In SEC 2027 G3 Chemistry (K324), Paper 3 assesses practical skills including planning, measuring and observing, presenting results, and drawing or evaluating conclusions. The paper lasts 1 hour 50 minutes and makes up 20% of the qualification. Planning accounts for 15% of the practical assessment.
A fair-test plan should tell another student what to change, what to measure and what to keep the same. It should also say how the results would answer the question. This guide walks through one paper-based example so you can practise the decisions before writing the method.
- Name the independent and dependent variables in the context of the investigation.
- Define exactly how the measurement will be taken.
- Plan a table and state what pattern would support your conclusion.
Turn the question into a comparison
Suppose an exercise asks how the concentration of an acid affects the rate at which it reacts with calcium carbonate. Start with “I will vary the acid concentration and measure the time to collect a fixed volume of carbon dioxide.” A time is easier to compare when the gas volume is fixed across trials.
Think through the direction of the result before writing anything about equipment. If two trials collect the same volume and the first takes less time, the average rate of gas collection over that interval was higher in the first. The comparison tells you why you are measuring time in the first place.
Use the concentrations, volumes and apparatus specified by the question. This is a written planning exercise. Practical work belongs in a supervised laboratory with the school’s procedures.
Make the procedure repeatable
Imagine the worksheet provides acid solutions at three concentrations and a gas syringe setup. Use the same volume of acid, the same mass of calcium carbonate and chips of comparable size. Keep the starting temperature the same. Different chip sizes can change the exposed surface area, which affects the rate and confuses the comparison.
Decide when timing starts and ends. In a school-provided method, use a consistent start point tied to mixing and a fixed gas volume as the endpoint. Account for gas lost while the apparatus is being closed. If the prompt supplies a way to mix the reactants in a closed system, explain how it solves that measurement problem.
Use an endpoint that all conditions can reach with the quantities provided. If one concentration never produces the chosen gas volume, “time to the endpoint” cannot be compared across every trial. When the question permits, an alternative is to measure the gas collected after the same short interval for all conditions.
Prepare a table before collecting results
For each concentration, allow room for the individual times and the mean time. Put units in the headings, for example “Acid concentration / mol per dm³” and “Time to collect fixed volume / s”. State the endpoint volume in the heading or accompanying instructions.
Here is a fictional set of times from a worksheet. At the lowest concentration, the times are 44, 46 and 45 seconds. At the middle concentration, they are 31, 30 and 32 seconds. At the highest concentration, they are 22, 21 and 23 seconds. Their mean times are 45, 31 and 22 seconds respectively.
| Acid concentration | Trial 1 (s) | Trial 2 (s) | Trial 3 (s) | Mean (s) |
|---|---|---|---|---|
| Lowest | 44 | 46 | 45 | 45 |
| Middle | 31 | 30 | 32 | 31 |
| Highest | 22 | 21 | 23 | 22 |
A conclusion could say “As the acid concentration increased across the three tested conditions, the mean time to collect the same gas volume decreased from 45 s to 22 s. This indicates a higher average rate of gas collection over the measured interval.” Keep the conclusion within the range you actually tested.
Explain an unexpected result
Change the highest-concentration results to 22, 21 and 39 seconds. The 39-second run stands out from the other two. Look for a recorded reason such as a connection that was loose or a delay in starting the timer. If the question allows another run, repeat the condition and state how you will handle the unusual value.
Distinguish an unusual result from a systematic issue. A consistent leak could lower the volume registered on several runs. Repetition alone would not fix that. Explain what you would change in the setup or procedure and how it would improve the measurement.
A parent or study partner can read a plan and ask “Could I repeat this without guessing what you meant?” Check the start and stop points, what stays fixed and what would count as evidence. Rewrite the one sentence that would make the biggest difference.
Frequently asked questions
Is planning 15% of the whole subject?
The syllabus assigns 15% to planning within the practical assessment. Paper 3 itself contributes 20% of the qualification.
Can I just say to repeat and average?
Name what you repeat and what you average. If a result looks unusual, use the observations and task instructions to decide how to handle it.
Will every practical question require a written plan?
The syllabus says one or more questions may include planning. Follow the task you are given.
Official information
SEAB’s 2027 SEC G3 Chemistry syllabus (K324), Scheme of Assessment and Practical Assessment. Checked 8 November 2026.