Most Common Topics on Leaving Cert Chemistry

Organic chemistry takes up more of the recent Leaving Cert Chemistry papers than any other topic. On the 22 Higher and Ordinary Level papers from 2015 to 2025 it holds 84 of the 242 questions and 4,200 of the 12,100 printed marks, and it reached every one of the 22. Seven further topic areas also reached all 22, so bare presence separates almost nothing. The sharper pattern in this data is positional. Chemistry printed its 11 questions in the same order every year, and several of those numbered slots held the same topic for the whole eleven years.

What is in the sample

The sample is every Higher and Ordinary Level Chemistry paper from 2015 to 2025 inclusive, broken into the questions printed on it. Eleven years at two levels gives 22 papers and 242 questions, 121 at each level.

Chemistry prints an unusually regular paper. Every one of the 22 papers set 11 questions worth 50 marks each, numbered 1 to 11. Questions 1 to 3 are Section A and questions 4 to 11 are Section B. Section A is the experiments section, where each question describes a laboratory procedure from the course.

Eleven questions at 50 marks each, on 22 papers, is the 12,100 printed marks used as the denominator throughout. Section A accounts for 3,300 of them and Section B for 8,800.

No candidate answered 12,100 marks, or even 550 on one paper. The rubric decides how much of the printed paper reaches a script, and it moved twice in these eleven years.

The 2021 and 2022 cut to six questions is one of the assessment adjustments the Exam Material Archive flags on papers from 2021 onwards, made after learning was disrupted during Covid. The compulsory two-from-Section-A requirement went at the same time and has not come back. A candidate in 2025 could in principle have answered eight Section B questions and no experiment question at all.

A different course is examined from 2027

All 22 papers were set on the 2009 Chemistry syllabus, whose final examination is June 2026. Chemistry is one of the Tranche 1 subjects under Senior Cycle Redevelopment, introduced to schools in September 2025. The SEC says its April 2025 sample papers "are applicable to candidates sitting the Leaving Certificate examinations in 2027" (SEC Senior Cycle Redevelopment FAQ). The sample papers themselves carry 2027 exam codes.

The assessment changes with it. From 2027 the written paper is worth 300 marks over 2 hours 30 minutes, printing seven questions of 50 marks each. Question 1 is compulsory and any five of the other six are answered. Alongside it sits the Chemistry in Practice Investigation, worth 200 marks, which is 40 percent of the subject (SEC sample investigation brief). Sections A and B are gone.

Read everything below as a record of chemistry content under the old syllabus. The section splits and the question-number patterns belong to a paper format whose last outing is June 2026.

From tags to thirteen strands

Each question was labelled during extraction with freeform tags as fine as le chatelier's principle, flame tests or fractional distillation. The 22 papers produced 277 distinct tags, 139 of which occur on a single paper, so counting at that grain is mostly counting one-offs.

Those labels roll up into 13 strands derived from the Chemistry syllabus. The same 13 organise the Chemistry topic pages, so each row in the table below links to the page holding exactly the questions it counts.

Chemistry questions mix strands heavily. Of the 242 questions, 68 sit in one strand, 102 in two, 54 in three, 13 in four, and 2 in five or more. The remaining 3 are the Higher Level short-answer question from 2023, 2024 and 2025, which ranges over the whole course and carries no strand. A question's full 50 marks are credited to each strand it touches, so the mark shares below total well above 100 percent.

Every paper carried at least eleven of the thirteen strands

A paper counts once in the papers column if the strand appeared on it at all, whether in one question or four. Marks are the printed marks on those questions, and share is that figure over 12,100.

Twelve of the thirteen strands turned up in every one of the eleven years, at one level or the other. Industrial chemistry is the single exception at seven years, and it is also the only strand below 16 papers. No individual paper dropped more than two strands. The leanest papers cover 11 of the 13, and four of the 22 cover all 13.

Weight is where the strands actually separate. Organic chemistry holds 4,200 printed marks against 500 for industrial chemistry, a gap of more than eight times, while both remained examinable on every paper in the sample.

The two levels weight the course differently

Splitting the 22 papers into 11 Higher and 11 Ordinary changes two rows noticeably.

