reported alongside the grade as a pass or not classified, judged by the teacher against criteria shared by all the English boards.

Behind the practicals sits a list of apparatus and techniques, lettered AT a to AT l. Every student must use each one by the end of the course, and the twelve practicals are chosen so that between them they cover the list. A practical that asks for a reflux set-up is assessing AT d as well as the chemistry.

The endorsement is judged against the Common Practical Assessment Criteria, CPAC: five statements that a teacher must see a student meet, consistently and by the end of the course, in the lab book and at the bench. One good practical does not meet a criterion. The teacher looks for it run after run, and the lab book is the evidence.

Other boards: PAGs, core practicals and research skills

OCR A arranges the same skills as twelve Practical Activity Groups, PAG 1 to PAG 12, and Edexcel as sixteen core practicals. Both are assessed against the same five CPAC criteria and reported as the same pass or not classified. The experiments overlap heavily: titration, calorimetry, rates, test-tube analysis, organic preparation and chromatography appear on every list. Cambridge International has no endorsement and examines practical skill directly, in a laboratory examination, Paper 3, and in a written paper on planning, analysis and evaluation, Paper 5.

OCR's PAG 12 is research skills, and it asks for three things the other boards leave inside CPAC 5. Reading a method critically means checking that it gives every quantity, concentration and condition needed to repeat it, and its hazards. Citing a source means giving enough for a reader to find it: the author, the title, the date, and the publisher or the web address with the date it was read.

Judging a claim means asking who made it, what evidence supports it, whether the data could support it, and whether other workers have checked it. A figure from a supplier's advertisement and a measured value in a peer-reviewed journal do not carry the same weight, and a written PAG 12 task expects that difference to be stated.