Experiment questions — also called fair test questions or Section B open-ended questions — appear in every PSLE Science paper and are worth significant marks. Yet many students lose marks here not because they don't know the science, but because they don't know how to structure their answers. This guide fixes that.
📋 What You Will Learn
- 🔬 The 3 types of variables — IV, DV, CV explained clearly
- 📝 How to write a hypothesis the right way
- 🧪 How to write a method step by step
- ⚠️ The most common mistakes and how to avoid them
- 📊 How to spot an unfair test and explain what's wrong
- ✅ 10 fully worked experiment examples with model answers
- 🏆 Marking scheme tips — what examiners actually look for
The 3 Types of Variables — Manipulated, Responding & Controlled (PSLE Notes)
Every PSLE experiment question tests your understanding of variables. There are always exactly three types:
IV = I Change it. DV = I measure it. CV = I keep it the same.
The aim of the experiment always tells you the IV and DV: "To find out how [IV] affects [DV]."
How to Write a PSLE Science Hypothesis — Formula, Examples & Common Mistakes
A hypothesis is a prediction of what you expect to happen, with a reason. It must follow this structure:
"I predict that [IV change] will cause [DV result] because [scientific reason]."
| ❌ Weak Hypothesis | ✅ Strong Hypothesis (Full Marks) |
|---|---|
| "The plant will grow more." | "I predict that the plant given more water will grow taller because water is needed for photosynthesis and cell growth." |
| "The car will go faster." | "I predict that the car on the smooth surface will travel a greater distance because smooth surfaces have less friction, so less energy is lost." |
| "The candle will go out." | "I predict that the candle under the smaller jar will go out first because there is less oxygen available for combustion." |
| "The ice will melt faster." | "I predict that the ice in the warm room will melt faster because the higher temperature provides more heat energy to break the bonds holding water molecules in the solid state." |
How to Write a Science Method for PSLE — Step-by-Step Notes
A method describes what you do step by step. It must be clear enough that someone else could repeat the experiment exactly. Use numbered steps and mention:
- What materials and equipment you will use
- How you will set up the experiment (including quantities and measurements)
- What you will change between trials (the IV)
- What you will keep the same in each trial (the CVs)
- What you will measure and how (the DV)
- How many trials you will run (repeat for reliability)
- Not saying how much of something to use (e.g. "add water" instead of "add 100 ml of water")
- Forgetting to mention repeating the experiment
- Not saying how the DV will be measured (e.g. "observe the plant" instead of "measure the height of the plant in cm")
- Changing more than one variable between trials
How to Spot an Unfair Test — PSLE Questions, Traps & Model Answers
A common PSLE question gives you a described experiment and asks: "Is this a fair test? Explain your answer."
Step 1: Identify the IV (what is being changed).
Step 2: Check if anything ELSE is different between the test groups.
Step 3: If YES — it is NOT a fair test. State what should have been kept the same.
Answer format: "This is not a fair test because [name the variable that was not controlled] was different between the two groups. To make it fair, [that variable] should be kept the same in both trials."
10 Worked PSLE Experiment Examples — Variables, Hypothesis & Method
No, this is not a fair test. The student changed two variables — the amount of sunlight AND the amount of water given to each plant. Since different amounts of water were used, we cannot be sure whether any difference in plant height was caused by the different light levels or by the different amounts of water.
To make it a fair test: The same amount of water (e.g. 150 ml) should be given to all three plants. Only the amount of sunlight should differ.
I predict that the toy car will travel the shortest distance on the rough carpet because rough surfaces have more friction than smooth surfaces. More friction means more force opposing the car's motion, causing it to stop sooner.
What the results would show: Smooth tiles = greatest distance. Wooden floor = medium distance. Rough carpet = shortest distance.
The best insulator is the material whose cup shows the smallest drop in temperature after 20 minutes. If wool keeps the water hottest, we conclude wool is the best insulator because it traps the most heat energy and slows heat loss to the surroundings.
The nail will move most strongly when closest to the magnet (1 cm). As distance increases, the magnetic force decreases. At 5 cm, the nail may not move at all, showing that magnetic force decreases with distance.
I predict that the plant under red and blue light will produce the most oxygen bubbles because chlorophyll absorbs red and blue light most efficiently for photosynthesis. Green light is mostly reflected by chlorophyll, so the plant under green light will produce the fewest bubbles.
I predict that sugar will dissolve fastest in the hottest water (80°C) because higher temperatures give water molecules more energy. The faster-moving water molecules collide with the sugar particles more frequently, breaking them apart more quickly.