Thermochemistry appeared on all 11 Higher Level papers and on 5 of the 11 Ordinary Level papers, missing Ordinary Level in 2018, 2019, 2020, 2022, 2024 and 2025. It is the widest level gap in the data.

Laboratory skills goes the other way. It reached all 11 Ordinary Level papers and 8 of the 11 Higher Level papers, with 37 Ordinary Level questions against 15 Higher Level. Part of that gap is tagging, not content. The Section A questions on the 2019, 2021 and 2023 Higher Level papers describe experiments, but their tags name the chemistry and not the technique. Those three papers therefore fall outside the strand while still setting three experiment questions each.

Everything else is close. Organic chemistry has 43 Higher Level questions and 41 Ordinary Level, and atomic theory has 29 and 38.

Section A is the experiments section

Section A holds 3 of the 11 questions on every paper, 66 questions and 3,300 printed marks in total. The split below counts questions, and the rows are sorted by Section A.

Titrations are the cleanest case in the whole dataset. All 24 volumetric analysis questions are in Section A and none are in Section B. Twenty of the 22 papers set exactly one, and the 2017 and 2024 Higher Level papers set two. If you have practised titrations, you have practised a Section A question and nothing else.

Bonding and equilibrium sit at the opposite end. Chemical bonding has 43 of its 44 questions in Section B, and all 24 equilibrium questions are in Section B, so neither arrived attached to a laboratory procedure.

The same question numbers carried the same topics

Because the paper prints 11 questions in the same order every year, a strand can be traced to a slot rather than just to a paper. Several slots held the same strand for the whole eleven years.

• Question 6 covered organic chemistry on all 22 papers, and so did question 8. No other strand holds any slot at that rate.
• Question 5 covered atomic theory and the periodic table on 21 of the 22 papers, the exception being 2018 Ordinary Level.
• Question 10 covered stoichiometry and the mole on 20 of the 22, missing both 2016 papers.
• In Section A, question 1 or question 2 was a titration on every one of the 22 papers.

Question 11 is the loose one. Water and environmental chemistry, equilibrium, and redox each turned up there about half the time, and no strand held it more than 11 times out of 22.

Treat those slot counts as a description of how the old paper was laid out, not as a rule about the next one. They also stop being useful in 2027, when the numbered eleven-question paper goes.

What the choice rules took back out

Shares of printed marks measure the paper, not a script. In most of these years a candidate picked eight of the eleven questions, so a topic could sit on the paper and never be answered.

Working through each paper under its own rubric, most strands were avoidable every year. Equilibrium, redox, rates of reaction, thermochemistry, industrial chemistry, and acids and bases could each be dodged completely on all 22 papers while still answering the required number of questions.

Organic chemistry could not. On 15 of the 22 papers there were not eight questions free of it, so a full selection had to include at least one organic question. Laboratory skills closed off a selection on 10 of the 22, and atomic theory on 4, all four of those Ordinary Level.

That is the limit of the share column. It measures the paper as printed, and it overstates the exposure of anyone who steered around a topic.

Where the old topics sit in the 2027 specification

The redeveloped Chemistry specification is built from a unifying strand, The Nature of Science, and four contextual strands, Nature of Matter, Behaviour of Matter, Interactions of Matter, and Matter in our World. Those four hold sixteen numbered units between them.

Ten of the thirteen strands map onto named units without much argument, though atomic theory and bonding each span a pair of them.

The remaining three do not fit a single unit, and the biggest strand in the table is one of them.

Organic chemistry is split in two. The new specification puts hydrocarbons in unit 2.4, under Behaviour of Matter. It puts the reactivity of organic compounds in unit 4.2, under Matter in our World, two strands away. Heats of combustion, which the old syllabus taught alongside fuels, sit in 3.1 with the rest of thermochemistry. Our Organic Chemistry, Hydrocarbons and Fuels strand keeps those questions in one block, matching the old exam layout, so its 4,200 marks are not any single unit's worth of material.

Laboratory skills has no contextual unit. Its content belongs to unit U2, Investigating in chemistry, which sits in the unifying strand and runs through all four contextual strands. It also belongs to the Chemistry in Practice Investigation, where the 200 marks now are.

