A testable hypothesis names what you will change, what you will measure, and the direction you expect. Without all three, an experiment cannot test it.
This lesson belongs to biological investigation. The next skill, identifying variables and controls, depends on it.
What goes into a hypothesis?
Use one frame and fill the gaps: “When the [manipulated variable] increases, the [responding variable] increases or decreases.” Then check that both variables can be measured.
| Part | Question to ask | Example |
|---|---|---|
| Manipulated variable | What will I change? | Light intensity |
| Responding variable | What will I measure? | Number of gas bubbles per minute |
| Direction | Up, down or no change? | Increases |
Worked example: vague versus testable
Situation. A student investigates how light affects photosynthesis in a water plant.
Vague: “Light affects photosynthesis.”
This cannot be tested properly. It does not say how light changes, what to measure or which way the result should go.
Testable: “The higher the light intensity, the higher the rate of photosynthesis, measured by the number of oxygen bubbles released per minute.”
The second version names the manipulated variable (light intensity), the responding variable (bubbles per minute) and the direction (higher). A teacher can now design a method from it.
The mistake that costs marks
The slip is to state only one variable: “Temperature affects enzyme activity.” It feels scientific but leaves out what you measure.
Add the measure: “As temperature increases from 20 °C to 40 °C, the time taken for the starch to be digested decreases.” The hypothesis now names temperature, time taken and the direction.
Check yourself
A student wants to test whether the concentration of sugar solution affects the mass of potato strips after 30 minutes. Write a testable hypothesis.
Answer
“The higher the concentration of sugar solution, the greater the decrease in mass of the potato strips after 30 minutes.”
The manipulated variable is sugar concentration, the responding variable is change in mass, and the direction is given.
What to study next
Move on to identifying variables and controls, then try the biological investigation practice. If you want a teacher to rewrite your hypotheses with you, see online one-to-one Biology tuition.