Paper 6 Skills (Part 2): Planning experiments

01 What planning questions ask

Paper 6 (Alternative to Practical) often includes questions that ask you to plan an investigation rather than carry one out in the lab. Typical tasks include:

• Identifying the independent, dependent, and control variables.
• Stating variables that must be kept constant for a fair test.
• Choosing and justifying apparatus (e.g. pipette vs measuring cylinder).
• Writing a clear, numbered method with safety precautions.
• Designing a results table with correct headings and units.

Your theory notes (e.g. rates of reaction in Chapter 6) explain chemical principles. This module shows how to turn those ideas into a practical plan. Apparatus detail is covered further in Chapter 12.

02 Independent, dependent & control variables

Independent variable (IV)

The factor you deliberately change between experiments. Only one independent variable in a fair test.

Dependent variable (DV)

The factor you measure to see how it responds to the independent variable.

Control variables

All other factors that could affect the rate or outcome. These must be kept constant.

Fair test

Only the independent variable changes; all control variables remain the same.

03 Identifying variables

Exam-style scenario: A student investigates how the concentration of hydrochloric acid affects the rate of reaction with magnesium ribbon. They use 0.5 mol/dm³, 1.0 mol/dm³, 1.5 mol/dm³, 2.0 mol/dm³, and 2.5 mol/dm³ acid. Each time they use a 5 cm strip of magnesium and collect the hydrogen gas produced in a gas syringe for 2 minutes.

Independent variable

Concentration of hydrochloric acid (mol/dm³).

Dependent variable

Volume of hydrogen gas collected in 2 minutes (cm³), or rate of reaction.

Control variables (any two acceptable answers)

• Mass or length of magnesium ribbon (5 cm each time)
• Volume of acid used
• Temperature of acid and room
• Same size of gas syringe and same collection time (2 minutes)
• Same surface area of magnesium (same width of ribbon)

See Chapter 6 for how concentration, temperature, surface area, and catalysts affect rate.

04 Fair tests & controlled variables

In chemistry rate experiments, these are commonly kept constant:

• Temperature — use a thermostatic water bath or carry out all trials at room temperature on the same day.
• Mass or length of metal — e.g. always 0.5 g of zinc or 5 cm of magnesium ribbon.
• Surface area of solids — same size of marble chips or same mesh size of powder; do not switch between lumps and powder.
• Volume of liquid reactants — e.g. 25 cm³ of acid in every trial.
• Concentration of other reactants — if acid concentration is the IV, the metal must be in excess and identical each time.
• Catalyst mass — if testing temperature, use the same mass of manganese(IV) oxide in each flask.

Why it matters: If you change concentration and temperature together, you cannot tell which factor caused the faster reaction.

05 Control experiments

A control experiment shows that the measured effect is due to the reaction or factor being tested.

Example — acid and metal

Place magnesium in water instead of acid — no hydrogen is produced, confirming that acid is required.

Example — catalyst

Repeat the decomposition of hydrogen peroxide without manganese(IV) oxide — much slower reaction, showing the catalyst increases rate.

Example — indicator in titration

A blank titration (without the analyte) is not always required at IGCSE, but you may need to explain why a control proves the reactants cause the observed change.

When asked to “suggest a control”, describe a set-up identical to the experiment except that the key factor is absent or set to zero.

06 Range, repeats & concordant results

Range

Use at least 5 different values of the independent variable (e.g. five acid concentrations). Choose a range that shows a clear trend.

Repeats

Repeat each condition at least 3 times and calculate a mean. Discard clear anomalies before calculating the mean.

Concordant results

Results that are close together (within a few tenths of a cm³ for titres, or similar for gas volumes). In titration, concordant titres are used to calculate the mean titre — see Chapter 12 Part 2.

Reliability

Improved by good technique, accurate apparatus, and repeating. If results vary widely, identify a source of error (e.g. gas escaping, incomplete mixing, temperature drift).

07 Choosing & justifying apparatus

Paper 6 may ask you to select apparatus and explain your choice:

• Measuring cylinder — approximate volumes; quick but less accurate (e.g. ±1 cm³).
• Volumetric pipette — delivers one fixed volume very accurately (e.g. 25.0 cm³) for preparing solutions or titrations.
• Burette — measures variable volumes accurately to 0.05 cm³; used in titrations.
• Balance — measure mass of solid reactants; tare (zero) before use; keep away from drafts.
• Gas syringe — collect and measure gas volume directly; more accurate than counting bubbles or displacement over water if gas is soluble.
• Stopwatch — measure time; limited by human reaction time.
• Thermometer — monitor temperature; read at eye level to avoid parallax error.

Justification example: “Use a gas syringe rather than counting bubbles because volume is measured directly in cm³ and is more accurate.”

Full apparatus detail: Chapter 12 Part 1.

08 Writing a method

Structure your plan as follows:

1. Aim — what you are investigating (one sentence).
2. Variables — IV, DV, and controls listed clearly.
3. Apparatus — list with sizes (e.g. 100 cm³ conical flask, 50 cm³ gas syringe).
4. Method — numbered steps; state how controls are kept constant.
5. Safety — identify hazards and precautions:
   • Dilute acids — corrosive; wear safety goggles; wipe spills.
   • Hydrogen gas — flammable; no naked flames; ventilate.
   • Heat — use tongs; label hot apparatus.
6. Results table — columns with headings and units, e.g. “Temperature / °C”, “Volume of gas / cm³”.

Use imperative verbs (Add, Measure, Record, Repeat). Do not include predicted results or a conclusion in the method.

09 Full worked example

Exam-style question: Plan an investigation to find how temperature affects the rate of reaction between magnesium ribbon and dilute hydrochloric acid.

Independent variable

Temperature of the acid (e.g. 20°C, 30°C, 40°C, 50°C, 60°C using a water bath).

Dependent variable

Time taken to collect 20 cm³ of hydrogen (s), or volume of gas in a fixed time (cm³).

Control variables

• Length and mass of magnesium ribbon (e.g. 5 cm each trial)
• Volume and concentration of hydrochloric acid (e.g. 25 cm³ of 2.0 mol/dm³)
• Same apparatus (same flask, same gas syringe)
• Same method of warming (thermostatic water bath)

Outline method

1. Set up a conical flask on a tripod with a thermostatic water bath at 20°C.
2. Measure 25 cm³ of 2.0 mol/dm³ HCl into the flask and allow it to reach 20°C.
3. Add a 5 cm strip of magnesium and immediately attach a gas syringe.
4. Start a stopwatch and record the time to collect 20 cm³ of gas.
5. Repeat at 30°C, 40°C, 50°C, and 60°C, using fresh acid and a new magnesium strip each time.
6. Repeat the whole experiment three times at each temperature and calculate mean times.

Safety

Acid is corrosive — wear goggles. Hydrogen is flammable — no flames, keep flask vented. Hot water bath — handle with care.