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Knowing the core practicals is only half the challenge. The other half is knowing how to answer exam questions about them. Examiners do not simply ask you to recite a method — they ask you to apply your practical knowledge, evaluate methods, identify errors, and suggest improvements.
This lesson teaches you the specific techniques for each type of practical question, with model answers showing exactly what the examiner is looking for.
These questions ask you to write out the procedure for a core practical. They are typically worth 4–6 marks.
"Measure 5.0 g of marble chips using a balance and place them in a conical flask. Measure 50 cm³ of hydrochloric acid at 1.0 mol/dm³ using a measuring cylinder and add it to the flask. Immediately connect a gas syringe to the flask and start a stopwatch. Record the volume of gas collected every 30 seconds for 5 minutes. Repeat the experiment using 0.5 mol/dm³ and 0.25 mol/dm³ acid, keeping the mass and size of marble chips and the volume of acid the same each time."
| Mistake | Example | Correction |
|---|---|---|
| Too vague | "Add the acid to the chips" | "Add 50 cm³ of 1.0 mol/dm³ HCl" |
| Missing measurements | "Heat the acid" | "Heat the acid to 40°C using a water bath" |
| No mention of repeats | Just one trial | "Repeat each concentration three times and calculate a mean" |
| Wrong order | Connecting gas syringe after the reaction starts | "Immediately connect the gas syringe" |
Exam tip: When describing a method, imagine you are writing instructions for someone who has never done the experiment. Include every detail: what equipment, what quantities, what order, and how measurements are taken.
These questions test whether you understand experimental design.
| Variable | Definition | How to Answer |
|---|---|---|
| Independent variable (IV) | What you deliberately change | "The concentration of hydrochloric acid" |
| Dependent variable (DV) | What you measure | "The volume of gas produced" or "The time taken for the reaction to complete" |
| Control variables | What you keep the same to make it a fair test | "Mass of marble chips, volume of acid, temperature, size of marble chips" |
Question: A student investigates the effect of temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid. Identify the independent, dependent, and control variables.
"The independent variable is the temperature of the sodium thiosulfate solution. The dependent variable is the time taken for the cross to disappear (or for the solution to become opaque). The control variables are the volume and concentration of both solutions, and the size of the cross drawn on the paper."
Exam tip: For control variables, always name at least two specific things. "Everything else stays the same" earns zero marks.
These questions ask you to identify weaknesses in a method and propose specific changes.
| Bad Improvement | Why It Fails | Good Improvement |
|---|---|---|
| "Be more careful" | Too vague — not a method improvement | "Use a pipette instead of a measuring cylinder to measure the acid more accurately" |
| "Do it again" | Does not explain why | "Repeat the experiment three times at each concentration and calculate a mean to improve reliability" |
| "Use better equipment" | Too vague | "Use a data logger with a temperature probe instead of a thermometer to record temperature continuously and more precisely" |
| "Make sure it is a fair test" | Does not say how | "Use marble chips of the same size by sieving them to control surface area" |
Question: A student measures the temperature change during a neutralisation reaction using a glass beaker and a thermometer. Suggest two improvements to the method.
"Improvement 1: Use a polystyrene cup instead of a glass beaker. Polystyrene is a better insulator than glass, so less heat would be lost to the surroundings, making the temperature measurement more accurate.
Improvement 2: Use a lid on the polystyrene cup to reduce heat loss by evaporation and convection from the top of the liquid."
Exam tip: Every improvement you suggest should follow the pattern: what to change + why it helps. An improvement without a reason often earns only half marks.
These questions give you data or observations and ask you to explain them using your chemistry knowledge.
Question: In an electrolysis experiment using copper sulfate solution and copper electrodes, the student found that the cathode gained 0.32 g in mass while the anode lost 0.30 g. Explain these results.
"At the cathode, copper ions (Cu²⁺) from the solution are reduced by gaining two electrons, forming copper atoms that are deposited on the electrode — this is why the cathode gains mass. At the anode, copper atoms lose two electrons and become copper ions (Cu²⁺) that dissolve into the solution — this is why the anode loses mass. The cathode gained slightly more mass than the anode lost because the masses may differ slightly due to measurement uncertainty, or some impurities may have been deposited alongside the copper."
These are often the highest-tariff practical questions (4–6 marks). They require critical thinking about the quality of the experiment.
| Aspect | What to Consider |
|---|---|
| Sources of error | What could have caused inaccurate results? |
| Reliability | Were repeats done? Were results consistent? |
| Accuracy | Were appropriate instruments used? Was precision sufficient? |
| Anomalies | Were there any outliers? Should they be excluded? |
| Improvements | How could the method be made better? |
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