The importance of using clear English in IB chemistry
Why using clear English is important when communicating in chemistry. This blog gives examples of two past IB questions where the language used to phrase the questions can lead to different interpretations creating difficulty in knowing which answer(s) are required.
Why language is important
The importance of using the correct language of chemistry itself is covered in the section on the Language of Chemistry. However it is equally important to be able to write IB chemistry using language that is clear and completely free of ambiguity, particularly as many students are non-native speakers of English. Even if a student's ability to speak and write English is excellent there are subtle differences between the English spoken and written in different parts of the world. This creates some challenges when writing this website which ends up as a bit of a mixture of various types of English. Essentially I try to follow the language used in the guide so, for example, I use colour, sulfur, formulas and aluminium whereas other speakers of English may use color, sulphur, formulae and aluminum. In the new Student access part of the site one of the top headings is labelled Review/revision - this is because review and revision have very different meanings in US English compared to UK English. Usually individual words do not cause too many problems, but sentence construction can.
Constructing unambiguous questions
1. Phrasing the question clearly
A teacher1 contacted me about a question on this year's HL TZ3 Paper 2. The question was part (b)(ii) of Question 3.
One of her students asked, "How can the pressure increase while both the volume and temperature are constant, as there is no way for that to happen?"
When I look at the question it seems to me that it can be interpreted in different ways. It could be interpreted as the volume remaining constant as both the pressure and temperature are increased. Normally one would expect from applying the gas laws that changing both the pressure and temperature for a fixed mass of gas would also change the volume so in sense this is an impossibility. Or, is the examiner actually asking how increasing the pressure will affect the equilibrium position and the value of the equilibrium constant if both the volume and temperature are kept constant at the same time? This is how the student read it. However you cannot keep both the volume and temperature constant at the same time if you increase the pressure - as the student said, "There is no way for that to happen". An alternative interpretation from how the sentence is worded is that "constant" applies to both the volume and the temperature, but not both at the same time. This might be best interpreted as two separate questions. (1) How does increasing the pressure at constant volume affect ....? and (2) How does increasing the pressure at constant temperature affect ....?
The markscheme has not yet been made available but I suspect it is likely that the two separate questions are what was intended. One mark might be given for writing “The equilibrium position shifts to the reactant side since there will be less moles of gas” and the other mark for writing "the value of the equilibrium constant does not change as its value only changes when the temperature changes". It will be interesting to see the published markscheme and the subject report when they are released.
Update: October 2026
This is what is written in the subject report for this question:
“(b)(ii) Surprisingly, many students struggled with explaining how increasing the pressure affects the equilibrium position and constant. Some students gained one mark for stating the effect on both without explanation. The average mark was 0.7 marks out of 2. A couple of teachers mentioned that the experimental setup of how the pressure was increased while temperature and volume were kept constant was not clarified”
It would have caused much less confusion if the question had been split into two parts. In fact it is possible to keep the volume constant if the pressure and temperature are both increased at the same time. If you increase the pressure, the volume will get smaller. If you increase the temperature, the volume will get bigger. If you do both of these at the same time so that they affect the volume by the same amount then the volume will remain constant. However students who interpreted the question in this way would find it difficult to predict how this would affect the position of equilibrium.
2. Wrongly using a word
Sometimes a single word can be used wrongly which will affect the answer required. Consider the following question2 on one of the 2023 specimen papers.
HL Paper 2 Qu3(b)(ii).
The markscheme gives one mark for ΔGrecharge > ΔGdischarge
The word "absolute" in the question is presumably being used to show that ΔG with a positive value is greater than ΔG with a negative value. This is confirmed by the accompanying teachers notes where it states "No data is given - the question tests the concept that discharge reactions are spontaneous (negative ΔG), while recharge reactions are non-spontaneous (positive ΔG).
However this is the wrong use of the word "absolute". The + and − values are a convention, they are not absolute. The absolute value (or modulus) |ΔG| is the non-negative value of ΔG without regard to its sign. The absolute amount of energy change is the same for recharging and discharging so the correct answer is ΔGrecharge = ΔGdischarge. In fact perhaps an even better answer would be |ΔGrecharge| = |ΔGdischarge|.
The two examples above demonstrate how important it is for IB examiners to carefully consider not only correct chemistry when writing examination questions but also word them so they cannot be misinterpreted.
1,2. I am grateful to Anthi Kotsiliou and Susie Wong who brought these two questions to my attention.