Revise Writing a Hypothesis & Choosing a Method for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I
Writing a Hypothesis & Choosing a Method
🧭 Big idea: A hypothesis is not a guess about what will happen — it is a testable prediction with a direction and a reason. "Temperature affects enzyme activity" cannot be tested; "increasing temperature to 40 °C will increase the rate because molecules collide more often" can.
Summary — What This Topic Covers
What makes a hypothesis testable rather than just a statement
Giving a prediction a direction and a biological reason
Choosing the independent variable range and number of levels
Deciding what to measure, and how to turn it into a rate
Justifying a method rather than just describing it
1. What Makes a Hypothesis Testable
Key Term
A hypothesis is a testable statement predicting how the dependent variable will respond to a change in the independent variable, supported by a biological reason.
✗ "Temperature affects the rate of reaction."
— no direction, no reason, cannot be shown false
✗ "The enzyme will work better in the heat."
— "better" is not measurable
✓ "As temperature increases from 10 °C to 40 °C, the rate of oxygen
production will increase, because molecules gain kinetic energy and
collide with the active site more frequently."
Direction — say increase, decrease, or the shape of the relationship
Range — name the values you will actually use
Reason — the biology that explains the prediction
Falsifiable — a result that would prove it wrong must be possible
Common mistake
Predicting only what happens, never why. The biological reason is usually worth as much as the prediction itself.
2. Choosing the Range and the Levels
The rule
Use at least five values of the independent variable, spread across a range wide enough to show the trend — including, where relevant, values either side of an optimum.
Too narrow a range and every result looks the same. Too few values and a curve cannot be distinguished from a straight line. If the relationship might have an optimum — as enzyme activity does — the range must extend past it on both sides, or you will report "rate increases with temperature" and miss denaturation entirely.
✗ 35, 36, 37, 38 °C too narrow — no visible trend
✗ 20, 60 °C only two points — no shape
✓ 10, 20, 30, 40, 50, 60 °C wide range, six levels, spans the optimum
Worth remembering
A preliminary experiment is the standard justification for a chosen range, and naming one earns credit: "a preliminary run showed no product below 10 °C, so the range starts there."
3. Deciding What to Measure
The dependent variable has to be something you can put a number on. Vague outcomes cannot be graphed.
INSTEAD OF MEASURE
───────────────────────────────────────────────────────────────
"how fast it goes" volume of gas in cm³ per minute
"how much it changed" % change in mass
"the colour change" absorbance on a colorimeter
"how well it grew" diameter of the clear zone in mm
"how much starch was digested" time in s for iodine to stop
turning blue-black
Turning a measurement into a rate
rate = change ÷ time. Where you measure the time taken for something to happen, rate is proportional to 1 ÷ time. Plotting 1/t rather than t turns a curve into something readable.
Easy marks
Percentage change, not absolute change, whenever the starting values differ — potato cylinders are never all exactly the same mass, so % change is the only fair comparison.
4. Justifying the Method, Not Just Describing It
High-mark planning answers explain why each step is there. Every controlled variable should come with the reason it matters.
✗ "Keep the pH the same."
✓ "Use a pH 7 buffer throughout, because a change in pH would alter the
charges on the active site and change the rate independently of
temperature."
✗ "Repeat three times."
✓ "Repeat three times at each temperature and take a mean, so that the
effect of random variation between potato cylinders is reduced."
What mark schemes look for
The word "because" is doing the work. A method step without a reason is a description; with a reason it is a justification, and the higher band needs justification.
Practice Questions
Practice Question 1
A student writes: "Light intensity affects photosynthesis." Explain why this is not an acceptable hypothesis and rewrite it.
Practice Question 2
Explain why a student investigating enzyme activity between 35 °C and 40 °C might reach a misleading conclusion.
Practice Question 3
A student measures the time taken for a solution to turn from blue-black to brown. Explain how to convert this into a rate and why that is preferable.
Practice Question 4
Rewrite this method step as a justification: "Use the same volume of enzyme each time."
What to Memorise
Hypothesis = direction + range + reasonMust be falsifiableAt least five values of the IVRange must span any optimumDV must be numericalrate ∝ 1 ÷ time% change when starting values differEvery control needs a "because"
Concepts Checklist
Exam Tips
What mark schemes look for
Direction, range, reason. A prediction with no biological explanation sits in the lowest band however well written it is.
The trap
A range that stops at the optimum. If the biology has a peak, your range must go past it on both sides or your conclusion will be wrong.
Easy marks
"A preliminary experiment showed…" justifies almost any choice of range or concentration, and takes one sentence.
Worth remembering
Where the measurement is a time taken, the examiner is usually looking for 1/t. Plotting raw time is rarely the intended answer.