From 2027, O-Level Combined Science is sat as SEC G3 Science K326, K327 or K328. The papers and their marks, how much each half counts, two worked questions with marking points, the mistakes that cost grades, and what it means for JC H2 sciences.
From 2027, O-Level Combined Science is sat as SEC G3 Science under three new codes: K326 (Physics, Chemistry), K327 (Physics, Biology) and K328 (Chemistry, Biology), which replace 5086, 5087 and 5088. Every candidate sits four papers worth 200 marks: a 40-question multiple choice paper (20%), one structured paper for each of the two sciences (32.5% each) and a 1 hour 30 minute practical test set by SEAB (15%). Each science carries close to half of a single grade. In the 2028 Post-Secondary Education Exercise (PSE), that grade counts as one Science subject for JC and polytechnic admission.
The three combinations and the papers each one sits
The school fixes your child's combination at Sec 3. For the topics inside each science, see our guides to Chemistry, Physics and Biology. The pure or combined decision itself is covered in our Sec 1 subject combinations guide, so this guide sticks to doing well in the subject your child already has.
2027 SEC code
Combination
2026 O-Level code
Structured papers sat
K326
Science (Physics, Chemistry)
5086
Paper 2 Physics, Paper 3 Chemistry
K327
Science (Physics, Biology)
5087
Paper 2 Physics, Paper 4 Biology
K328
Science (Chemistry, Biology)
5088
Paper 3 Chemistry, Paper 4 Biology
Paper
What it is
Time
Marks
Weighting
1
Multiple choice, 40 questions on both sciences
1 h
40
20.0%
2
Structured and free response, Physics
1 h 15 min
65
32.5%
3
Structured and free response, Chemistry
1 h 15 min
65
32.5%
4
Structured and free response, Biology
1 h 15 min
65
32.5%
5
Practical test, both sciences
1 h 30 min
30
15.0%
Each structured paper has a compulsory Section A of 55 marks, ending with a 10-mark question, and a Section B of 10 marks where your child answers one of two questions. The Periodic Table is printed in Papers 1 and 3 for the Chemistry combinations.
How much each science counts toward the one grade
Weighting of each paper in K326 Science (Physics, Chemistry)
Paper 1 MCQ, both sciences20 %
Paper 2 Physics structured32.5 %
Paper 3 Chemistry structured32.5 %
Paper 5 Practical, both sciences15 %
02040
SEAB, K326, K327 and K328 G3 Science syllabus for 2027. K327 and K328 swap in Paper 4 Biology.
Each science gets 32.5% from its own structured paper and about 10% from its half of the MCQ. The practical has questions on both sciences, and SEAB does not publish how its 30 marks divide, so each half lands near 50%. The certificate shows one grade, so a weak Physics half pulls down a strong Chemistry half.
How the practical is assessed from 2027
G3 or G2 Combined Science: what changes
Past papers from one level are poor practice for the other.
G3 and G2 Combined Science from 2027
Point
G3 (K326, K327, K328)
G2 (K223, K224, K225)
Practical exam
Yes, Paper 5, 15%
No, experimental skills are about 10% of the written marks
Weight of each science
Close to half
Exactly 50%
MCQ
40 questions across both sciences
20 questions for each science
Structured paper
65 marks per science
30 marks per science
Counts for JC L1R4
Yes
No, L1R4 subjects must be G3
A worked Chemistry question: the limiting reactant
The syllabus says stoichiometry questions may involve limiting reactants, and that percentage yield and purity calculations are not required. We wrote this question in the style of a structured question.
Question. A student adds 2.4 g of magnesium to 50 cm³ of 1.0 mol/dm³ hydrochloric acid. The equation is Mg + 2HCl gives MgCl₂ + H₂. (Ar: Mg = 24)
(a) Show which reactant is in excess. [3] (b) Calculate the volume of hydrogen produced at room temperature and pressure. [2]
Working. Moles of Mg = 2.4 ÷ 24 = 0.100 mol. Moles of HCl = 1.0 × (50 ÷ 1000) = 0.0500 mol. The ratio is 1 Mg to 2 HCl, so 0.100 mol of Mg would need 0.200 mol of HCl. Only 0.0500 mol is present, so HCl is limiting and magnesium is in excess. Moles of H₂ = 0.0500 ÷ 2 = 0.0250 mol. Volume = 0.0250 × 24 = 0.600 dm³, or 600 cm³.
Marking point
What earns it
Where students drop it
M1
Moles of Mg = 0.100
No working shown
M2
Moles of HCl = 0.0500
Multiplying by 50 instead of 0.050 dm³
M3
Comparing with the 1 to 2 ratio, HCl limiting
Stating "HCl is limiting" with no comparison
M4
Moles of H₂ from HCl, 0.0250
Working from magnesium and getting 2.4 dm³
M5
0.600 dm³ with the unit
Leaving out the unit
These marking points are ours, written the way mark schemes usually reward working. The trap is M4: a student who works from magnesium gets four times the right answer.
A worked Physics question: a series and parallel circuit
Question. A 6.0 V battery with no internal resistance is connected to a 4.0 Ω resistor in series with a parallel pair of 6.0 Ω and 3.0 Ω resistors.
