Lesson focus: Variables are the bridge between a scientific idea and a fair testA test where you change only one thing and keep everything else the same, so you know what caused any difference..
Identify independent, dependent and control variableSomething you keep the same in an investigation so the test is fair, such as using the same amount of water each time. and explain why control variables matter.
Build on earlier learning
Scientific investigationsRevisit when a fair test is the appropriate type of scientific enquiry.
Planning an investigation?
Design an investigation with CORMMSSA step-by-step Logic Lab guide to planning a fair test, with a worked example and practice.
You will be successful when you can…
- Identify independent, dependent and control variables.
- Choose a useful range for the independent variable.
- Explain how an uncontrolled variable weakens a conclusion.
- Distinguish a control variable from a control group.
Words that carry the science
- independent variable
- The factor deliberately changed in an investigation.
- dependent variable
- The outcome measured or observed.
- control variable
- A relevant factor kept constant to support a fair comparison.
- control group
- A comparison group that does not receive the tested treatment.
- confounding variable
- An uncontrolled factor that offers another explanation for a result.
Use variables to isolate an effect
01Change, measure and keep the same
The independent variableThe one thing you choose to change in an investigation. is deliberately changed. The dependent variableThe thing you measure or observe in an investigation to see if it changes. is measured. Relevant control variableSomething you keep the same in an investigation so the test is fair, such as using the same amount of water each time. are kept as constant as reasonably possible.
In “How does water temperature affect dissolving time?”, temperature is independent and time is dependent. Water volume, solute mass, particle size and stirring method are control variableSomething you keep the same in an investigation so the test is fair, such as using the same amount of water each time.: keep them the same. Particle size matters because small crystals dissolve faster than big ones.
02Control variables protect the comparison
If hotter trials are also stirred faster, either temperature or stirring could explain shorter dissolving times. Stirring is then a confounding variableAnother factor you did not control that could also change the result, so you cannot be sure what caused it..
A control variable is a condition held constant. A control groupA group that does not get the thing being tested, so you have something fair to compare the results with. is a comparison group, often without the tested treatment. They are different ideas.
03Use a range, not only two values
A sensible range can reveal the shape of a relationship and expose unusual values. Values should be safe, achievable and spaced closely enough to show change.
Repeats estimate random variation at each value; they do not replace a well-chosen range.
A fair-test claim depends on all three links.
- Changeindependent variableThe one thing you choose to change in an investigation.: choose safe values across a range.
- Keep constantcontrol variableSomething you keep the same in an investigation so the test is fair, such as using the same amount of water each time.: protect the comparison from rival explanations.
- Measuredependent variableThe thing you measure or observe in an investigation to see if it changes.: use the same defined method each time.
- InterpretAttribute a pattern cautiously to the changed factor.
Fair test: what changes a pendulum’s swing?
Pendulum A is the control. Change pendulum B. A fair test changes only one variable (the independent variable) and keeps the others the same, so any change in the time for a swing is caused by that variable.
Controls
Control variables for A: 50 cm, 100 g, 10°. Timing 10 swings and dividing by 10 makes the reading more accurate.
Identify the hidden variable
A student tests lamp distance and plant growth. The nearest plant also receives twice as much water.
- independent variableThe one thing you choose to change in an investigation.: distance from lamp.
- dependent variableThe thing you measure or observe in an investigation to see if it changes.: growth, defined by a measurement such as height increase.
- Water volume also changed and can affect growth.
- Keep water volume the same or measure and control it in the design.
How to make the reasoning visible: Name how the uncontrolled factor could affect the outcome, not merely that it is “unfair”.
Dissolving investigation
Plan variables for testing how water temperature affects the time for 5.0 g of sugar to disappear.
| Role | Variable | How handled |
|---|---|---|
| Independent | Water temperature / °C | 20, 30, 40, 50, 60 |
| Dependent | Dissolving time / s | Same endpoint and timing rule |
| Control | Water volume / mL | Use 100 mL each time |
| Control | Sugar mass and particle size | 5.0 g from same batch |
| Control | Stirring | Same speed or no stirring |
Why should all the sugar crystals be the same size? (Particle size means how big each crystal is.)
Why is testing five temperatures better than testing only 20 °C and 60 °C?
Name two control variables for the dissolving investigation, and say why each must be kept the same.
Check your understanding
Answer from memory, then check against the suggested answer.
Which variable do you change on purpose?
Which variable do you measure?
Why do you keep the control variables the same?
What is the dependent variable?
What is a control group?
A toy car rolls down a ramp onto different surfaces. You measure how far it travels. Name two control variableSomething you keep the same in an investigation so the test is fair, such as using the same amount of water each time. (things you keep the same).
Scientists studying plants in a field cannot control the weather. How can they still collect useful evidence?
A student tests seeds watered with fertiliser. Why should they also grow some seeds with plain water?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student tests how the height of a ramp affects the speed of a trolley. They use the same trolley each time and let it go the same way each time.
Why should the student use the same trolley each time?
(1)Why should the trolley be let go, not pushed, the same way each time?
(1)Finish the sentence: “Keeping these things the same means that any change in speed is caused by …”
(1)Total for this question: 3 marks
Command word: Do not stop at “to make it fair”. Say what could change the result if you did not keep it the same.