13.6 - Drawing Conclusions

What a conclusion question looks like

The mark for a conclusion isn't for repeating a number from the table - it's for answering the question the experiment was set up to ask, using your results.

A typical example

Solution A and solution B are two samples of dilute hydrochloric acid at different concentrations. Each is titrated separately against 25 cm³ of the same aqueous potassium hydroxide:

• (a) Deduce which solution, A or B, was the more concentrated. [1]
• (b) Deduce how many times more concentrated it was than the other. [1]

How to answer each part:

1. (a) Say which one, using the data - the acid that needed the smaller volume to reach the end point is the more concentrated.
2. (b) Turn the comparison into a number by dividing one titre by the other.

Put together, the conclusion is: "solution A was more concentrated, because it needed a smaller volume to react completely, and it was twice as concentrated as solution B."

A typical question

(a) Deduce which solution of dilute hydrochloric acid, A or B, was the more concentrated. [1]

(b) Deduce how many times more concentrated this solution was than the other one. [1]

State a conclusion that answers the question

• A conclusion answers the question the experiment set out to ask, using the results you have.
• A conclusion always names the two things being compared or linked, not just a bare reading:
  • "The titre was 12.5 cm³" is a reading.
  • "Solution A needed less acid, so it was the more concentrated solution" is a conclusion.

The three shapes a conclusion can take

1. A comparison - Says which of two measured things is greater, using the data (e.g. solution A needed a smaller volume to reach the end point than B, so A is more concentrated).
2. A direction - Links a changed variable to a measured one (e.g. "as temperature increases, solubility increases").
3. A number - Gives a figure worked out from the data (e.g. if A's titre was half of B's, A was twice as concentrated).

A conclusion is not an explanation

• A conclusion says what the results show.
• An explanation says why they show it.
• Mark schemes treat these as two separate points, each worth its own mark.

Example - Predict and explain

Aqueous sodium carbonate is added in increasing amounts to a fixed volume of aqueous barium nitrate. Predict what happens if even more sodium carbonate is added, and explain your answer:

1. The prediction (conclusion) - The precipitate height stays the same, at the value it has already levelled off to.
2. The explanation - All the barium nitrate has reacted, so adding extra sodium carbonate leaves nothing for it to react with.

The conclusion states what happens to the measurement; the explanation states the chemistry behind it.

Common exam questions

Question 1

Two solutions of acid are each titrated against the same volume of alkali. Solution A needs 12.5 cm³ and solution B needs 25.0 cm³. What conclusion can you draw?

Solution A is more concentrated than solution B, because a smaller volume of A was needed to react with the same amount of alkali; A is twice as concentrated as B, since B needed exactly double the volume.

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Question 2

A student answers "the volume of solution A used was 12.5 cm³". Why does this not score as a conclusion?

Because it only states a single reading and does not say what that reading means for the question asked, which was which solution is more concentrated; a conclusion has to answer the question, not just repeat a number from the table.

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Question 3

A solubility graph shows solubility rising steadily as temperature rises. Write a conclusion.

As the temperature increases, the solubility increases. The reverse wording, that solubility decreases as temperature decreases, describes the same relationship and scores equally well.

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Question 4

A precipitate's height rises then stays constant as more reagent is added. A student is asked to predict what would happen with even more reagent, and to explain the prediction. What does each part need?

The prediction needs to say that the height would stay the same, at the value it has already reached. The explanation needs to say why: the other reactant has already been fully used up, so there is nothing left for the extra reagent to react with.

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Question 5

Why does giving only the explanation, with no stated outcome, not earn full marks on a "predict and explain" question?

Because the prediction and the explanation are marked as two separate points; a correct reason with no stated result only answers half of what was asked.

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Question 6

A student is given the results of two experiments and asked to deduce which used the more concentrated acid, then how many times more concentrated it was. What kind of conclusion is each part?

The first is a comparison, saying which of the two is greater based on the results. The second turns that comparison into a number, worked out by dividing one result by the other.