(a) Calculate the effective resistance of the circuit. [2] (b) Calculate the current in the 3.0 Ω resistor. [3] (c) Calculate the energy transferred in the 4.0 Ω resistor in 2.0 minutes. [2]
Working. Parallel pair: 1/R = 1/6.0 + 1/3.0 = 3/6.0, so R = 2.0 Ω. Total = 4.0 + 2.0 = 6.0 Ω. Current from the battery = 6.0 ÷ 6.0 = 1.0 A. p.d. across the pair = 1.0 × 2.0 = 2.0 V. Current in the 3.0 Ω resistor = 2.0 ÷ 3.0 = 0.67 A. p.d. across the 4.0 Ω resistor = 4.0 V. Energy = VIt = 4.0 × 1.0 × 120 = 480 J.
Marking point
What earns it
Where students drop it
(a)
2.0 Ω for the pair, then 6.0 Ω in total
Adding 6.0 and 3.0 to get 9.0 Ω
(b)
1.0 A, then 2.0 V across the pair, then 0.67 A
Using the full 6.0 V across the pair
(c)
4.0 V across the resistor, then 480 J
Using t = 2.0 instead of 120 s
The mistakes that cost Combined Science marks
Eight mistakes to check for in every mock paper
Chemistrycm³ and dm³ mixed upDivide cm³ by 1000 before using mol/dm³. Many wrong mole answers are out by exactly 1000.
ChemistryNo limiting reactant checkWhen two amounts are given, compare them with the equation's ratio before working out any product.
ChemistryTime spent on percentage yieldThe syllabus says percentage yield and purity calculations are not required. Revise reacting masses and gas volumes instead.
PhysicsParallel resistors addedUse 1/R = 1/R₁ + 1/R₂ for branches. The answer must be smaller than the smallest branch.
PhysicsMinutes in E = VItConvert to seconds. For cost questions, use kW and hours to get kW h.
BothUnits and significant figuresSEAB says misusing units or significant figures is liable to be penalised.
Paper 5Skipping the extension questionThe modification or extension part carries 10% to 20% of the practical marks.
BothRevising only the stronger halfOne grade covers both sciences, and the weaker half usually has more marks to gain.
A Sec 4 revision timetable for the 2027 SEC
Schools set their own prelim dates. SEAB's SEC calendar puts practicals first, then written papers from October to November, with results in January.
Nov to Dec 2026Close the Sec 3 gapsRedo the Sec 3 topics with the lowest marks, starting with the mole concept.
Jan to Mar 2027Keep pace in Term 1One timed structured question per science each week, marked against marking points.
Mar 2027 holidaysFirst full MCQ40 questions in 1 hour, with every wrong answer logged by topic.
Apr to May 2027Practical skillsTables with units, graphs, gradients and the modification question. Mid-year exams show the weaker half.
Jun 2027 holidaysFull mock papersPaper 1 and both structured papers, timed, then a week on the error log.
Jul to Aug 2027Practical and prelimsPaper 5 comes before the written papers. Most schools hold prelims around Term 3.
Sep 2027Fix prelim errorsRework every lost mark and practise choosing the Section B question quickly.
Oct to Nov 2027Written papersShort daily recall of definitions and formulae. No new topics.
How Combined Science counts in the 2028 PSE
From 2028, JC admission uses L1R4: English or Higher Mother Tongue plus four relevant subjects, with a gross total of 16 or below for JC and 20 for Millennia Institute. MOE's Science list names Biology, Chemistry, Physics and the three Combined Science subjects. Each combination is one subject with one grade, so it fills one slot only. MOE's own worked example uses G3 Science (Physics, Chemistry) at B3 as the R3 subject. Our L1R4 guide walks through the full calculation.
For polytechnics, the ELR2B2 net total must be 22 or below (24 for Nursing), and MOE's examples use G3 Science (Physics/Chemistry) as a best subject. In the 2026 JAE, Combined Science counted as a relevant subject for science and engineering courses. MOE will update the 2028 course lists in mid-2027.
If your child wants H2 sciences later
MOE sets no common rule for which grade a student needs to take an H2 science. Each JC publishes its own guidance, and the figures below come from two JCs' pages for recent O-Level intakes.
Wants two H2 sciencesCatholic JC listed A1 in the matching Combined Science as one route. Jurong Pioneer JC prefers a distinction in Combined Science with both relevant sciences.
A Combined Science student aiming for H2 sciences should plan for an A grade, not a pass. Bars differ between colleges and change each year, so check the subject combination pages of the JCs your child is considering.
Common questions
Does Combined Science count as one subject or two?
One. It has one grade, and MOE lists each combination as a single Science subject for the 2028 L1R4 and ELR2B2 aggregates.
Is there a school-based science practical assessment?
Not for G3. Paper 5 is a formal 1 hour 30 minute practical test worth 15%, set by SEAB. G2 Combined Science has no practical paper.
Which half is weighted more?
Neither, by design. Each science has a 32.5% structured paper and about half of the MCQ. SEAB does not publish the split of the practical marks.
Can a Combined Science student take H2 Chemistry in JC?
Many do. Catholic JC gave the same indicative B3 bar for Combined Science with Chemistry as for pure Chemistry. Other JCs set their own bars.
Is percentage yield tested in Combined Science Chemistry?
No. The syllabus says percentage yield and purity calculations are not required, though limiting reactant questions may be set.
When are the 2027 Combined Science exams?
SEAB holds practicals before the written papers, which run from October to November. Exact dates come with SEAB's timetable.
Written by Dion Tan, Chemistry & Science Tutor, The Singapore Syllabus. MSc (NUS), BSc Chemistry & Biological Chemistry (NTU) · 10 years' teaching experience. More about Dion
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