Less dense, not lighter: six words one chemistry paper refused

What the report says first

Paper 3 is the practical paper, and its examiner report is reliably the densest reading of the three, because practical questions are answered in words and words are where marks go missing. The June 2023 report says so in its opening paragraph.¹

That is the examiners’ own ranking of what went wrong: not the hard chemistry, and not the sums as such, but precision and terminology. The first question on the paper then illustrates it five times in a row.

Six rulings

1. Catalyst, not proton donor. Two thirds of students correctly named the role of the acid in an esterification as a catalyst. Some wrote oxidising agent or reducing agent, which are simply wrong. The interesting refusal is “proton donor”, which is nearly right, and the report explains exactly why nearly is not enough: it “was not allowed because, although the mode of catalytic action does involve proton donation, the proton is accepted back in a later stage of the mechanism.”¹ The acid does donate a proton. It also takes it back, and a substance that ends where it began has acted as a catalyst, which is the word that describes the whole story rather than its first step.

2. A positive answer, not a negative one. The stem said the mixture was flammable, and most students took the hint and named a water bath or an electric heater. Some wrote that you should not use a Bunsen burner. The report is blunt: “Note that a positive answer is needed so ‘not with a Bunsen burner’ did not score.”¹ Saying what you would not do names no method, and the mark was for the method.

3. Immiscible, not a difference in density. Only a quarter of students scored both marks on why two layers form. Many, the report says, thought “that a difference in density alone accounts for the formation of two layers. The key factor is that the organic and aqueous materials are immiscible.”¹ Density decides which layer is on top. Immiscibility is why there are two layers at all. Half the mechanism is half the marks.

4. Less dense, not lighter. And then, in the same sentence, the ruling this post is named for. The ethyl ethanoate forms the upper layer because it is “less dense (NOT lighter) than the aqueous layer”.¹ The capitals are the board’s. Lighter is a claim about mass, and a large enough volume of ethyl ethanoate is heavier than a small volume of water and still floats on it. The layers are decided by density, so the word is density.

5. Drying agent, not dehydration. Anhydrous calcium chloride is added to a wet organic product as a drying agent, and 64% of students said so. “A significant minority referred to ‘dehydration’ which is not an acceptable answer due to the alternate meanings of that term in Chemistry.”¹ In chemistry, dehydration is a reaction: an alcohol losing water to become an alkene, which happens to be the subject of the very next question. A drying agent removes water from a mixture. A dehydrating agent removes it from a molecule. The word points at the wrong one.

6. Displayed means displayed. In question 02.1 a displayed formula was asked for and only 36% gave one; the commonest reason for the lost mark was “that the OH group was not displayed”. The mirror image follows one part later, in a skeletal formula, where “the OH group must NOT be displayed”.¹ The two words name two conventions, and the mark is for using the one that was asked for.

What precision means to an examiner

None of the six refused answers is chemically illiterate. A proton donor is a real thing the acid does. A negative answer shows the student knew about flammability. Density is genuinely involved in the layers. Lighter is what a person would say. Dehydration is a chemistry word. An OH group written as a group is how every textbook writes it in running text.

What they have in common is that each one stops short of the thing the mark is for, or points at something next to it. The examiner has a list of statements they may credit, and the question they ask of a script is not “does this student understand?” but “is the creditable statement on the page?”. Precision, in the report’s sense, is the habit of writing the statement the mark is for rather than a true statement in its neighbourhood.

The report offers one more example of that habit in question 05.2, about polarity, where “many answers referred to differences in electronegativity without explicitly stating which atoms the difference was between or which way round the difference was”.¹ Electronegativity is a property of an atom. A difference has two ends. An answer that does not say which two atoms and which way round has described the idea and left the chemistry out.

What to write instead

• When a question asks for a role, name the role that describes the whole story: catalyst, not the first thing the substance does.
• When a question asks how, say what you would do, never what you would avoid. A method is a positive statement.
• When two liquids separate, give both halves: they are immiscible, so there are two layers, and the upper one is less dense.
• Drying is what you do to a wet product. Drying agent. Keep “dehydration” for the reaction it names.
• A displayed formula shows every atom and every bond, including the O to H. A skeletal one shows neither.
• Any time you write “difference in electronegativity”, follow it with the two atoms and which is greater.

Credited and refused

All from the one report, all AQA, all June 2023. Where the report gave a reason it is the reason below.