9.8 - Planning an Investigation
What a planning question looks like
- The N5 exam asks you to plan an investigation, and the assignment has you plan and carry out one of your own.
A typical question
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.
There's no single "right" experiment
- What's judged is not a model method but whether your plan contains the ingredients any good plan has.
- There are more possible marks across those ingredients than you strictly need, so run through the checklist and give each part.
The ingredients of a good plan
- The variable you change
- The variable you measure
- The variables you keep the same
- The apparatus
- The method
- The repeats
- A safety precaution
Because these ingredients are the same whatever the experiment, a plan becomes a checklist you work through rather than a blank page you face.
The ingredients of a plan
- A plan that scores works through the same ingredients every time - turn each one into a sentence about the experiment in front of you and you have a full answer.
- Written out as sentences, these ingredients form a complete plan. Keep each one short: the marks are spread across all the ingredients, so a plan that runs the whole checklist briefly beats one that writes a paragraph on the method and forgets the constants and the repeats.
The ingredients, worked through an example
The example below has the aim of finding how temperature affects how fast dye rises up a celery stick.
| Ingredient | What it means | Worked example (dye up celery) |
|---|---|---|
| The variable you change | The one thing you change, given a range of about five values in even steps | Temperature: 10, 20, 30, 40, and 50 °C |
| The variable you measure | The thing you measure, and how you measure it | Distance the dye rises up each stick after 30 minutes, in mm |
| Controlled variables | The things kept the same for a fair test, each named specifically | Same length and type of celery; same colour, concentration, and volume of dye; same time in the dye |
| Apparatus | The right instrument for each variable measured or controlled | A ruler to measure the distance, a water bath to hold each temperature, a thermometer to check it |
| Method | Enough detail to follow, including any step that lets things reach temperature | Stand each stick in the 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 |
| Repeats | Repeat at each value and take a mean, for reliability | Three sticks at each temperature, then the mean distance |
| Safety | A hazard together with a way to reduce it | The knife is sharp, so cut on a flat surface, away from your fingers |
What makes the test fair and reliable
- Fair test - 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 the temperature and nothing else - a fair test with a valid result.
- Reliable result - Repeating and taking a mean is what makes the result reliable. A single reading might be an anomaly, and consistent values in the repeats show the result is reliable.
Choosing the right apparatus
- Choosing apparatus is part of any plan, and naming the right instrument for each variable scores where a vague answer does not.
- Work from the variable: ask what the experiment needs to measure or control, then name the standard piece of equipment for that job.
- The variable decides the apparatus, so always start from the variable.
The right apparatus for each job
| Job to do | The right apparatus | Example in an experiment |
|---|---|---|
| Measure a volume of liquid | Measuring cylinder (or syringe, or graduated pipette) | Measuring out 25 cm3 of hydrogen peroxide for a catalase experiment |
| Keep the temperature constant | Thermostatically controlled water bath | Holding an enzyme and its substrate at 40 °C |
| Measure temperature | Thermometer | Reading the temperature of the water bath |
| Measure mass | Balance | Weighing a potato cylinder before and after soaking in a sugar solution |
| Measure time | Stopclock (stopwatch) | Timing how long starch takes to disappear |
| Measure length or distance | Ruler | Measuring how far a dye has risen up a stalk of celery |
| Measure a volume of gas | Gas syringe, or a measuring cylinder inverted over water | Collecting the carbon dioxide from respiring yeast |
Two things make the answer score
- Name a specific instrument, not a vague one - "a measuring cylinder" scores where "a container" does not.
- Match the instrument to the precision the job needs - A measuring cylinder measures a volume accurately, where a beaker only gives a rough amount.
Improving a method by choosing better apparatus
- The same logic answers "how would you improve this method?" questions that ask for better apparatus.
- Each time, swap a rough judgement for a proper measuring instrument:
- If the method counted gas bubbles by eye, improve it by measuring the volume of gas with a gas syringe.
- If it judged a mass roughly, improve it by weighing it on a balance.
Common exam questions
Question 1
A student plans an investigation into the effect of ethanol concentration on the growth of cress seedlings. Name the variable to change, one variable to control for validity, and say how to make the results reliable.
The variable to change is the ethanol concentration, given a range of values. A variable to control for validity is any factor kept the same, such as the number of seeds, the volume of solution, the temperature, or the light. To make the results reliable, repeat at each concentration and take a mean.
Question 2
Name the kinds of thing a good plan should include.
The variable to change with a range of values; a measurable variable to measure; the variables kept constant to make it a fair test; the apparatus and method; repeats and a mean for reliability; and a safety precaution.
Question 3
Why is "use different temperatures" a weak way to describe the variable you change 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 4
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 5
Name a piece of apparatus a student could use to measure out 25 cm3 of a liquid accurately.
A measuring cylinder (a syringe or graduated pipette is also accepted); a beaker only gives a rough amount, so it does not score.
Question 6
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".