22.6 - Drawing Conclusions
What a conclusion question looks like
- Every Cambridge practical paper asks you to state a conclusion from the results.
- It's usually worth one mark, sometimes two if a second, separate conclusion is wanted.
- It's the part examiners flag most often as thrown away.
A typical question
A conclusion question gives you results (often in a table) and asks you to draw a link from them. For example:
A student counted the number of gas bubbles released each minute by yeast at two temperatures, 18 °C and 42 °C. The results are shown in the table. State a conclusion the student can make from these results. [1]
How to earn the mark
- The mark is not for repeating a number off the table.
- It's for a sentence that ties the two variables together. To earn it, your answer must do two things:
- Name both variables - The temperature and the rate (the number of bubbles), not just one of them.
- Give the direction of the link - As the temperature rises, the rate rises.
- Put together, that's a conclusion: "the higher the temperature, the higher the rate of respiration".
- This one idea - linking the variable you changed to the variable you measured - is the whole subtopic.
Link the variable you changed to the one you measured
- A conclusion answers the question the experiment set out to ask: how does the thing you changed affect the thing you measured.
- So a conclusion always mentions both variables and says which way they move together.
- The most reliable way to write one is to fill in a single frame:
As the [independent variable] increased, the [dependent variable] increased (or decreased).
- For the yeast experiment that gives "as the temperature increased, the rate of respiration increased".
- Cambridge accepts either wording - "the higher the temperature, the greater the rate of respiration" scores just as well.
- The mark scheme also allows the reverse argument (marked ora on the scheme), so "the lower the temperature, the lower the rate" is also fine.
The trap - Describing a single reading
- The trap is to describe a single reading instead. "The number of bubbles went up" only mentions the thing you measured, and never names the temperature.
- It doesn't link the two variables, so it doesn't score as a conclusion. A conclusion has to carry both names.
A conclusion is not the same as a reading
- Many papers have a separate part that asks you to read a value straight off the table or graph, such as "how many bubbles were released at 42 °C". That is a reading, and the answer is just the number.
- The conclusion part wants the relationship, not the number - so don't answer one with the other.
Evidence for your conclusion
- Sometimes the question goes one step further and asks for the evidence for your conclusion, or asks you to justify it.
- The answer is the readings or observations that show the link, quoted from the results - for the yeast experiment, the number of bubbles per minute at 18 °C set against the number at 42 °C, or, in a test that uses iodine, that the liquid stayed brown and did not turn blue-black.
- The syllabus wording is that conclusions are "justified by reference to observations and data", and the paper asks it as "State the evidence to support your conclusion".
- Writing the conclusion out a second time earns nothing here - the mark is for the data.
Common exam questions
Question 1
An experiment shows that a piece of potato loses more mass the more concentrated the sugar solution it sits in. Write a conclusion.
As the concentration of the sugar solution increased, the mass lost by the potato increased. Both variables are named and the direction is given.
Question 2
A student concludes "the number of bubbles was 40". Why does this not score as a conclusion?
Because it quotes a single reading and never mentions the variable that was changed; a conclusion must link the two variables, for example "as the light intensity rose, the number of bubbles rose".
Question 3
What is the difference between "read the number of bubbles at 30 °C" and "state a conclusion"?
The first asks for a single value read straight off the table, so the answer is just a number; the second asks how the two variables are related, so the answer links the temperature to the rate.
Question 4
An enzyme works fastest at 40 °C and slower at temperatures above and below it. Why can you not write "the higher the temperature, the faster the enzyme"?
Because the relationship is not a simple rise; the rate increases up to 40 °C and then falls, so the conclusion must say the rate was highest at 40 °C and lower on either side.
Question 5
A question asks for a second, different conclusion. Why does "as temperature decreases, the rate decreases" not earn it?
Because it is the first conclusion written backwards, not a new point; a second conclusion has to add something different, such as identifying the temperature at which the rate was greatest.
Question 6
A question asks you to state the evidence for your conclusion. What should the answer contain?
The readings or observations from the results that show the link, such as the number of bubbles per minute at each of the two temperatures; the conclusion itself, written out again, scores nothing here.