Both groups of seeds should germinate similarly. Seeds do NOT need light to germinate — they only need water, warmth, and air. If both groups germinate equally, we conclude that light is not needed for germination. Light is needed for photosynthesis AFTER the seedling emerges, not for germination itself.
Common mistake: Students often confuse germination with photosynthesis. Seeds germinate using stored food in the cotyledon — they do not need light at this stage.
Tube A (water + air): Rusts — both water and air present. Tube B (water, no air): Does NOT rust — air absent. Tube C (air, no water): Does NOT rust — water absent. Tube D (salt water + air): Rusts fastest — salt speeds up rusting.
Conclusion: Both water AND air (oxygen) are needed for iron to rust. Removing either one prevents rusting. Salt water speeds up rusting.
Hypothesis: The candle under the largest jar will burn longest because the larger jar contains more air (oxygen). Combustion requires oxygen — more oxygen available means the candle can burn for longer before oxygen runs out.
Expected results: Small jar = goes out first. Medium jar = goes out second. Large jar = burns longest.
(a) State the independent variable and dependent variable.
(b) Name TWO variables that must be kept constant to make this a fair test.
(c) Write a hypothesis for this experiment.
(d) Describe the method Maya should follow.
(a) Variables:
Independent variable: The length of the nichrome wire (10 cm / 20 cm / 30 cm / 40 cm).
Dependent variable: The brightness of the bulb.
(b) Two controlled variables:
1. The same battery (same voltage) must be used in each trial.
2. The same type and thickness of nichrome wire must be used in each trial.
(Also acceptable: same bulb, same connecting wires.)
(c) Hypothesis:
I predict that the longer the nichrome wire, the dimmer the bulb will glow. This is because a longer wire has greater resistance, which reduces the current flowing through the circuit and causes the bulb to glow less brightly.
(d) Method:
1. Set up a circuit with one battery, one bulb, and connecting wires.
2. Connect a 10 cm piece of nichrome wire into the circuit.
3. Close the switch and observe and record the brightness of the bulb.
4. Replace the nichrome wire with a 20 cm piece of the same wire. Repeat step 3.
5. Repeat for 30 cm and 40 cm pieces of nichrome wire.
6. Keep the battery, bulb, and all other conditions the same in every trial.
7. Repeat the entire experiment 3 times and take the average brightness to ensure reliable results.
What PSLE Examiners Look For — Marking Scheme & Tips for Full Marks
| Question Part | What Gets Full Marks | Common Mistake |
|---|---|---|
| IV | Specific variable + the values tested (e.g. "length of wire: 10, 20, 30, 40 cm") | Too vague: "the wire" |
| DV | State what is measured AND how (e.g. "brightness of bulb, observed visually / measured with LDR") | Forgetting how it is measured |
| CVs | Name at least 3 specific variables kept the same | Writing "all other variables kept the same" — earns 0 marks |
| Hypothesis | Prediction + scientific reason linked to the IV and DV | Prediction without reason |
| Method | Numbered steps, specific quantities, mentions repeating | Vague steps, no measurements, no repeats |
| Conclusion | "The results show that [IV change] causes [DV result] because [reason]" | Just stating the result without linking to the science |
🏆 Key Rules to Memorise
- 🔄 IV = the ONE thing you change. Never change two things at once.
- 📏 DV = what you measure. Always say HOW you measure it.
- 🔒 CV = keep at least 3 specific variables the same. Name them!
- 📝 Hypothesis = prediction + reason. No reason = lose a mark.
- 🔁 Always mention repeating the experiment for reliability.
- ⚖️ Use the same quantities (same volume, same mass, same time) across all trials.
- 📊 Conclusion must link result back to the science — not just restate the numbers.
- ❌ Never write "all other variables kept the same" — always name them specifically.
- 3 types of variables: Manipulated (MV, what you change), Responding (RV, what you measure), Controlled (CV, what stays the same)
- A fair test changes only 1 variable at a time — all others are controlled
- Hypothesis format: 'If [MV] increases, then [RV] will increase/decrease, because [science reason]'
- Method: state starting conditions → change the MV → measure the RV → list CVs → state number of repeats
- Repeat the experiment to improve reliability and reduce the effect of random errors
- Unfair test = two or more variables changed simultaneously — identify which CV was not controlled
- Examiners award marks for: specific MV, specific RV, measurable quantities, and scientific explanation
Practice Questions
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