Required Practicals Across the Three Sciences

OCR Gateway Combined Science A assesses your practical work entirely within the six written papers — there is no separate practical exam. Across the course you carry out a set of required practical activities, often called PAGs (Practical Activity Groups), spanning biology, chemistry and physics. Questions about them can appear on any of the six papers, and they can ask you to describe a method, identify variables, spot sources of error, interpret results, evaluate an investigation, suggest improvements, or do a calculation from practical data. Because the skills are common to all three sciences, learning them once pays off everywhere.

By the end of this lesson you should be fluent in the language of working scientifically, know the standard required practicals in each science and their most common exam question, and be able to identify apparatus, variables and errors confidently.

Required-practical questions mostly target AO1 (recalling a method and apparatus) and AO2 (applying it to identify variables or errors), rising to AO3 when you evaluate an investigation or suggest improvements.

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 reach for your working-scientifically vocabulary.

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The Language of Working Scientifically

Almost every practical question, in any science, rewards the same handful of ideas. 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 do not 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).
• Accuracy vs precision vs validity: accurate = close to the true value; precise = readings close together; valid = actually measures what it claims to, with variables controlled.

Term	Definition	Watch out for
Accuracy	How close a measurement is to the true value	A precise result can still be inaccurate (e.g. a balance reading 2 g too high every time)
Precision	How close repeated measurements are to each other	Precision alone does not prove correctness
Repeatability	Same person, same method, similar results	Improved by repeats and controlling variables
Reproducibility	Different people/equipment, similar results	Stronger evidence than repeatability alone

Exam Tip: "Suggest one improvement to this method" is one of the most frequent practical questions in every science. 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".

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Biology Required Practicals

The activities below are the standard set you meet in the biology topics. Always check your specification and your school's practical record for the exact activities you completed.

Practical	Aim	Key variables	Common exam question
Microscopy	Observe and draw cells	IV = magnification; DV = detail seen	Calculate actual size or magnification; draw a labelled diagram
Osmosis in plant tissue	Effect of concentration on potato mass	IV = concentration; DV = % change in mass	Explain mass change; calculate % change
Food tests	Identify starch, sugars, protein, lipid	IV = food sample; DV = colour change	Describe the test and positive result for each molecule
Enzymes	Effect of pH or temperature on amylase	IV = pH/temperature; DV = time to digest starch	Explain the rate curve using denaturation
Photosynthesis	Effect of light intensity on pondweed	IV = lamp distance; DV = bubbles per minute	Explain why the rate levels off (a limiting factor)
Sampling (quadrats/transects)	Estimate abundance or distribution	IV = location; DV = number/% cover	Estimate a population; explain random sampling

A worked reminder of two frequent biology moves:

• Food tests: iodine → blue-black for starch; Benedict's heated in a water bath → brick-red for reducing sugar; Biuret → violet/purple for protein; the emulsion test (ethanol then water) → cloudy white for lipid. Always run a distilled-water control.
• Microbial growth (antiseptics/antibiotics): use aseptic technique; incubate at 25 °C in a school lab (not 37 °C, to avoid culturing human pathogens); measure the clear zone of inhibition.

Exam Tip: For any biology enzyme or photosynthesis practical, the mark-winning explanation is almost always denaturation (active site changes shape) or a limiting factor (the rate can only go as fast as its slowest-supplied requirement). Have both phrases ready.

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Chemistry Required Practicals

Practical	Aim	Key variables	Common exam question
Making a soluble salt	Prepare a pure, dry salt	Technique-based (no IV/DV)	Describe the method: excess base → filter → evaporate → crystallise
Titration	Find a neutralisation volume	IV = volume of one solution; DV = end point	Describe the method; read a burette; calculate concentration
Temperature changes	Exothermic vs endothermic reactions	IV = reaction/type; DV = temperature change	Interpret a temperature–time graph
Rates of reaction	Effect of concentration on rate	IV = concentration; DV = time for a change	Calculate rate; explain using collision theory
Chromatography	Separate and identify a mixture	IV = substance; DV = distance moved / Rf	Calculate an Rf value; identify components
Electrolysis	Products at the electrodes	IV = solution; DV = products formed	Predict products; link to reactivity

Two frequent chemistry moves worth rehearsing:

• Rate from the "disappearing cross" method: place a flask over a paper cross, add acid to sodium thiosulfate, and time how long the cross takes to vanish. Rate = 1 ÷ time. Control temperature, volumes and the cross.
• Rf value: $R_f = \dfrac{\text{distance moved by the spot}}{\text{distance moved by the solvent}}$Rf​=distance moved by the solventdistance moved by the spot​ — always a value between 0 and 1 with no units.

Exam Tip: In a titration, name the equipment precisely — burette, pipette, conical flask, white tile — and describe the end point (a permanent colour change of the indicator). "Container" and "when it changes colour a bit" will not score.

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Physics Required Practicals