Use of apparatus and techniques — OCR A-Level Chemistry

This statement requires you to choose and use apparatus that records five kinds of measurement: mass, time, volume of liquids, volume of gases and temperature. For each, match the instrument to the size and precision of the reading. A balance records mass, with the choice of balance depending on the precision needed. A stopwatch or timer records time, started and stopped at defined events. A burette, pipette or measuring cylinder records the volume of a liquid, chosen by the precision required. A gas syringe records the volume of a gas collected. A thermometer records temperature, read at eye level with the scale fully immersed where appropriate. In a multiple-choice question, identify the quantity being measured, then select the apparatus whose range and resolution suit it, rejecting options that measure a different quantity or give insufficient precision.

This practical technique covers indirect heating of a reaction mixture using a water bath, an electric heater or a sand bath, rather than a naked Bunsen flame. A water bath suits gentle, controlled warming of a flask or test tube to a chosen temperature, for example heating an alcohol with a carboxylic acid and acid catalyst under reflux so the mixture warms evenly without local overheating. An electric heater such as a heating mantle fits a round-bottomed flask and gives steady, flame-free heating. A sand bath spreads heat around a vessel for higher, more even temperatures. The choice depends on the required temperature, the flammability of the substance and the need for even, controllable heating. Assessment asks you to select the appropriate method for a described situation and to justify the choice by reference to control, safety and evenness of heating.

This technique covers three ways of measuring pH. A pH chart or universal indicator paper is compared against a colour scale to give an approximate pH, typically to the nearest whole number. A pH meter gives a numerical reading from an electrode dipped into the solution and is more precise than indicator paper. A pH probe connected to a data logger records pH continuously and can log changes over time, for example during a titration or when monitoring a reaction. The choice depends on the precision needed and whether a single reading or a continuous record is required. Assessment asks you to select the appropriate method for a described purpose and to recognise the trade-off between convenience, precision and continuous monitoring.

This row is a course-level requirement, not a single exam question. It asks you to become competent in choosing and handling laboratory apparatus across a range of experimental techniques, with the specific techniques listed in the sub-clauses that follow (such as titration with burette and pipette). Treat it as a checklist for your practical work: for each technique, learn which apparatus is appropriate, how it is set up, how it is read and how it is cleaned and stored. When you carry out a practical, name the apparatus, justify the choice (for example a burette for delivering variable, measured volumes), record readings to the correct precision and describe the technique accurately. Assessment comes through your practical work and through questions that ask you to describe or explain apparatus and technique, so practise writing precise descriptions rather than vague ones.

A titration uses a pipette to deliver one accurately known volume of the solution being analysed into a conical flask, and a burette to add the other solution in measured, variable volumes until the end-point is reached. You rinse the pipette with the solution it will deliver and the burette with the solution it will contain, then fill the burette, remove air from the jet and record the initial reading at eye level against the bottom of the meniscus. You add the titrant with swirling, slowing near the end-point as the indicator colour changes, and record the final reading. The titre is the difference between final and initial readings. Repeating to obtain concordant titres improves reliability. Questions test your understanding of apparatus choice, rinsing, reading the meniscus and calculating from the titre.

Distillation separates a volatile liquid from a mixture by boiling it and condensing the vapour, so the thermometer bulb sits level with the side-arm and the condenser carries water in at the lower inlet and out at the upper outlet. Heating under reflux boils a reaction mixture while the condenser returns condensed vapour to the flask, allowing prolonged heating without loss of solvent; the flask is clamped to a retort stand above the heat source and the open top of the condenser is left unsealed. In both techniques the round-bottomed flask is held by a clamp on a retort stand, never by hand, and joints are greased where appropriate. Distillation collects a distillate; reflux keeps the mixture together.

Qualitative tests identify ions or functional groups from an observable change such as a precipitate, colour or gas. For cations, add dilute sodium hydroxide solution and record the precipitate colour and its behaviour in excess: Cu²⁺ gives a blue precipitate, Fe²⁺ a green precipitate, Fe³⁺ a brown precipitate, and Al³⁺ a white precipitate that dissolves in excess. For sulfate ions, acidify with dilute hydrochloric acid, then add barium chloride solution; a white precipitate of barium sulfate confirms sulfate. For halides, acidify with dilute nitric acid and add silver nitrate: chloride gives a white precipitate, bromide cream and iodide yellow. For organic groups, bromine water decolourises with an alkene, and warming with Tollens' reagent gives a silver mirror with an aldehyde. Record the reagent, the observation and the conclusion for each test.

