What you need to know
- Unit 3Identify the independent, dependent and controlled variables, and state how the other variables are kept the same so the test is fair.
The specification states that Unit 3 (WBI13) is a written paper of 1 hour 20 minutes, out of 50 marks, and that calculators may be used. These lessons teach how to answer. They are not a mark scheme, and they do not replace the Core Practical guides.
By the end, you should be able to…
- Identify the independent, dependent and controlled variables in a familiar or unfamiliar plan.
- State how a control variableA variable kept the same throughout an investigation so that it does not affect the result. is kept the same, not merely name it.
- Distinguish a controlled variable from a control experiment.
Build your understanding
Three jobs, three names
The independent variable is the factor you change on purpose. The dependent variable is the outcome you measure. A controlled variable, also called a control variableA variable kept the same throughout an investigation so that it does not affect the result. in the specification, is a factor you keep the same so that it does not become a second explanation.
Write the trio in that order before you describe apparatus. In an illustrative enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. investigation, temperature may be the independent variable, the time to reach a stated end-point the dependent variable, and pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline., enzyme volume and substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. concentration the controlled variables. Swapping the first two makes the rest of the plan describe the wrong experiment.
A name is not yet a control
“Control temperature” does not tell an examiner what you would do. State the value, or the way it is held, and the instrument. An illustrative sentence is: “Keep the water bath at 35 °C, checked with a thermometer, so temperature is not a second independent variable.”
If several factors could affect the outcome, you do not have to list the universe. Name the factors that could reasonably change the dependent variable in this method, and say how each is held constant. A fair test is that argument, not the phrase “fair test” on its own.
Control experiment is a different idea
A control experiment is a comparison set up without the factor under test, or with a known treatment, so you can see what the measurement does in that condition. A water blank in a Benedict’s series and a boiled-enzyme tube are control experiments. They are not controlled variables.
In an unfamiliar plan, decide which meaning the question wants. “Identify the controlled variables” asks for factors kept the same. “Describe a suitable control” asks for the comparison tube, dish or transectA line across a habitat along which samples are taken, to show how organisms change with conditions.. Using one word for both usually misses the second idea. This is study guidance, not a published tariff.
NOT: a control variable is not the same thing as a control experiment.
- State the independent variable and how it will be changed
- State the dependent variable and its unit
- Name at least two factors you will keep constant
- A control experiment lacks the active treatment
Two traps this diagram blocks
- Do not change two factors and still call the test fair.
- Do not call every unchanged detail a control experiment.
From the diagram alone
Question
Distinguish a control variable from a control experiment in an enzyme investigation. [3]
Show a model answer
A control variable, such as substrate volume or pH, is kept constant so it does not cause differences between tubes. A control experiment is a comparison that lacks the active treatment, for example boiled enzyme, so you can see that the working enzyme caused the result.
| What you were asked | Write this | Not this |
|---|---|---|
| Independent variable | The factor you change, and the values or range if they are given | The thing you measure |
| Dependent variable | The outcome, including the unit and how it is detected | “The result” with no measurement |
| Controlled variable | A named factor, the value or method that keeps it constant, and why that matters | The word “control” alone |
| Control experiment | The comparison that lacks the treatment, or the known standard | A variable you merely kept the same |
Make your reasoning visible
Change one factor in the sentence
Start with “Change …, measure …, keep … the same by …”. If the sentence changes two factors, the test is no longer fair.
Use the question’s context
A novel investigation still uses this trio. Do not drag in a core practical that the question did not ask about.
Key explanations
- The dependent variable is what you measure to judge the effect.
- Control variables are factors you keep the same so they do not explain the result.
- Changing two factors at once means you cannot tell which one caused a difference.
- Calling every constant “the control” mixes the two meanings.
Avoid this misconception
The claim: A control variable and a control experiment are the same thing.
The correction: A control variableA variable kept the same throughout an investigation so that it does not affect the result. is held constant. A control experiment is a comparison missing the factor under test, or a known standard. Name which one the question wants.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative student investigates the effect of pH on the time taken for a starch–amylase mixture to lose its blue-black colour with iodine. Identify the independent and dependent variables.
Show suggested answer
The independent variable is pH. The dependent variable is the time taken for the blue-black colour to disappear.
Practice question
Retrieval and explanation · suggested answer
Name two variables that should be controlled in that investigation, and state how one of them is kept the same.
Show suggested answer
Enzyme concentration or volume, substrate concentration or volume, and temperature should be controlled. For example, use the same volume of amylase solution from the same stock in every tube, so the amount of enzyme is not a second independent variable. Temperature can be kept at one stated water-bath temperature.
Practice question
Retrieval and explanation · suggested answer
Why is a tube of starch with water instead of amylase a control experiment rather than a controlled variable?
Show suggested answer
It is a comparison that lacks the enzyme, so any colour change can be compared with the tubes that contain amylase. It is not a factor being held at the same value across the pH series.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
An unfamiliar question says a student changed both light intensity and the temperature of pondweed and then concluded that light increases the rate of bubble production. Explain why that conclusion is not supported, and state one change that would make a fair test of light.
Show a hint
Two altered factors can both explain the same outcome.
Explore the reasoning
Light intensity and temperature were both changed, so a change in bubble production cannot be attributed to light alone. Keep temperature at one stated value, measured in the same way each time, and change only light intensity. The assumption is that other relevant factors, such as carbon dioxide and the plant used, are also kept the same.
Recall this lesson
Lesson recall questions
Question 1 of 5: A candidate plans to investigate how temperature affects the initial rate of an amylase-catalysed starch hydrolysis. State the independent variable and the dependent variable.
Unit 3 planning
Practice question
Retrieval and explanation · suggested answer
A candidate plans to investigate how temperature affects the initial rate of an amylase-catalysed starch hydrolysis. State the independent variable and the dependent variable.
Lesson: P2: Apparatus and the measurements you will take
Try this from memory first.
Beyond the classroomA comparison group is not a controlled variableResearch · health · Enrichment, not required recall
Clinical-trial guidance describes comparison groups so that a treatment can be judged against another condition. That comparison is closer to a control experiment than to a school “control variableA variable kept the same throughout an investigation so that it does not affect the result.” such as a water-bath temperature. The trial context is enrichment. It is not a Unit 3 required example, and it is not medical advice.
Back to the spec: Unit 3 planning: say which factors change, which are measured, and which comparison lets you interpret the outcome.
World Health Organization: clinical trials questions and answersQuestion
A report says a new treatment was given and patients improved. Why is that not yet a fair comparison?
Explore a possible response
There is no comparison group and no statement of what else changed. Improvement could have happened without the treatment. A fair comparison needs a stated comparison condition and control of other relevant differences.