Industrial chemistry is the strand with no home at all. On the old syllabus it sat outside the core, as the option "1A. Additional Industrial Chemistry", and it is the one strand here that reached fewer than 16 of the 22 papers. The new specification has no industrial unit and no options, and it carries sustainability, health and technology as cross-cutting themes instead. Where industrial processes are examined from 2027, they will arrive inside some other unit.

One wrinkle sits inside a row that otherwise maps cleanly. Gas laws were taught with the mole on the old syllabus and our tagging counts them under Stoichiometry, the Mole, Formulas and Equations. The new specification gives them a unit of their own, 2.3 Behaviour of gases, in a different strand from quantifying matter.

A unit title confirms that a content area survives, and it is silent on depth, which the learning outcomes and the SEC sample papers set. Separately, the new paper prints seven questions and marks 300, against eleven and 400 on the old one, so the space any topic occupied before tells you nothing about its share of the new paper.

What the table cannot tell you

All 13 strands stay examinable in any given year, and eleven years of counts constrain the twelfth in no way at all. The State Examinations Commission has told candidates to treat any claim of advance knowledge of a paper as unreliable, and has described predictions as spurious. The quote and its date are in our post on whether Leaving Cert questions repeat.

The bottom row is where a table like this gets misused. Industrial chemistry is the thinnest strand at 10 questions across 22 papers, and it still took two questions on the 2018 Higher Level paper, at question 9 and question 11. It was absent from both 2025 papers, which tells you nothing about 2027.

One thing the counts describe with no inference at all is your supply of practice material. These are the papers you revise from, so a thin row is a strand you have had few chances to attempt. Industrial chemistry gives you 10 questions and 500 marks to work with. Organic chemistry gives you 84 questions and 4,200 marks. That imbalance is already inside your preparation, whatever next June holds.

Turning the table into revision

Work one strand at a time, across consecutive years, closed book, and mark every answer against the official scheme before moving on. Our guide on studying with past papers sets out that routine. Three or four questions into a strand, the chemistry is familiar and the presentation is not, and a single question cannot produce that.

Begin with whichever strand you have covered least, and let the section split decide the format. For laboratory work, volumetric analysis holds 24 Section A questions and laboratory skills and techniques holds another 38. For written theory, chemical bonding and chemical equilibrium between them have 67 Section B questions and 1 in Section A.

We have run the same count for Leaving Cert Biology, the other Tranche 1 science subject, and for Higher Level maths.

FAQ

What is the most common topic on Leaving Cert Chemistry? Organic chemistry, on both measures. Over the 22 Higher and Ordinary Level papers from 2015 to 2025 it takes 84 of the 242 questions and 4,200 of the 12,100 printed marks. It reached all 22 papers, and each of them placed an organic question at number 6 and another at number 8.

What comes up every year in Chemistry? Twelve of the thirteen strands turned up in every year from 2015 to 2025, and eight were on all 22 papers. Industrial chemistry is the one exception, reaching 9 papers spread over 7 years.

What is the least common topic? Industrial chemistry, at 10 questions and 500 of the 12,100 printed marks. Thermochemistry is next by marks at 950, and it reached all 11 Higher Level papers.

What comes up in Section A of the Chemistry paper? Section A is the experiments section, three questions on every paper. All 24 titration questions in the sample are there, along with 38 of the 52 laboratory skills questions and 24 organic chemistry questions.

Can I use this to predict the Chemistry paper? No. Every part of the course can be examined in any year, and the State Examinations Commission has warned against anything claiming to know a paper ahead of time. A frequency table earns its keep by showing you where your practice is thin.

Why do the mark shares add to more than 100 percent? Because 171 of the 242 questions sit in two or more strands, and every strand receives the whole 50 marks of a question it touches.

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All 242 questions counted above are on the Leaving Cert Chemistry topic pages, filed under these 13 strands and then by year, each one carrying its marking scheme. Free PDF packs cover every strand at Higher and Ordinary Level, and nothing there asks you to sign up. If you would rather sit a whole paper under timed conditions, begin at the past paper finder.