Filtration separates an insoluble solid from a liquid or solution. In gravity filtration the mixture is poured through a filter funnel holding filter paper; the liquid that passes through is the filtrate and the solid left behind is the residue. Fluting the paper (folding it into a concertina and opening it into a cone) increases the exposed surface and lets liquid pass faster than a plain cone, and the paper sits snugly against the funnel. For filtration under reduced pressure, a Buchner funnel fitted with a flat filter paper sits on a Buchner flask connected to a suction pump; the reduced pressure pulls liquid through quickly and the solid is collected and dried on the paper. Choose the method by the quantity of solid and the speed needed.

A standard solution is one of accurately known concentration, made in a volumetric flask of fixed capacity, such as 250 cm³. Weigh the solid accurately, dissolve it in a small volume of solvent in a beaker, and transfer the solution to the flask through a funnel, rinsing the beaker and glassware into the flask so no solute is lost. Add solvent until the level is near the graduation mark, then add it dropwise with a dropping pipette until the bottom of the meniscus sits on the mark, with the eye level with the mark. Stopper and invert the flask repeatedly to mix thoroughly. The flask is calibrated to contain its stated volume at a specified temperature, so read it at room temperature.

Acid–base indicators are weak acids whose conjugate forms have different colours, so each changes colour over a characteristic pH range. Choosing an indicator means matching that range to the sharp vertical section of the titration curve at the equivalence point. Strong acid with strong alkali gives a very steep section around pH 7, so phenolphthalein (range roughly pH 8.2–10.0) or methyl orange (roughly pH 3.1–4.4) both work. Weak acid with strong alkali has an equivalence pH above 7 (for example about 8.7 for ethanoic acid and sodium hydroxide), so phenolphthalein suits it but methyl orange does not. Strong acid with weak alkali has an equivalence pH below 7, so methyl orange suits it but phenolphthalein does not. Weak acid with weak alkali has no sharp jump, so no simple indicator gives a reliable end-point and a pH meter is preferred.

This practical-skills statement introduces purification techniques that the course develops through laboratory work. The published statement is truncated after 'purification of:', so treat the listed techniques as those your centre teaches and do not invent a fixed list. Typically you will meet recrystallisation to purify an impure solid, using a minimum of hot solvent, cooling to crystallise, then filtering under reduced pressure and washing with cold solvent. You will also meet distillation, including fractional distillation, to purify liquids, and drying agents followed by decanting or filtration. Learn to select apparatus, set it up safely and explain why each step improves purity. Assessment is through practical work and written questions on technique and purpose, not a memorised mark scheme.

Recrystallisation purifies an impure solid by dissolving it in the minimum volume of hot solvent, then cooling so the purified solid crystallises while most impurities stay dissolved or are removed by filtration. The learner selects the correct solvent (one in which the product is much more soluble hot than cold), dissolves the solid in the minimum hot solvent, allows slow cooling to form crystals, then filters under reduced pressure using a Büchner funnel and washes the crystals with a little cold solvent. Slow cooling gives larger, purer crystals; rapid cooling traps impurities. The solid is then dried between filter papers or in an oven. Assessment asks the learner to choose the correct step, apparatus or reason at each stage.

A liquid product is often isolated from a reaction mixture by separating two immiscible layers in a separating funnel. The learner pours the mixture into the funnel, allows the layers to settle, then runs off the lower layer through the tap and pours the upper layer out of the top. The organic layer is identified by its density or by a test, and the aqueous layer is often discarded. The organic layer is then dried with a drying agent such as anhydrous sodium sulfate or calcium chloride, filtered to remove the drying agent, and the pure liquid obtained by distillation. Assessment asks the learner to choose the correct order of operations, the correct layer to keep, or the purpose of each step.

