📅 Last Reviewed: July 2026 · ✍️ By: PSLE Hero Editorial Team
The Singapore Primary Science Olympiad (SPSO) is an annual national science competition organised by NUS High School of Mathematics and Science. It is open to Primary 5 students and covers Biology, Chemistry, and Physics concepts that go beyond the PSLE syllabus. The 2026 Theory Round took place on 19 May at participants’ own schools. This guide explains what parents and students can verify from official sources, then provides five original SPSO-style science questions with worked solutions. These are not the official 2026 question paper, answer key, or past-year paper.
Check the official SPSO introduction and competition homepage at NUS High School for verified updates.
There is no verified official SPSO 2026 answer key on this page. Students can use the five original practice questions below to review transferable scientific reasoning skills, but should not treat them as a reconstruction of the 2026 paper. Parents should check with the teacher-in-charge at their child’s school once results are released via email.
SPSO may help a student demonstrate scientific aptitude, but the competition is not directly affiliated with the DSA exercise. Outstanding performance may be noted, but a medal does not guarantee a DSA offer from any secondary school.
For parents: SPSO is enrichment, not a replacement for school science or PSLE preparation. Ask the school science department about selection and registration. Choose practice that builds scientific curiosity rather than exam pressure. The concepts tested go beyond the P5 PSLE syllabus, so some topics will be unfamiliar.
For students: Try each question without looking at the solution first. Read carefully — many SPSO questions have multiple correct answers, and a single wrong choice costs marks. Identify the science concept being tested, apply what you know, then compare your thinking with the worked solution and note one improvement.
They are different assessments. SPSO tests biology, chemistry, and physics thinking at a level above the MOE primary syllabus, while the PSLE Science examination assesses the five core themes (Diversity, Cycles, Systems, Interactions, Energy) from the SEAB 2026 syllabus. SPSO practice can deepen a student’s conceptual understanding and strengthen scientific reasoning, but it does not guarantee a PSLE result and should not replace syllabus-based revision.
See the SEAB 2026 PSLE Science syllabus and examination format for verified PSLE information.
These are 5 original Science Olympiad-style sample questions inspired by common primary science reasoning ideas. They are educational examples created for this page, not official SPSO questions, past papers, or an answer key. Each worked solution focuses on the scientific concept and a reasoning approach students can explain and check.
Combustion requires oxygen. When a candle burns in a sealed jar, it consumes available oxygen until the concentration drops too low to sustain the flame. Understanding what happens to the air inside after the flame goes out tests both chemistry and physics reasoning.
A lit candle is placed inside a glass jar. The jar is then sealed airtight with a lid. The candle continues burning for a short while before the flame goes out on its own.
Which of the following statements correctly explains what happens to the air inside the jar after the flame goes out?
A) The oxygen in the jar has been completely used up, leaving only nitrogen.
B) The oxygen level has dropped below the level needed to sustain combustion, and the proportion of carbon dioxide has increased.
C) All the air in the jar has been converted into carbon dioxide.
D) The nitrogen in the jar reacted with the candle wax to extinguish the flame.
Why A is wrong: Combustion does not use up all oxygen. Air is ~21% oxygen, ~78% nitrogen. The candle flame goes out when oxygen drops to ~16% — not zero. Oxygen is still present.
Why B is correct: When the candle burns, it uses oxygen and produces carbon dioxide and water vapour. Oxygen drops below the combustion threshold (~16%), nitrogen stays unchanged (unreactive), and CO2 increases.
Wax + Oxygen → Carbon dioxide + Water vapour(combustion equation)
Why C is wrong: Nitrogen (78% of air) is largely unreactive under normal combustion. It remains in the jar throughout. Not all air converts to CO2.
Why D is wrong: Nitrogen is an unreactive gas under normal conditions. It does not react with candle wax or participate in combustion.
Key principle: Combustion requires fuel, oxygen, and heat (the fire triangle). When oxygen drops below ~16%, the reaction stops — even though oxygen is still present. This is why a flame in a sealed container goes out before all oxygen is consumed.
PSLE connection: The PSLE syllabus covers that burning requires oxygen (Interactions theme). SPSO-style questions extend this to gas proportions and what happens to each gas. Practise identifying which gas changes and which stays the same.
Whether an object floats depends on its density relative to the liquid. Adding dissolved substances such as salt increases water density. This principle is central to many SPSO-style physics and chemistry questions.
An egg sinks to the bottom of a glass of plain water. A student adds salt to the water and stirs until the salt dissolves completely. After adding enough salt, the egg rises and floats in the middle of the glass.
Select ALL statements that are correct:
A) The mass of the egg increased after salt was added to the water.
