The Required Practicals (PAGs)
OCR Gateway Chemistry assesses your practical work entirely within the two written papers — there is no separate practical exam. Across the course you carry out a set of practical activities (often called PAGs — Practical Activity Groups), and questions about them can appear on either paper. You can be asked to describe a method, identify variables, explain or interpret results, evaluate the investigation, suggest improvements, or do a calculation from practical data.
By the end of this lesson you should know the aim, key apparatus and method, and the variables for each of the standard practical activities, recognise the most common exam question on each, and be fluent in the titration and concentration calculations that arise from them.
Exam Tip: Practical questions are not always flagged as "practical" — they appear as ordinary structured or 6-mark questions. Recognise them from the context (apparatus, a method, a results table) and apply your working-scientifically vocabulary.
The Language of Working Scientifically
Almost every practical question rewards the same handful of ideas, so learn them as a checklist you can apply to any investigation:
- Independent variable (IV): the one thing you deliberately change.
- Dependent variable (DV): the thing you measure.
- Control variables (CV): everything you keep the same to make it a fair test.
- Repeats and a mean: repeating each measurement and taking a mean reduces the effect of random error and improves repeatability.
- Anomalies: results that don't fit the pattern; identify them and exclude them from the mean.
- Resolution: the smallest change an instrument can measure (e.g. a balance reading to 0.01 g, a burette to 0.05 cm3).
- Accuracy / precision / validity: accurate = close to the true value; precise = readings close together; valid = actually measures what it claims to, with controlled variables.
Exam Tip: "Suggest one improvement to this method" is one of the most frequent practical questions. Strong answers name a specific control variable the method missed, add repeats and a mean, or use equipment of higher resolution — never just "be more careful".
The Required Practicals — Aim, Method, Variables and the Common Question
The list below covers the kinds of practical activities you meet in OCR Gateway Chemistry. Treat them as the standard set; always check your specification and your school's practical record for the exact activities you completed.
Making a soluble salt (C3)
- Aim: prepare pure, dry crystals of a soluble salt (e.g. copper sulfate) from an insoluble base and an acid.
- Method: add excess insoluble base (e.g. copper oxide) to warm acid (e.g. sulfuric acid) and stir until no more reacts (excess ensures all the acid is used); filter off the excess solid; gently evaporate the filtrate to the point of crystallisation; leave to crystallise; dry the crystals (e.g. between filter paper).
- IV/DV/CV: for a yield investigation, IV = acid volume/concentration; DV = mass of salt; CV = temperature, base used.
- Common question: describe each step in order and explain why excess base is used and why it is then filtered off.
Titration (C3)
- Aim: find the volume of acid that exactly neutralises a known volume of alkali (and hence a concentration).
- Method: use a pipette to measure a fixed volume of alkali into a conical flask; add a few drops of indicator (e.g. phenolphthalein or methyl orange); fill a burette with acid; add acid, swirling, until the indicator just changes colour (end-point); record the titre; repeat until concordant results (within 0.10 cm3) and take a mean.
- IV/DV/CV: IV = volume of acid added; DV = colour change at end-point; CV = volume and concentration of alkali, indicator.
- Common question: calculate the concentration of the acid or alkali from the mean titre; explain why repeats and concordant titres are used.
Temperature/energy change of reactions (C3/C5)
- Aim: measure the temperature change in a reaction (e.g. neutralisation, displacement, dissolving) to classify it as exothermic or endothermic.
- Method: measure a fixed volume of one reactant into an insulated cup (polystyrene cup reduces heat loss); record the start temperature; add the second reactant; stir and record the highest (or lowest) temperature; find the temperature change.
- IV/DV/CV: IV = the reactant/variable changed (e.g. mass of metal, concentration); DV = temperature change; CV = volumes, starting temperature, insulation.
- Common question: state whether the reaction is exothermic or endothermic and explain how the temperature change shows this; suggest how to reduce heat loss.
Rates of reaction (C5)
- Aim: investigate how a factor (concentration, temperature, surface area, catalyst) affects the rate of reaction.
- Method: depending on the reaction — gas volume (collect gas in a gas syringe and time the volume produced), mass loss (place on a balance and record mass as gas escapes), or turbidity / "disappearing cross" (time how long a precipitate takes to obscure a cross, e.g. sodium thiosulfate and acid).
- IV/DV/CV: IV = the factor tested; DV = volume of gas / mass lost / time for the cross to disappear; CV = the other factors (volume, concentration, temperature, mass).
- Common question: explain results using collision theory; calculate a rate; sketch how the curve would change with a higher concentration or temperature.
Chromatography and Rf (C2)
- Aim: separate and identify the substances in a mixture (e.g. inks or food dyes).
- Method: draw a pencil start line on chromatography paper; spot the samples; stand the paper in solvent below the line; let the solvent rise; mark the solvent front; calculate Rf for each spot.
- IV/DV/CV: IV = the substance/sample; DV = distance moved / Rf; CV = solvent, paper, temperature, distance of start line.
- Common question: calculate Rf=distance moved by solventdistance moved by spot and explain why the start line is drawn in pencil (pencil does not dissolve and move).
Electrolysis (C3/C4)
- Aim: investigate the products formed at the electrodes when an ionic compound (molten or in solution) is electrolysed.
- Method: place the electrolyte in a cell with inert (carbon/graphite) electrodes; connect to a d.c. supply; observe and test the products at the cathode (negative) and anode (positive).
- IV/DV/CV: IV = the electrolyte (or electrode/concentration); DV = products at each electrode; CV = current/voltage, time, electrode type.
- Common question: predict and identify the products (e.g. hydrogen and oxygen from dilute acid; metal/hydrogen at the cathode; halogen/oxygen at the anode) and test for the gases.
Identifying ions / qualitative analysis (C4)
- Aim: identify the ions present in an unknown compound.
- Method: flame tests for some metal ions (lithium red, sodium yellow, potassium lilac, calcium orange-red, copper green); sodium hydroxide to form coloured metal hydroxide precipitates (copper(II) blue, iron(II) green, iron(III) brown); tests for anions — carbonate (fizzes with acid, CO₂ turns limewater milky), sulfate (white precipitate with barium chloride), halides (cream/white/yellow precipitates with silver nitrate).
- IV/DV/CV: IV = the unknown sample; DV = colour of flame/precipitate or gas produced; CV = volume of reagent, method.
- Common question: identify the ion from a described observation, and write the test and the positive result.
Analysis and purification of water (C6)
- Aim: analyse a water sample and purify it (e.g. by distillation).
- Method: test the pH of water samples; measure dissolved solids by evaporating a known volume to dryness and weighing the residue; distil water to purify it (heat to evaporate, then condense the pure water).
- IV/DV/CV: IV = the water sample/source; DV = pH, mass of dissolved solids; CV = volume tested, method.
- Common question: explain why distilled water is purer (only water evaporates and condenses; dissolved solids are left behind) and describe how to find the mass of dissolved solids.