AQA GCSE Combined Science Required Practicals: The Complete Guide
AQA GCSE Combined Science Required Practicals: The Complete Guide
Required practicals are one of the most reliable sources of marks in AQA GCSE Combined Science Trilogy -- and one of the most under-revised. AQA confirms that at least 15% of marks across your papers assess practical skills. With 420 marks across six papers, that is a minimum of 63 marks tied to your understanding of practicals.
There is no practical exam. Every practical mark is earned through written papers. You need to know the methods, variables, expected results, sources of error, and improvements for each practical -- not just vaguely remember doing it in a lesson. This guide covers all 21 required practicals and explains what examiners are looking for.
How Required Practicals Appear in the Exam
AQA tests required practicals in several ways:
- Describe the method -- write out steps in a logical order, naming equipment and explaining how to ensure a fair test.
- Identify variables -- state the independent variable (what you change), dependent variable (what you measure), and control variables (what you keep the same).
- Analyse data -- read tables or graphs, calculate means, identify anomalies, and describe trends using figures from the data.
- Evaluate the method -- identify sources of error and suggest specific improvements.
- Apply to unfamiliar contexts -- use your understanding of a required practical to interpret an experiment you have not seen before.
The last type separates the higher grades. If you understand the principles behind each practical, applying them to a new scenario is straightforward. If you have only memorised the steps, those questions become very difficult.
Biology Required Practicals (6 Practicals)
Combined Science Trilogy includes six of the eight biology required practicals from the full GCSE Biology specification.
| # | Practical | Independent Variable | Dependent Variable | Key Control Variables |
|---|---|---|---|---|
| 1 | Microscopy | N/A (observational) | Cell structures observed | Staining technique, microscope settings |
| 2 | Osmosis in plant tissue | Sucrose concentration | Percentage change in mass | Potato size, solution volume, temperature, time |
| 3 | Food tests | Food sample tested | Colour change observed | Sample volume, reagent volume, heating time |
| 4 | Effect of pH on amylase | pH (buffer solutions) | Time for starch breakdown | Temperature, enzyme and starch concentration |
| 5 | Photosynthesis (light intensity) | Distance of lamp | Oxygen bubbles or gas volume | Temperature, CO2 concentration, pondweed type |
| 6 | Reaction time | Factor tested (e.g. caffeine) | Distance ruler falls | Same person, hand, ruler, starting position |
1. Microscopy
Aim: Observe plant and animal cells, prepare slides using staining techniques, and calculate magnification.
Key exam focus: The formula magnification = image size / actual size. Converting between mm, micrometres, and nanometres. Why staining is used (cell structures are transparent without it). The difference between magnification and resolution.
Common mistakes: Forgetting unit conversions before calculating. Confusing resolution with magnification.
2. Osmosis in Plant Tissue
Aim: Investigate the effect of sucrose concentration on potato cylinder mass.
Key exam focus: Using percentage change in mass, not absolute change. Identifying the isotonic point on a graph (where the line crosses zero). Explaining osmosis as water movement through a partially permeable membrane from dilute to concentrated solution.
Common mistakes: Using absolute change instead of percentage change. Not blotting potato pieces before re-weighing. Omitting "partially permeable membrane" from osmosis descriptions.
3. Food Tests
Aim: Test for sugars, starch, proteins, and lipids in food samples.
Key exam focus: Benedict's reagent turns blue to green/yellow/orange/brick-red for reducing sugars (requires heating in a water bath). Iodine turns brown-orange to blue-black for starch. Biuret reagent turns blue to lilac/purple for protein. The ethanol emulsion test produces a cloudy white layer for lipids.
Common mistakes: Forgetting that only Benedict's test requires heating. Describing the Biuret result as "pink" rather than "lilac/purple."
4. Effect of pH on Amylase
Aim: Investigate how pH affects the rate of starch breakdown by amylase.
Key exam focus: Buffer solutions control pH. Rate is calculated as 1/time. A water bath keeps temperature constant. At extreme pH the enzyme is denatured -- the active site changes shape permanently and the substrate no longer fits.
Common mistakes: Saying the enzyme is "killed" (correct term: denatured). Forgetting to mention buffer solutions in the method.
5. Effect of Light Intensity on Photosynthesis
Aim: Investigate how light intensity affects the rate of photosynthesis using pondweed.
Key exam focus: The inverse square law -- light intensity is proportional to 1/d squared, so halving the distance quadruples intensity. Sodium hydrogen carbonate provides constant CO2 so it does not become a limiting factor. At high intensity the rate plateaus as another factor becomes limiting.
Common mistakes: Stating that halving distance halves intensity (it quadruples it). Forgetting sodium hydrogen carbonate in the method.
6. Reaction Time
Aim: Measure reaction time using the ruler drop test.
