9.6 - Drawing Conclusions
What a conclusion question looks like
A conclusion question asks you to state the relationship between the quantities tested in an experiment.
A typical question
A student measured the average boiling point of sodium chloride solution at three different concentrations. The aim was to find out how the concentration of a sodium chloride solution affects its boiling point. Suggest a conclusion based on the results.
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A conclusion must:
- Name both quantities - The concentration of the solution and its boiling point.
- Give the direction of the link, in the same order as the aim.
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Put together: "as the concentration of the sodium chloride solution increases, the boiling point increases".
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Always write the conclusion with reference to the aim - link the two named quantities and check that it answers what the aim set out to find.
A typical question
A student measured the average boiling point of sodium chloride solution at three different concentrations. The student's aim was to find out how the concentration of a sodium chloride solution affects its boiling point. Suggest a conclusion for the experiment based on the student's results.
Writing a conclusion that answers the aim
- A conclusion answers the aim by stating how the quantity that was changed affected the quantity that was measured.
- It always names both quantities and states the direction of the relationship between them.
A frame for writing a conclusion
As the [quantity that was changed] increases, the [quantity that was measured] increases (or decreases).
For example: "as the concentration of the sodium chloride solution increases, the boiling point increases".
Rules for a valid conclusion
- Put the changed quantity first - "the concentration increases as the boiling point increases" gets no marks because it reverses cause and effect.
- Do not just quote a single reading - Stating "the boiling point was 108 °C" does not link the two quantities.
- Stay within the data collected - Do not claim the trend applies to untested substances or beyond the range of values tested.
Conclusion versus explanation
- A conclusion states what the results show.
- An explanation states why it happens, using underlying chemistry theory.
Example - Comparing two catalysts
In an investigation into how two catalysts affect the rate of a reaction, where catalyst B gives a faster rate than catalyst A:
- Conclusion - The reaction is faster with catalyst B than with catalyst A. This comes directly from the results and answers the aim.
- Explanation - Catalyst B provides a reaction pathway with a lower activation energy than catalyst A, so a greater proportion of collisions are successful. This uses chemistry knowledge that isn't in the results table.
Matching the command word
- "State a conclusion" requires the trend or relationship shown by the results, naming both quantities.
- "Explain why" requires the chemistry reason behind the result (such as collision theory or activation energy).
Common exam questions
Question 1
A student's aim was to find out how the concentration of a sodium chloride solution affects its boiling point. Suggest a conclusion for the experiment.
As the concentration of the sodium chloride solution increases, the boiling point increases. Both quantities are named and the direction is given in the same order the aim tested them.
Question 2
Why does "the concentration increases as the boiling point increases" not earn the same credit as "as the concentration increases the boiling point increases"?
Because it states the cause and effect the wrong way round; the quantity that was deliberately changed, the concentration, must come first in the sentence.
Question 3
A student concludes "the average boiling point was 105 °C." Why does this not score as a conclusion?
Because it states a single reading and never mentions the concentration; a conclusion has to link the two quantities, for example by saying how the boiling point changed as the concentration changed.
Question 4
An investigation tested acid concentrations from 0.5 to 2.0 mol/l and found the rate of reaction increased throughout. Why would "the rate will keep increasing at any concentration" go too far as a conclusion?
Because the results only show what happened between 0.5 and 2.0 mol/l; claiming the same pattern continues at concentrations outside that range is not something the data can support.
Question 5
An investigation compares the rate of a reaction with two catalysts and finds catalyst B is faster. State a conclusion, and then give an explanation for it.
Conclusion: the reaction is faster with catalyst B than with catalyst A. Explanation: catalyst B lowers the activation energy of the reaction more than catalyst A does, so more particles have enough energy to react when they collide.
Question 6
A question asks a student to explain why increasing the temperature increases the rate of reaction, and the student answers "the rate goes up as the temperature goes up." Why does this not answer the question?
Because it restates the trend rather than explaining it; an explanation needs the chemistry reason, such as particles having more energy and colliding more frequently and more successfully as the temperature rises, not just a repeat of what the results show.