B) The density of the salt water is greater than the density of the egg.
C) The upward force (upthrust) on the egg increased when salt was added.
D) The volume of the egg changed when the egg began to float.
A — Incorrect. Salt dissolves in the water, not in the egg. The mass of the egg does not change.
B — Correct. Dissolving salt in water increases the solution’s mass without significantly changing volume, so density increases. When salt water density exceeds egg density, the egg floats.
Density = Mass / Volume(fundamental relationship)
More dissolved salt = greater mass per unit volume = higher density(key mechanism)
C — Correct. Upthrust equals the weight of liquid displaced. When salt water is denser, the weight of displaced liquid increases for the same volume, so upthrust on the egg increases until it equals the egg’s weight.
D — Incorrect. The egg’s volume does not change. Dissolving salt in surrounding water has no effect on the egg’s dimensions.
Key principle: An object floats when upthrust (upward force from liquid) equals or exceeds the object’s weight. Upthrust depends on liquid density and the volume of liquid displaced. Increasing liquid density increases upthrust for the same submerged volume.
PSLE connection: The PSLE syllabus covers forces including upthrust under the Interactions theme. SPSO questions extend this to density comparisons and multiple-correct-answer formats. Practise identifying which variable changes and which stays the same.
Plants carry out both photosynthesis (using light energy to make food) and respiration (releasing energy from food) simultaneously. The balance between these processes changes with light availability. SPSO questions often test whether students understand which process dominates under different conditions.
A potted plant is placed in a sealed transparent box with a carbon dioxide sensor. The sensor records CO2 concentration every hour.
• From 6 am to 6 pm: CO2 concentration steadily decreases.
• From 6 pm to 6 am: CO2 concentration steadily increases.
Which of the following correctly explains both observations?
A) The plant only photosynthesises during the day and only respires at night.
B) During the day, the rate of photosynthesis exceeds the rate of respiration, so net CO2 decreases. At night, only respiration occurs, so CO2 increases.
C) During the day, plants absorb CO2 through their roots, causing the level to fall. At night, plants release CO2 through their leaves.
D) CO2 is produced during photosynthesis and absorbed during respiration.
Why A is wrong: Plants respire continuously, 24 hours a day, not only at night. Respiration releases energy from food and occurs in all living cells at all times. Only photosynthesis requires light and stops in darkness.
Why B is correct: During daylight, both processes occur simultaneously. Photosynthesis uses CO2 faster than respiration produces it, so net CO2 falls. At night, photosynthesis stops (no light) while respiration continues, so CO2 builds up.
Daytime: Rate of photosynthesis > Rate of respiration = net CO2 uptake(O2 released)
Night-time: Photosynthesis = 0, Respiration continues = net CO2 release(O2 consumed)
Why C is wrong: Plants absorb carbon dioxide through stomata (mainly on leaves), not through roots. Roots absorb water and dissolved minerals.
Why D is wrong: This reverses the correct gas exchange. Photosynthesis uses CO2 (and produces O2). Respiration produces CO2 (and uses O2).
Key principle: Both processes occur simultaneously in plant cells. We observe the net effect. During bright daylight, photosynthesis dominates. In darkness, only respiration continues. At the compensation point, rates are equal and CO2 does not change.
PSLE connection: The PSLE syllabus covers photosynthesis and respiration under the Systems and Energy themes. SPSO extends this to interpreting sensor data and reasoning about rates. Practise reading data tables and graphs alongside conceptual explanations.
Different materials conduct heat at different rates. SPSO questions on this topic often present experimental data and ask students to draw conclusions, compare materials, or identify fair-test variables — skills that overlap with PSLE experimental questions.
Five rods of equal length and diameter, made of different materials, are heated at one end using the same heat source for 5 minutes. A temperature sensor is placed at the other end of each rod. Results:
Copper: 72 °C | Aluminium: 58 °C | Iron: 41 °C | Glass: 28 °C | Plastic: 24 °C
Select ALL statements that are supported by this data:
A) Copper is a better thermal conductor than aluminium.
B) Plastic is a thermal insulator, so it does not conduct any heat at all.
C) The experiment is a fair test because the rods are of equal length and diameter.
D) Glass conducts heat better than plastic in this experiment.
A — Correct. Copper recorded 72 °C vs aluminium 58 °C under identical conditions. A higher temperature means heat travelled more quickly. Copper is a better thermal conductor than aluminium.
B — Incorrect. Plastic recorded 24 °C — above starting room temperature (~22 °C). Some heat did transfer through plastic. Insulators slow heat transfer; they do not stop it entirely. This data does not support the claim of zero heat conduction.