Key exam focus: Sources of error -- visual cues from the dropper, anticipation by the catcher. Why repeats and a mean improve reliability. Computer-based tests as a more precise alternative.
Common mistakes: Writing "human error" without being specific. Confusing accuracy with reliability.
Chemistry Required Practicals (8 Practicals)
Combined Science Trilogy includes all eight chemistry required practicals.
| # | Practical | Independent Variable | Dependent Variable | Key Control Variables |
|---|---|---|---|---|
| 7 | Making a soluble salt | N/A (preparation) | Purity of salt produced | Volume of acid, type of base |
| 8 | Electrolysis | Aqueous solution used | Products at electrodes | Electrode material, voltage |
| 9 | Temperature changes | Reactants used | Temperature change | Volume, concentration, insulation |
| 10 | Rates of reaction | Concentration/temperature/surface area | Gas volume or reaction time | All other factors constant |
| 11 | Chromatography | Substances separated | Rf values | Solvent, paper type, baseline position |
| 12 | Identifying ions | Ion being tested | Flame/precipitate colour | Testing method |
| 13 | Water purification | N/A (technique) | Purity of water | Apparatus used |
| 14 | Neutralisation and pH | Volume of acid/alkali added | pH of solution | Concentration, temperature, indicator |
7. Making a Soluble Salt
Aim: Prepare a pure, dry sample of a soluble salt from an acid and an insoluble base.
Key exam focus: Excess base ensures all acid has reacted. Filtration removes excess base, then evaporation or crystallisation produces the solid salt. You must name the correct acid and base for a given salt.
Common mistakes: Not explaining the purpose of excess base. Confusing the order of filtration and evaporation.
8. Electrolysis
Aim: Investigate electrolysis of aqueous solutions using inert electrodes.
Key exam focus: Predicting products at anode and cathode. Writing half-equations. In concentrated halide solutions the halogen forms at the anode; in dilute solutions oxygen forms instead.
Common mistakes: Mixing up anode (positive, oxidation) and cathode (negative, reduction). Forgetting the concentration rule for halide ions.
9. Temperature Changes in Reactions
Aim: Measure temperature change in reactions to classify them as exothermic or endothermic.
Key exam focus: A polystyrene cup acts as an insulator to reduce heat loss. Energy profile diagrams. Exothermic reactions release energy and the temperature rises; endothermic reactions absorb energy and the temperature falls.
Common mistakes: Not mentioning a lid to reduce heat loss through evaporation. Confusing temperature change direction with energy transfer direction.
10. Rates of Reaction
Aim: Investigate how concentration, temperature, surface area, or a catalyst affects reaction rate.
Key exam focus: Rate graphs and calculating rate from the gradient of a tangent. Collision theory -- increasing concentration increases collision frequency, so rate increases.
Common mistakes: Confusing rate with time (faster rate = shorter time). Drawing tangents incorrectly on curved graphs.
11. Chromatography
Aim: Separate and identify substances using paper chromatography.
Key exam focus: Rf = distance moved by substance / distance moved by solvent front. A pencil baseline is used because ink would dissolve in the solvent. Comparing Rf values with known standards identifies unknowns.
Common mistakes: Measuring from the bottom of the paper rather than from the pencil baseline.
12. Identifying Ions
Aim: Identify metal and non-metal ions using chemical tests.
Key exam focus: Flame test colours: lithium (crimson), sodium (yellow), potassium (lilac), calcium (orange-red), copper (green). Precipitate colours with NaOH: aluminium (white, redissolves in excess), calcium (white), magnesium (white), copper(II) (blue), iron(II) (green), iron(III) (brown). Tests for carbonates (acid + limewater), sulfates (barium chloride), halides (silver nitrate).
Common mistakes: Confusing white precipitates -- aluminium hydroxide redissolves in excess NaOH. Mixing up iron(II) green and iron(III) brown.
13. Water Purification
Aim: Analyse and purify water samples using distillation, filtration, and evaporation.
Key exam focus: Potable water is safe to drink but may contain dissolved minerals. Pure water contains only water molecules. Water treatment involves sedimentation, filtration, and chlorination. Distillation is needed for desalination.
Common mistakes: Confusing potable water with pure water.
14. Neutralisation and pH
Aim: Investigate pH changes during neutralisation and carry out a titration.
Key exam focus: Titration method using a burette, pipette, and phenolphthalein or methyl orange (not universal indicator). Concordant results agree within 0.10 cm cubed. Calculating concentrations from titration data.
Common mistakes: Using universal indicator for titrations. Not discarding the rough titre when calculating a mean.
