Writing a Hypothesis & Choosing a Method

Revise Writing a Hypothesis & Choosing a Method for Biology 3 (IAL) WBI13 (AS Level) — revision notes, 2 practice questions and instant AI marking.

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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

time taken 1 ÷ time

Easy marks

4. Justifying the Method, Not Just Describing It

why

Practice Questions

What to Memorise

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