KS3 LOGIC LAB · PLANNING

Design an investigation.

When a question says “plan an investigation”, it wants to know you can run a fair test. One word helps you remember everything: CORMMSS.

What is a fair test?

In a fair test you change one thing, measure what happens, and keep everything else the same. Then, if the results change, you know what caused it.

Independent variable

The one you change.

Yeast example: Temperature of the water bath.

Dependent variable

The one you measure.

Yeast example: Number of bubbles in one minute.

Control variables

The ones you keep the same, so the test is fair.

Yeast example: Mass of yeast, volume of sugar solution, type of sugar.

A control is different again: an extra test where the thing you are testing is missing, such as a tube with no yeast. It shows that your result really comes from the thing you are testing.

The seven letters: C·O·R·M·M·S·S

Write one sentence for each letter. That gives a full plan, and makes it easy to check you have not missed anything.

Change
Pick the one thing you will change on purpose. Use at least three different values, like 20 °C, 30 °C and 40 °C. Sometimes add a control: a test where the thing is missing, so you can compare.
For example: Change the temperature of the water: 20, 30, 40 and 50 °C.
Organism
If you use living things, make them as alike as you can: the same type, and the same age or size.
For example: Use the same type of yeast, from the same packet.
Repeat
Do each test more than once, then work out the mean (add the results and divide by how many there are).
For example: Do each temperature three times and find the mean.
Measure (what)
Say what you will measure and how you will measure it. Name the equipment and the unit.
For example: Count the bubbles of gas that come out of the tube.
Measure (when)
Say when you will measure, or for how long.
For example: Count for one minute, after waiting two minutes for the yeast to warm up.
Same (1)
Name something you will keep the same, and say how.
For example: Use the same mass of yeast: 2 g, weighed on a balance.
Same (2)
Name a second thing you will keep the same, and say how.
For example: Use the same volume of sugar solution: 20 cm³, measured with a measuring cylinder.

Worked example · illustrative values

Worked example: does temperature change how fast yeast respires?

The question

Plan an investigation to find out how temperature affects how quickly yeast respires. Yeast makes carbon dioxide gas when it respires, so a faster rate gives more bubbles.

Change: I will put the tubes in water baths at 20, 30, 40 and 50 °C. As a control, I will set up one tube with sugar solution but no yeast, to show the bubbles come from the yeast.
Organism: I will use the same type of dried yeast, all from the same packet.
Repeat: I will do each temperature three times and work out the mean number of bubbles.
Measure (what): I will count the bubbles of gas coming out of the end of a delivery tube into a beaker of water.
Measure (when): I will leave each tube in its water bath for 5 minutes to warm up, then count the bubbles for 1 minute using a stopwatch.
Same (1): I will use the same mass of yeast, 2 g, weighed on a balance.
Same (2): I will use the same volume and concentration of sugar solution, 20 cm³ of 5% solution, measured with a measuring cylinder.

Every letter of CORMMSS has its own sentence. Each “same” says how it is kept the same, not just “keep it the same”. The numbers here are illustrative: your teacher may use different ones.

Check your plan

I named the one thing I will change, with at least three values.
I added a control if it helps the comparison.
If I used living things, they are the same type, and the same age or size.
I will repeat each value and calculate a mean.
I said what I will measure, with the equipment and the unit.
I said when I will measure, or for how long.
I named two things to keep the same, and how I will keep each one the same.

Have a go

Plan each one with CORMMSS. Use the hints if you get stuck, then compare your plan with the ideas.

Practice 1

Pondweed and light

Your turn

Plan an investigation to find out how the distance of a lamp affects how fast pondweed makes oxygen bubbles.

Write your plan on paper first, one sentence per letter. Then open the hints or the points.

Hints

C: what is the one thing you change? Give at least three distances.
M1 and M2: what can you count, and for how long?
S: what else could change the number of bubbles? Think about the water and the plant.

Ideas for a good plan

C: put the lamp at 10, 20, 30 and 40 cm, measured with a ruler.
O: use the same type of pondweed, cut to the same length.
R: repeat each distance three times and find the mean.
M1: count the oxygen bubbles given off.
M2: wait 2 minutes at each new distance, then count for 1 minute.
S1: keep the water temperature the same, for example with a beaker of water between the lamp and the plant, checked with a thermometer.
S2: use the same lamp and bulb each time.

Practice 2

Cress and plant food

Your turn

Plan an investigation to find out how the amount of plant food in the water affects how tall cress seedlings grow.

Write your plan on paper first, one sentence per letter. Then open the hints or the points.

Hints

C: you need at least three amounts, plus one with no plant food at all.
O: how will you make the seeds alike?
M2: how many days will you let them grow?

Ideas for a good plan

C: water with 0, 1, 2 and 4 drops of plant food per 100 cm³.
O: use cress seeds of the same type, from the same packet.
R: grow several seedlings in each dish and find the mean height.
M1: measure the height of each seedling with a ruler, in mm.
M2: measure after 7 days.
S1: give each dish the same volume of water each day, measured with a measuring cylinder.
S2: keep all the dishes in the same place, so they get the same light and temperature.

Practice 3

Potato in salty water

Your turn

Plan an investigation to find out how the concentration of salt solution affects the mass of potato pieces.

Write your plan on paper first, one sentence per letter. Then open the hints or the points.

Hints

C: list at least three concentrations, and include pure water.
M1: what do you weigh, and what do you work out?
S: think about the size of the potato pieces.

Ideas for a good plan

C: use salt solutions of 0, 2, 4 and 6%.
O: cut all the pieces from the same potato.
R: use three pieces in each solution and find the mean change.
M1: weigh each piece on a balance before and after, then work out the change in mass.
M2: leave the pieces in the solutions for 30 minutes.
S1: cut the pieces to the same size with a cork borer and a ruler.
S2: use the same volume of solution, 20 cm³, in each tube.

Learn more in your lessons

You will use CORMMSS again at GCSE and A level, where the plans get more detailed.