Physics Required Practicals (7 Practicals)
Combined Science Trilogy includes seven of the eight physics required practicals.
| # | Practical | Independent Variable | Dependent Variable | Key Control Variables |
|---|---|---|---|---|
| 15 | Specific heat capacity | Energy supplied | Temperature change | Mass, insulation |
| 16 | Thermal insulation | Insulating material | Rate of cooling | Starting temperature, water volume, container |
| 17 | Resistance | Wire length | Resistance (V/I) | Wire material, diameter, temperature |
| 18 | I-V characteristics | Voltage | Current | Component type, temperature |
| 19 | Density | Object tested | Density (mass/volume) | Measurement technique |
| 20 | Force and extension | Force applied | Extension | Same spring, measurement reference point |
| 21 | Acceleration | Force applied | Acceleration | Total system mass, friction compensation |
15. Specific Heat Capacity
Aim: Measure specific heat capacity by heating a material and recording energy input, mass, and temperature change.
Key exam focus: E = mc(delta T), where delta T is the change in temperature. Heat loss to surroundings is the main error, causing calculated values to exceed the true value. Insulation improves accuracy.
Common mistakes: Not insulating the block. Forgetting that energy supplied is not the same as energy absorbed by the material.
16. Thermal Insulation
Aim: Compare the effectiveness of different insulating materials by measuring cooling rates.
Key exam focus: Controlling starting temperature, water volume, and container type. Interpreting cooling curves -- a slower gradient indicates better insulation.
Common mistakes: Not keeping starting temperature consistent. Confusing rate of cooling with final temperature.
17. Resistance
Aim: Investigate how wire length affects resistance.
Key exam focus: R = V/I at each length. Proportional relationship between length and resistance. The wire must not overheat -- use low currents and switch off between readings.
Common mistakes: Not cooling the wire between readings. Placing the ammeter in parallel or voltmeter in series.
18. I-V Characteristics
Aim: Investigate current-voltage characteristics of a resistor, filament lamp, and diode.
Key exam focus: Correct graph shapes: straight line for resistor, curve for filament lamp (resistance increases with temperature), current in one direction only for diode.
Common mistakes: Drawing a straight line for the filament lamp. Plotting current on the wrong axis.
19. Density
Aim: Measure density of regular objects, irregular objects, and liquids.
Key exam focus: Density = mass / volume. Displacement method for irregular shapes. Unit conversions between g/cm cubed and kg/m cubed.
Common mistakes: Not reading the meniscus at eye level. Forgetting to subtract initial water volume in displacement measurements.
20. Force and Extension (Hooke's Law)
Aim: Investigate the relationship between force and extension for a spring.
Key exam focus: Plotting a force-extension graph. The linear region where Hooke's law applies. Spring constant k = F/e. The limit of proportionality where the graph stops being linear.
Common mistakes: Measuring total length rather than extension. Not identifying the limit of proportionality.
21. Acceleration (Newton's Second Law)
Aim: Investigate the relationship between force, mass, and acceleration.
Key exam focus: F = ma. Compensating for friction by tilting the ramp. The total system mass includes both trolley and hanging masses. Plotting acceleration against force.
Common mistakes: Not compensating for friction. Not recognising that mass must be transferred from trolley to hanger to keep total system mass constant.
How to Write About Practicals in the Exam
Extended response questions on practicals are typically worth 6 marks. To maximise your score, include:
1. Name the apparatus. Be specific -- "250 cm cubed beaker," not "container."
2. Describe the method step by step. Include specific quantities: volumes, masses, concentrations, distances, and time intervals.
3. State your variables. Independent variable, dependent variable, and at least two control variables with an explanation of how each is kept constant.
4. Ensure reliability. Repeat at least three times, calculate a mean, and discard anomalous results.
5. Present results. State the graph type and what goes on each axis.
6. Suggest improvements. Be specific: "Use a data logger instead of a stopwatch to reduce timing errors" earns marks, while "use more accurate equipment" does not.
Common Mistakes on Practical Questions
Writing "human error." Too vague to earn marks. Name the specific problem: "Difficulty judging the exact colour change means the recorded time could be inaccurate."
Confusing accuracy, precision, and reliability. Accuracy is closeness to the true value. Precision is how close repeated measurements are to each other. Reliability means results are consistent when repeated. Examiners expect correct use of these terms.
Not quoting data. When a question provides results, reference specific values. "The rate increased from 0.02 to 0.05 cm cubed per second" is stronger than "the rate increased."
Forgetting units. Every calculation needs a unit. Every graph axis needs a label with a unit.
Describing without explaining. "The rate increased because particles had more kinetic energy, collided more frequently, and a greater proportion exceeded the activation energy" earns more marks than "the rate increased because it was hotter."
Prepare with LearningBro
LearningBro's AQA Combined Science Exam Prep course includes practice questions on every required practical, structured by topic so you can target your weakest areas. For more on how AQA papers work, see our guides to AQA mark schemes and AQA exam command words.