Paper 6 Skills (Part 2): Variables & planning

01 What Paper 6 expects

Paper 6 tests your ability to plan and analyse investigations, not just recall theory. Many questions ask you to:

• State the independent, dependent, and control variables in a given experiment.
• Identify variables that must be kept constant to ensure a fair test.
• Suggest a suitable range of values and number of repeats.
• Write or improve a method with clear, numbered steps.

Your theory notes (e.g. osmosis in Chapter 3, enzymes in Chapter 5, photosynthesis in Chapter 6) explain what happens in biology. This module explains how to investigate it in an exam.

02 Independent, dependent & control variables

Independent variable (IV)

The factor you deliberately change to test its effect. There is only one independent variable in a fair test.

Dependent variable (DV)

The factor you measure to see how it is affected. It depends on the independent variable.

Control variables

All other factors that could affect the result. These must be kept constant so you can be confident that any change in the dependent variable is caused by the independent variable alone.

Fair test

An investigation in which only the independent variable is changed and all control variables are kept the same.

03 Identifying variables in a scenario

Exam-style scenario: A student investigates how temperature affects the activity of amylase. Equal volumes of starch solution and amylase are mixed in test tubes placed in water baths at 20°C, 30°C, 40°C, 50°C, and 60°C. Every 30 seconds, a drop is tested with iodine solution until the starch is digested.

Independent variable

Temperature of the water bath (or of the reaction mixture).

Dependent variable

Time taken for starch to be digested (or rate of digestion).

Control variables (any two acceptable answers)

• Volume of starch solution
• Volume of amylase solution
• Concentration of starch
• Concentration of amylase
• pH of the solutions
• Same source/type of amylase
• Same method of testing with iodine

Tip: Read the method carefully. The variable that has different values listed (20°C, 30°C…) is usually the independent variable. The variable described as “measured”, “recorded”, or “timed” is usually the dependent variable.

04 Variables to keep constant

When stating control variables, be specific and link them to the investigation:

• Volume — e.g. 10 cm³ of each solution in every test tube.
• Concentration — e.g. same molarity of sugar solution for all potato cylinders.
• Organism / tissue type — e.g. same species of plant, same age of leaf discs.
• Size or surface area — e.g. potato cylinders cut to the same length and diameter with a cork borer.
• Equipment and technique — e.g. same lamp wattage, same distance from the light source, same size of measuring cylinder.
• Time allowed — if comparing uptake over a fixed period, the time must be the same for all trials.

Why it matters: If you change temperature and volume at the same time, you cannot tell which factor caused a change in the result. Keeping controls constant isolates the effect of the independent variable.

05 Control experiments

A control experiment is a comparison set-up where the factor being tested is absent or at a standard level, to show that the observed effect is genuine.

Negative control

A set-up where the expected process does not occur, proving that measured changes are due to the process being studied.

Example — photosynthesis

In the aquatic plant investigation (Chapter 6):

• Foil on a leaf — stops light reaching part of the leaf; no starch is made there (negative control for light).
• Soda lime — absorbs CO&sub2;; no starch is made when CO&sub2; is removed (negative control for carbon dioxide).

Example — enzyme activity

Boiled enzyme (denatured) mixed with substrate — no reaction occurs, showing that a living enzyme is required.

In Paper 6, you may be asked to suggest a control or explain why a control is needed: it confirms that the dependent variable only changes when the independent variable is present.

06 Range, repeats & reliability

Range of the independent variable

Use at least 5 different values spread across a sensible range (e.g. 0%, 2%, 4%, 6%, 8% salt concentration). Avoid gaps that are too large.

Repeats

Repeat each condition at least 3 times and calculate a mean. This improves reliability and allows you to spot anomalies.

Anomalies

Results that do not fit the pattern. Identify them on a graph or table and exclude them from the mean with a reason (e.g. air bubble trapped in syringe, leaf disc still floating due to incomplete air removal).

Reliability

Repeatable, consistent results. If repeats are very different, the method may need improving (e.g. use a thermostatically controlled water bath instead of a beaker of hot water).

07 Writing a method

A good Paper 6 method follows a clear structure:

1. Aim — one sentence stating what you are investigating.
2. Variables — state IV, DV, and at least two controls.
3. Equipment — list apparatus with sizes where relevant (e.g. 25 cm³ measuring cylinder).
4. Method — numbered steps in logical order; include how to keep variables constant.
5. Safety — relevant hazards (e.g. iodine stain, sharp cork borer, hot water bath).
6. Results table — columns for IV and DV with headings and units (e.g. “Salt concentration / %”, “Change in mass / g”).
7. Graph — state the type (usually IV on x-axis, DV on y-axis, line graph for continuous data).

Writing tips: Use imperative verbs (Measure, Cut, Record, Repeat). Say how much and how long. Do not include results or conclusions in the method.

08 Full worked example

Exam-style question: Plan an investigation to find how the concentration of salt solution affects osmosis in potato tissue.

Independent variable

Concentration of salt solution (e.g. 0%, 0.5%, 1.0%, 1.5%, 2.0%).

Dependent variable

Change in mass of potato cylinders (initial mass minus final mass, or percentage change in mass).

Control variables

• Same type of potato
• Same dimensions of cylinders (same cork borer, same length)
• Same volume of salt solution (e.g. 20 cm³ in each boiling tube)
• Same soaking time (e.g. 30 minutes)
• Same temperature (e.g. room temperature for all)
• Blotting dry with same technique before reweighing

Outline method

1. Cut five potato cylinders to 4 cm length using a cork borer.
2. Prepare salt solutions of 0%, 0.5%, 1.0%, 1.5%, and 2.0%.
3. Weigh each cylinder and record the initial mass.
4. Place one cylinder in each solution for 30 minutes.
5. Remove, blot dry gently, reweigh, and record final mass.
6. Repeat the entire experiment three times and calculate mean change in mass.

See Chapter 3 for the theory behind osmosis and why mass changes occur.

Before you submit a plan: check the Exam Traps and Examiner Report Insights in Sections 02–07 — they cover the mistakes most often seen in 0610 Paper 6 marking.