Planning an experiment question (SPM Chemistry)

The “plan an experiment” question is the most demanding part of SPM Chemistry Paper 3, the practical paper (Paper 3 is a practical test assessing science process skills) of SPM Chemistry, because it asks you to design a whole investigation, not just answer about one that is described. The good news is that the marks are given for a fixed set of headings, so if you follow the same structure every time you rarely miss one. This guide gives that structure, a template you can reuse, a worked example, and the errors to avoid.

What the question asks

A planning question gives you a problem, for example, “plan an experiment to investigate how the concentration of acid affects the rate of its reaction with marble chips”, and asks you to write a complete plan. It tests whether you can turn an aim into a fair, workable procedure and say how you would handle the results. The examiner awards marks for each part of the plan, so the safest approach is to answer under clear headings that cover every part.

The structure to follow every time

Answer under these headings, in this order:

1. Aim. State clearly what the experiment is to find out, usually taken straight from the question.
2. Hypothesis. Write a testable statement linking the manipulated and responding variables with a direction, such as “the higher the concentration, the faster the rate”.
3. Variables. Name the manipulated variable (what you change), the responding variable (what you measure), and the controlled variables (what you keep constant for a fair test).
4. Apparatus and materials. List everything you need, choosing apparatus that gives the accuracy the task requires.
5. Procedure. Write numbered steps that show how you change the manipulated variable over a range of values, how you keep the controlled variables constant, and how you measure the responding variable. Include repeating readings for reliability.
6. Tabulation of data. Show the empty table you would use, with headings and units for each column, ready to receive the readings.
7. Conclusion. State how you would use the results to accept or reject the hypothesis, for example, describing the expected trend.

If you cover all seven, you have covered the marks.

A reusable procedure template

Most rate, quantity or comparison experiments fit this shape: set up the apparatus; measure out a fixed amount of the controlled reactants; add the manipulated variable at its first value; measure the responding variable; repeat with the manipulated variable at several more values, keeping everything else the same; repeat each reading and average. Writing the procedure so that the fair test is visible, same volumes, same temperature, only the one variable changed, is exactly what earns the procedure marks.

Worked example

Problem: plan an experiment to investigate how the concentration of hydrochloric acid affects the rate of its reaction with excess marble chips.

• Aim: to find how the concentration of acid affects the rate of reaction.
• Hypothesis: the higher the concentration of the acid, the faster the rate of reaction.
• Variables: manipulated, concentration of acid; responding, rate (measured as the volume of gas collected in a fixed time, or the time for a fixed volume of gas); controlled, volume of acid, mass and size of marble chips, temperature.
• Apparatus: conical flask, delivery tube, gas syringe or measuring cylinder over water, stopwatch, measuring cylinder, balance.
• Procedure: measure a fixed volume of acid of the first concentration into the flask; add a fixed mass of marble chips and start the stopwatch; measure the volume of gas collected after a fixed time; repeat with acids of higher concentration, keeping the volume of acid, the mass and size of chips and the temperature the same; repeat each reading and average.
• Tabulation: a table with columns “Concentration of acid / mol dm⁻³” and “Volume of gas in fixed time / cm³” (with repeat and average columns).
• Conclusion: if the volume of gas in the fixed time increases as concentration increases, the rate increases with concentration, supporting the hypothesis.

Common errors

• Forgetting the controlled variables. A plan that changes the manipulated variable but does not say what is kept constant is not a fair test; always list the controlled variables and show how they are held constant in the procedure.
• A vague procedure. “React the acid and measure the rate” is not enough; give numbered steps with the quantities and what is measured.
• No range for the manipulated variable. You must change it over several values, not just two, so a trend can be seen.
• Leaving out repeats. Repeating readings and averaging improves reliability and is often a marked point.
• No empty table or no units. Draw the table with headings and units ready for the data.
• A conclusion that does not refer to the hypothesis. Say how the results would confirm or reject the prediction.

Tips for full marks

Write the seven headings down first, then fill each one, this makes sure you do not miss a section under time pressure. Make the fair test visible by naming the controlled variables both in the variables list and again in the procedure. Choose apparatus that matches the accuracy needed, and quote no invented numbers, describe what you would measure, not results you have not obtained. Keep the procedure in numbered steps so the method is easy to follow and mark.

How our teachers train this

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we practise planning questions against this seven-part checklist until a student can lay out a complete plan for any described problem, because the planning question carries some of the most valuable and most structured marks in the Paper 3 practical of SPM Chemistry. Once the structure is habit, the same student who once wrote a vague paragraph now produces a full plan with variables, a fair-test procedure and a ready table, the difference between a partial answer and full marks.