C — Correct. For a fair test, variables not being investigated must be controlled. Equal length and equal diameter are important controlled variables. The same heat source and same duration are also stated.
Fair test: Change only the independent variable (material type)(control all others)
D — Correct. Glass (28 °C) was higher than plastic (24 °C) under identical conditions. Glass conducted more heat in the same time.
Key principle: Thermal conductors allow heat to pass through quickly. Thermal insulators slow heat transfer. Comparing conductors fairly requires controlling all variables except material type.
PSLE connection: Fair-test questions appear in both PSLE Booklet B and SPSO. When data is given, always check what was kept the same and what was changed. Do not describe an insulator as completely stopping heat — say it slows heat transfer.
Electrical circuits are a key topic in SPSO. Questions often involve predicting what happens to bulb brightness or current when components are added, removed, or the circuit type changes.
Two identical bulbs, P and Q, are connected in series with a battery. A student then opens the switch connected to bulb Q so that bulb Q goes out.
Which of the following correctly describes what happens to bulb P when the switch to Q is opened?
A) Bulb P becomes brighter because more current flows through it.
B) Bulb P also goes out because no current can flow through the series circuit.
C) Bulb P becomes dimmer because the circuit now has less resistance.
D) Bulb P is unaffected because P and Q are in separate branches.
Why A is wrong: In a series circuit, there is only one path for current. Opening the switch creates a gap that stops all current flow. Bulb P does not receive more current — it receives none.
Why B is correct: In a series circuit, all components share the same single current path. If the switch connected to Q is opened, the circuit is broken. No current can flow, so bulb P also goes out.
Series circuit: one path = open switch anywhere = no current everywhere(key rule)
Switch at Q opens = gap in the loop = P and Q both go out(both bulbs off)
Why C is wrong: Bulb P goes out entirely, not dimmer. Opening Q’s switch breaks the entire circuit entirely.
Why D is wrong: "Separate branches" describes a parallel circuit, not a series circuit. In a parallel circuit, opening one branch leaves others unaffected. In series, there is only one shared path.
Key distinctions:
PSLE connection: Electrical systems are covered under the Systems theme. SPSO questions may involve predicting brightness changes when components are switched off. Practise drawing circuit diagrams and tracing the current path.
Q: What is the Singapore Primary Science Olympiad (SPSO)?
SPSO is an annual national science competition organised by NUS High School of Mathematics and Science. Open to Primary 5 students enrolled in MOE primary schools, with registration through participating schools. SPSO aims to generate interest in Biology, Chemistry, and Physics and to identify scientific talent. Check the official SPSO pages at NUS High School for the latest eligibility and dates.
Q: What is the SPSO 2026 format?
The 2026 edition was a Theory Round held on 19 May 2026 at participants’ respective primary schools. The paper is 75 minutes long with 30 single-answer MCQs and 10 questions with multiple correct responses (40 questions total). Registration fee was $25 inclusive of GST. Some editions have included a Practical Round; verify with your school whether this applies for your child’s year.
Q: When will SPSO 2026 results be released?
According to official SPSO information, results for the Theory Round are released in July. Schools are notified directly by email. Contact the teacher-in-charge at your child’s primary school — the organiser (NUS High School) does not accept telephone enquiries about results.
Q: What awards does SPSO give?
Individual medals are given based on performance. School trophies are typically awarded to the top 20 schools based on cumulative scores of their top 5 students. The organiser does not publish a fixed medal percentage allocation.
Q: How does SPSO help with PSLE Science?
They are different assessments. SPSO tests biology, chemistry, and physics above the MOE primary syllabus. PSLE Science assesses the five core themes from the SEAB 2026 syllabus. SPSO practice can deepen conceptual understanding and strengthen scientific reasoning — particularly for Booklet B open-ended questions — but is not a substitute for learning the PSLE syllabus.
Q: Does SPSO lead to DSA?
SPSO is not directly affiliated with the DSA exercise. Outstanding performance may be noted by schools, but there is no guaranteed DSA offer. Families should check each target secondary school’s current DSA criteria and application process independently.
Q: Is SPSO linked to IMSO?
According to competition information, SPSO serves as one of the platforms for identifying students for the International Mathematics and Science Olympiad (IMSO). The specific IMSO selection process is determined separately; an SPSO medal does not automatically lead to IMSO selection.
📚 Explore More PSLE Science Guides
The conceptual reasoning practised in SPSO-style questions — gas exchange, density, circuits, thermal conductivity — strengthens the scientific thinking you need for PSLE Booklet B. Keep practising on PSLE Hero.
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