22.8 - Planning an Investigation
What a planning question looks like
- Every Cambridge practical paper asks you to plan an investigation - usually the last part of Question 1, sometimes the start of Question 2.
- It's always worth 6 marks - more than any other single part - so it's where the most marks are won or lost.
- A typical question reads:
A student wants to find out how temperature affects the rate at which a coloured dye moves up a stick of celery. Plan an investigation the student could carry out. [6]
There's no single "right" method
- The examiner isn't marking a model experiment - there's no fixed correct answer.
- They're marking whether your plan contains the six ingredients any good plan has:
- The variable you change
- The variable you measure
- The variables you keep the same
- The method
- The repeats
- A safety precaution
- There are more than six possible marks across those ingredients, so you don't need every point - you need any six.
Treat it as a checklist, not a blank page
Because the ingredients are the same whatever the experiment, a plan becomes a checklist you run through rather than a blank page you face.
The six ingredients of an investigation plan
- A full-mark plan works through the same six ingredients every time. Turn each one into a sentence about the experiment in front of you and you have a complete answer.
- The marks are spread across all six, so a plan that runs the whole checklist briefly beats one that writes a paragraph on the method but forgets the constants and repeats.
The checklist
Worked through for an investigation into how temperature affects how fast dye rises up a celery stick:
| Ingredient | What it means | Example (dye up celery) | Marks |
|---|---|---|---|
| Independent variable | The one thing you change, given a range of values in even steps. The mark scheme credits at least two different values, but aim for about five | Temperature: 10, 20, 30, 40, and 50 °C | 1 |
| Dependent variable | The thing you measure, and how you measure it | Distance the dye rises up each stick after 30 minutes, in mm | 1 |
| Controlled variables | The things kept the same, each named specifically | Same length and type of celery; same colour, concentration, and volume of dye; same time in the dye | up to 3 |
| Control (where the experiment has one) | The same set-up with the factor under test removed, so you can show that nothing else caused the result | Not needed here, because temperature cannot be removed; in an enzyme experiment it would be a tube with boiled enzyme, or water in place of the enzyme | 1, where one applies |
| Method | Enough detail to follow, including any step that lets things reach temperature | Stand each stick in dye, hold each at its set temperature in a water bath, leave all for 30 minutes, then cut each stick and measure how far the dye has risen | 1-2 |
| Repeats | Repeat at each value and take a mean | Three sticks at each temperature, then the mean distance | 1 |
| Safety | A hazard together with a way to reduce it | The knife is sharp, so cut on a flat surface, away from your fingers | 1 |
What makes the investigation work
- Valid comparison - Keeping the controlled variables the same is the point of the plan. If only the temperature changes and everything else is held still, any difference in how far the dye rises must be down to temperature and nothing else.
- Reliable result - Repeating and taking a mean is what makes the result reliable. A single reading might be an anomaly, and repeats that give much the same value show the result can be trusted.
Common exam questions
Question 1
A planning question is worth 6 marks. Name the six kinds of thing a plan should include.
The independent variable with a range of values; a measurable dependent variable; the variables kept constant; the method; repeats and a mean; and a safety precaution.
Question 2
Why is "use different temperatures" a weak way to describe the independent variable in a plan?
Because it gives no range; a good answer states several specific values in even steps, such as 10, 20, 30, 40, and 50 °C.
Question 3
A student writes "keep everything else the same" for the controlled variables. Why does this not score?
The controlled variables must be named specifically, such as the same length of celery and the same volume and concentration of dye; a general phrase does not identify any of them.
Question 4
In a plan, why should the experiment be repeated and a mean taken?
Because a single reading could be an anomaly, and repeats that give much the same value make the result more reliable.
Question 5
A plan says "the knife is sharp" as its safety point. What is missing?
A way of reducing the hazard; the hazard alone scores nothing, so it needs something like "cut on a flat surface, away from your fingers".