Melting point apparatus gives a sharp, reproducible melting range that helps identify a solid and judge its purity. A pure solid melts sharply, usually within about 1–2 °C, at a value close to its literature melting point; an impure solid melts over a wider range and often below the literature value. In the practical, a small dry sample is packed into a capillary tube to a depth of a few millimetres, the tube is placed in the heated block and the temperature is raised slowly near the expected value. The learner records the temperature at which melting begins and ends, reports the range and compares it with a data book value. Slow heating near the melting point reduces error; a fast ramp gives a falsely wide or high reading.

Chromatography separates components by their different affinities for a stationary phase and a mobile phase. In paper chromatography the stationary phase is water held on the paper; in thin layer chromatography (TLC) it is a thin layer of silica or alumina on a plate. The mobile phase is a solvent that travels up the stationary phase by capillary action. A small spot of mixture is applied near the baseline, the solvent is allowed to rise, and each component travels a distance depending on its balance between the two phases. The retention factor Rf = distance travelled by component ÷ distance travelled by solvent front, and it is compared with known values to identify components.

This practical technique requires you to build a working electrochemical cell from two half-cells and measure its voltage with a voltmeter. Prepare each half-cell by dipping a clean metal electrode into a solution of its own ions, for example a copper electrode in copper(II) sulfate solution and a zinc electrode in zinc sulfate solution. Connect the two electrodes to the voltmeter terminals, then join the two solutions with a salt bridge, such as filter paper soaked in potassium nitrate solution, so the circuit is complete. Read the voltage once it settles and record it with the correct unit, the volt. Keep electrodes clean and avoid mixing solutions, because contamination changes the measured voltage. The technique is assessed by whether you assemble the cell correctly, complete the circuit and obtain a stable, sensible reading.

This practical technique covers handling solids and liquids safely and carefully, with specific attention to corrosive, irritant, flammable and toxic substances. For each hazard class, the learner selects controls that match the risk: corrosive substances are handled with eye protection, gloves and minimal contact, and spills are diluted and washed away; irritants are kept off skin and out of eyes; flammable liquids are kept away from naked flames and hot surfaces, with lids on containers and good ventilation; toxic substances are used in a fume cupboard or well-ventilated space with gloves and no mouth pipetting. Solids are transferred with a clean spatula or scoop, liquids poured slowly down a rod or via a funnel. The learner reads the hazard label and safety data before starting, uses the smallest sensible quantity, and disposes of waste as directed.

This technique requires measuring the rate of a reaction by at least two different methods, so the course teaches more than one experimental approach and expects you to choose and justify one. A continuous-monitoring method follows a changing quantity over time, for example the volume of gas collected in a gas syringe or the loss in mass on a balance, and the rate at any moment is the gradient of the tangent to the curve. A clock method times a visible change to a fixed endpoint, such as a precipitate hiding a cross, and the initial rate is taken as proportional to 1/t, the reciprocal of the time to that endpoint. The two methods give independent evidence of the same rate. Read the guidance to see which apparatus and which measurement each method uses, then practise recording readings, plotting the graph and calculating the gradient or the reciprocal time.

An initial-rate method measures how fast a reaction is at the very start, before concentrations have changed appreciably. A clock reaction is a common example: you mix the reactants and time how long it takes for a visible change, such as a colour appearing or a cross disappearing. Because the same visible end-point is used each time, the time taken is inversely related to the initial rate, so a shorter time means a faster reaction. By varying the concentration of one reactant while keeping others constant, you can see how initial rate depends on that concentration. The method assumes the measured time reflects the early stage of the reaction, so the change should be sharp and easy to judge.

A continuous monitoring method records a changing quantity repeatedly or continuously while a process runs, rather than taking one reading at the end. In chemistry this includes measuring gas volume collected over time with a gas syringe or inverted burette, following pH with a pH meter or data logger, tracking temperature with a thermometer or probe, and recording colour intensity with a colorimeter. The method suits rate experiments because it produces a series of readings from which a rate can be found, for example plotting gas volume against time and taking the gradient. Choosing it means matching the property that changes to a sensor or measuring device that can log it without interrupting the reaction.