Every investigation question has three parts: one thing you change, one thing you measure, and everything else you keep the same. A comparison is fair only when just the changed factor differs between the set-ups.
This lesson belongs to laboratory safety and investigation. It pairs well with reading measurements and reporting units.
How do I name the three kinds of variable?
Use three questions in this order. They work on any experiment, however long the description is.
- What does the experimenter change on purpose? That is the manipulated variable.
- What is measured or observed to see the effect? That is the responding variable.
- What must stay the same so the test is fair? Those are the fixed variables.
Always write the variable together with its unit or its description. “Temperature of water (°C)” is better than “temperature”.
Worked example: sugar dissolving
Aina wants to find out whether the temperature of water affects how fast sugar dissolves. She has three beakers.
| Beaker | Water | Temperature | Sugar | Stirring |
|---|---|---|---|---|
| A | 100 cm³ | 30 °C | 5 g | 10 times |
| B | 100 cm³ | 50 °C | 5 g | 10 times |
| C | 200 cm³ | 70 °C | 5 g | 10 times |
She records the time for all the sugar to disappear.
Manipulated variable: temperature of the water. It differs in every beaker on purpose.
Responding variable: the time taken for the sugar to dissolve, in seconds.
Fixed variables: mass of sugar, number of stirs and volume of water. But look at beaker C. It has 200 cm³, not 100 cm³, so the volume was not kept fixed.
What makes the comparison unfair?
Beaker C changed two things at once, the temperature and the volume of water. If C dissolves slowly, you cannot tell whether the extra water or the higher temperature caused it.
The correction is to use 100 cm³ of water in all three beakers. Then only temperature differs, and any change in time is linked to temperature alone.
The mistake that costs marks
A common slip is to write the responding variable as “the sugar” or “how well it dissolves”. The mark scheme wants something measurable, such as “time taken for the sugar to dissolve”.
| Wrong | Better |
|---|---|
| Responding variable: sugar | Responding variable: time taken for the sugar to dissolve (s) |
| Fixed variable: the water | Fixed variable: volume of water (100 cm³) |
| Manipulated variable: hot and cold | Manipulated variable: temperature of the water (°C) |
A quick test for a good answer: could another student repeat your experiment using only what you wrote?
A tip for paragraph-style questions
When the experiment is described in a paragraph, underline the verbs. “Changes”, “uses different” and “compares” point to the manipulated variable. “Measures”, “records” and “counts” point to the responding variable.
Words such as “the same each time” or “of equal size” point to the fixed variables. Use the graph evidence and fair-comparison lab to practise spotting them on new data.
Check yourself
A student tests whether the colour of a light affects the height of bean seedlings after 7 days. She uses red, green and blue light. Each pot gets 20 seeds, but the blue pot is placed on a sunny windowsill. Name the three kinds of variable and state what is unfair.
Answer
Manipulated variable: colour of the light. Responding variable: height of the seedlings after 7 days (cm). Fixed variables: number of seeds, amount of water, type of soil and duration of light.
Unfair: the blue pot also receives sunlight, so the light is not only blue. Place all the pots in the same dark room, lit only by the coloured lamps, at the same distance.
What to study next
Once you can name variables reliably, move on to evaluating a science claim from a supplied dataset and test yourself with the laboratory safety and investigation practice set.
If you want a teacher to go through your own investigation questions, see online one-to-one Science tuition.