Planning an Experiment

Components of a good plan

• Aim, a statement of what relationship is being investigated, usually starting with 'To investigate the effect of... on...'.
• Hypothesis, a testable prediction of how the manipulated variable affects the responding variable.
• Variables, the manipulated variable, the responding variable, and a list of controlled variables, each stated clearly.
• Apparatus and materials, a complete list of everything needed, including specific quantities or concentrations where relevant.
• Method, numbered steps describing exactly how the experiment is set up and carried out, in enough detail that another person could repeat it.
• Control, a setup without the factor being tested, or at a baseline level, used for comparison to show that the result is due to the manipulated variable.

Writing a fair-test method

1. State how each level of the manipulated variable will be set up, such as different concentrations or temperatures.
2. Describe exactly how the responding variable will be measured, including the instrument used and the unit.
3. State how the controlled variables will be kept constant across every setup.
4. Explain how the experiment will be repeated (usually at least twice more, for three readings in total) to calculate an average and improve reliability.
5. Describe how the results will be recorded, usually in a table with clear column headings.

Why a control is needed

A control setup does not include the factor being tested, or is kept at a fixed baseline level, so its result can be compared with the test setups. This comparison confirms that any change in the responding variable is caused by the manipulated variable, and not by some uncontrolled factor in the experiment.

Worked planning example

Problem statement. Plan an experiment to investigate the effect of light intensity on the rate of photosynthesis in a water plant such as Hydrilla.

Aim. To investigate the effect of light intensity on the rate of photosynthesis in Hydrilla, measured as the number of gas bubbles released per minute.

Hypothesis. The higher the light intensity, the higher the rate of photosynthesis, so more gas bubbles are released per minute, until the rate levels off at high intensity.

Variables. Manipulated: light intensity, varied by placing a lamp at set distances (for example 10, 20, 30, 40 and 50 cm). Responding: rate of photosynthesis, counted as bubbles per minute.

Controlled: the temperature of the water, the same piece of Hydrilla, and the concentration of sodium hydrogen carbonate solution used as a carbon dioxide source.

Apparatus and materials. Hydrilla shoot, beaker, sodium hydrogen carbonate solution, filter funnel, test tube, bench lamp, metre rule, thermometer, and a stopwatch.

Control and repeats. A set-up kept in darkness acts as the control, in which no bubbles should be produced. Each reading is repeated three times and the average number of bubbles per minute is calculated to improve reliability.

1. Place the Hydrilla shoot cut-end upwards in a beaker of sodium hydrogen carbonate solution and cover it with an inverted filter funnel and test tube.
2. Set the lamp 10 cm from the beaker and leave the plant for two minutes so the rate becomes steady.
3. Count the number of gas bubbles released in one minute, and record it.
4. Repeat the count twice more at the same distance and calculate the average.
5. Move the lamp to 20, 30, 40 and 50 cm in turn, repeating the counts at each distance while keeping the temperature constant.
6. Record all readings in a table and plot a graph of bubbles per minute against light intensity.

Worked planning questions

These short original tasks practise the separate skills a planning question rewards.

Task 1 (aim and hypothesis). An investigation tests how the concentration of amylase affects the time to digest starch. Write an aim and a hypothesis.

Model answer. Aim: to investigate the effect of amylase concentration on the time taken to digest starch. Hypothesis: the higher the amylase concentration, the shorter the time to digest the starch.

Task 2 (variables and control). For that investigation, state the responding variable and describe a suitable control. Model answer. Responding variable: the time for the starch to be fully digested, shown when iodine solution no longer turns blue-black.

Control: a tube with starch and distilled water but no amylase, in which the starch is not digested.

Task 3 (reliability). Explain how the plan makes the results reliable. Model answer. Each concentration is tested three times and the average time is calculated; repeating the tests lets anomalous readings be identified and reduces the effect of random error.

Including safety in your plan

A complete plan states the safety precautions that suit the apparatus and materials, because marks are given for a method that can be carried out safely.

• Name the hazard, such as hot water, a naked flame, or a sharp scalpel, and the precaution that controls it.
• Wear safety goggles when heating liquids or handling irritant solutions such as concentrated acids or alkalis.
• Handle living material and biological fluids carefully, and wash your hands after the experiment.
• Switch off and cool any heating apparatus before clearing away, and dispose of chemicals as instructed.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions