How to Perform in PSLE | Learner’s Guide Vol 0615 | Science: A Rubber Stopper Seals One Opening but the Whole Setup Is Airtight Only When Every Connection Is Sealed

One stopper cannot certify an entire apparatus

A correctly fitting rubber stopper can close the opening of a flask or tube. In a gas-collection setup, however, the whole apparatus is airtight only when every possible unintended path is sealed: around the stopper, through unused holes, beside inserted tubes, along connectors and through undamaged tubing. Airtightness belongs to the system, not to one visible part.

This guide belongs to the complete Learner’s Guide directory and continues the original PSLE Learning Guide. Use the eduKateSG Science hub for related study and the official SEAB PSLE formats examined in 2026 for current examination information.

The mechanism: gas follows available pressure paths

When gas is produced, it occupies space and may move through available openings. The intended route might lead through a delivery tube to a gas jar, water-displacement vessel, balloon or gas syringe. An unintended gap offers another route. Gas escaping there does not reach the measuring or collecting device.

A seal is local. A stopper seals the surface where it contacts the vessel opening. A hole through the stopper creates a new boundary. A tube through that hole creates an interface. A connector between tubes creates another interface. Each must be considered. The weakest unsealed path determines whether gas can escape.

Airtight does not mean every component is closed with no outlet. Many working setups require one controlled path for gas to leave the reaction vessel and enter a collector. The goal is to eliminate unintended paths while keeping the intended path open and correctly connected.

A five-step system audit

Where is gas produced?

Identify the reaction vessel and the space above the mixture. Use the diagram and stated method; do not invent hands-on tests or alter apparatus without supervision.

What is the intended path?

Trace the delivery route from vessel to collector without skipping connectors. Use the diagram and stated method; do not invent hands-on tests or alter apparatus without supervision.

Where are the boundaries crossed?

Mark stopper edges, holes, inserted instruments, tube joints and collector connections. Use the diagram and stated method; do not invent hands-on tests or alter apparatus without supervision.

Could any path leak or block?

A gap leaks; a pinch, liquid plug or obstruction blocks. Use the diagram and stated method; do not invent hands-on tests or alter apparatus without supervision.

How would failure affect evidence?

A leak can reduce collected volume; a blockage can stop flow or change pressure. Use the diagram and stated method; do not invent hands-on tests or alter apparatus without supervision.

Forty setup decisions

Setup 1: a flask mouth is closed by a correctly fitting stopper

Conclusion: that opening may be sealed. This is because airtightness still depends on holes, tubing and every other joint. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 2: a delivery tube passes through a stopper hole

Conclusion: the tube-stop interface must fit tightly. This is because gas can escape through a loose annular gap. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 3: two tubes pass through one stopper

Conclusion: both openings need secure fits. This is because one sound connection cannot compensate for another leak. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 4: the stopper is too small for the flask neck

Conclusion: the seal is unreliable. This is because a gap around the stopper creates an escape path. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 5: the stopper is forced into damaged glassware

Conclusion: the procedure is unsafe. This is because apparatus condition and teacher instructions take priority over completing the setup. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 6: gas bubbles appear at the intended outlet

Conclusion: gas is reaching that outlet. This is because this alone does not prove no gas escaped elsewhere. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 7: expected gas volume is lower than repeated trials

Conclusion: a leak is one possible explanation. This is because the learner should inspect connections before concluding less gas was produced. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 8: no bubbles appear after reaction begins

Conclusion: a blocked tube or leak may exist. This is because the evidence does not identify one cause without further checks. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 9: a stopper has an unused hole

Conclusion: the setup is not sealed unless the hole is properly closed. This is because an open path allows gas exchange. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 10: tubing is cracked near a connector

Conclusion: the whole pathway may leak. This is because sealing the flask mouth does not repair damaged tubing. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 11: a joint is submerged and bubbles emerge from it

Conclusion: the joint is leaking gas. This is because the unintended bubble location reveals an escape path. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 12: gas is collected by water displacement

Conclusion: all upstream joints must direct gas to the collector. This is because leaks reduce the amount reaching the collection vessel. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 13: two groups use different stopper fits

Conclusion: the comparison may be unfair. This is because sealing quality becomes an uncontrolled variable. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 14: the stopper is removed midway

Conclusion: the system becomes open. This is because gas can escape through the flask mouth. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 15: a stopper is present but tilted

Conclusion: visual presence is insufficient. This is because contact around the opening must actually close the path. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 16: the tube is pinched

Conclusion: flow may be restricted. This is because a tight seal and an open pathway are separate requirements. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 17: the gas syringe does not move

Conclusion: leak, blockage or little gas production are alternatives. This is because one observation cannot distinguish them immediately. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 18: a control setup uses the same stopper arrangement

Conclusion: the sealing condition is kept comparable. This is because matching apparatus reduces an alternative explanation. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 19: a connector becomes loose after warming

Conclusion: the seal may change during the trial. This is because connections should be checked within the approved procedure. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 20: petroleum jelly is suggested without instruction

Conclusion: do not add materials independently. This is because only approved sealing materials and methods should be used. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 21: a balloon is attached loosely to a flask mouth

Conclusion: gas may escape beside it. This is because the collection attachment must seal around the opening. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 22: a stopper closes a container holding liquid

Conclusion: it reduces exchange through that opening. This is because it does not automatically prevent evaporation or leakage through other paths. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 23: the stopper has a thermometer hole

Conclusion: the thermometer-stop interface must seal if airtightness matters. This is because the instrument creates another possible leak path. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 24: the apparatus gives inconsistent gas volumes

Conclusion: variable leaks may reduce reliability. This is because repeat results and inspect sealing before interpreting production. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 25: the same low reading occurs every time

Conclusion: a consistent leak could still cause systematic undercollection. This is because repeatability alone does not prove accuracy. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 26: the outlet tube ends outside the collector

Conclusion: gas is directed to the wrong place. This is because the pathway is connected but not to the intended destination. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 27: a stopper seals the reaction flask while the collector remains open as designed

Conclusion: the system may still function for displacement collection. This is because airtight does not always mean every vessel is completely closed; the intended gas path must be controlled. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 28: air is trapped initially in the apparatus

Conclusion: early collected gas may include displaced air. This is because sealing and sample identity are separate reasoning issues. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 29: a learner presses the stopper during measurement

Conclusion: this may change pressure or connection conditions. This is because apparatus should not be manipulated outside the stated method. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 30: a wet joint appears tight

Conclusion: appearance does not prove an airtight seal. This is because evidence should come from approved checks and consistent operation. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 31: a gas jar fills more slowly than expected

Conclusion: a leak is possible but not certain. This is because reaction rate, temperature, blockage and collection method also matter. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 32: the stopper is removed only after gas production ends

Conclusion: opening timing still changes what remains in the system. This is because interpret observations relative to the procedure. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 33: a tube reaches below liquid in the reaction flask accidentally

Conclusion: liquid may enter or block the path. This is because correct tube placement matters alongside sealing. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 34: all joints are tight but the stopper material is unsuitable

Conclusion: compatibility and safety may fail. This is because apparatus selection includes material suitability, not only fit. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 35: one group has longer tubing

Conclusion: volume and resistance differences may affect comparison. This is because keep the gas path comparable as well as sealed. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 36: soap solution is proposed for leak testing

Conclusion: use only if the teacher or approved procedure specifies it. This is because learners should not improvise tests with unknown substances. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 37: the reaction produces pressure in a fully closed setup

Conclusion: a sealed system may become hazardous. This is because never block intended outlets or create pressure without supervised design. The controlling idea is local seal. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 38: a diagram shows a stopper but no labels

Conclusion: infer only that it closes or supports connections. This is because do not claim airtightness without checking every path. The controlling idea is whole-system path. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 39: gas smell is noticed

Conclusion: stop work and alert the teacher. This is because sensory detection is not a safe leak-testing method. The controlling idea is fair comparison. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

Setup 40: the final explanation says ‘the stopper prevents all gas loss’

Conclusion: the claim is too absolute. This is because it seals one opening; the whole apparatus must be checked. The controlling idea is safety and evidence. A complete answer distinguishes what the stopper itself does from what the whole apparatus requires.

Trace the intended gas route and name the possible failure point. Do not claim that a single observation proves one cause when leakage, blockage, production rate and placement could produce similar results. Use cautious causal language and the evidence available in the question.

How leaks change measurements

If gas is collected for volume measurement, a leak before the collector can produce a value lower than the amount generated. If the leak varies across trials, results may scatter and reliability decreases. If the leak is steady, repeated results may agree yet remain systematically too low. Reliability and accuracy are therefore different.

Gas identity cannot be inferred from a stopper or from collection alone. Likewise, bubble rate at an outlet does not directly equal total gas production when some gas escapes elsewhere or bubble sizes differ. The apparatus condition determines what the observation can support.

Common misconceptions and targeted repairs

Misconception: A visible stopper means airtight

Repair: Draw every other opening and joint. Circle each possible boundary crossing. Then state how the corrected idea changes the interpretation of collected gas.

Misconception: No bubbles means no gas production

Repair: List alternatives: leak, blockage, wrong placement, slow reaction or no production. Then state how the corrected idea changes the interpretation of collected gas.

Misconception: Bubbles at the outlet prove perfect sealing

Repair: Recognise partial success: gas can reach the outlet while some escapes elsewhere. Then state how the corrected idea changes the interpretation of collected gas.

Misconception: Repeated readings prove accuracy

Repair: A consistent leak can create repeatable underestimates. Then state how the corrected idea changes the interpretation of collected gas.

Misconception: Tighter is always safer

Repair: Correct fit and approved setup matter. Forcing stoppers into glassware can be dangerous. Then state how the corrected idea changes the interpretation of collected gas.

Misconception: Airtight means completely closed

Repair: Replace this with controlled pathway: no unintended leaks, but the designed outlet remains open. Then state how the corrected idea changes the interpretation of collected gas.

Independent practice with explained answers

Practice 1

A stopper closes the flask. Is the whole setup proven airtight?

Practice 2

Gas escapes around a delivery tube. Which interface failed?

Practice 3

Why can a low collected volume suggest a leak?

Practice 4

Does repeating the same low result rule out leakage?

Practice 5

An unused stopper hole remains open. Consequence?

Practice 6

A tube is blocked but all joints are tight. Is the setup functioning?

Practice 7

Why use the same sealing arrangement for all groups?

Practice 8

Can bubbles at the outlet prove no other leak exists?

Practice 9

What should happen if glassware is damaged?

Practice 10

Why is ‘airtight’ a system property?

Practice 11

A thermometer passes through the stopper. What new risk appears?

Practice 12

What does a stopper definitely do when correctly fitted?

Practice 13

What does it not automatically prove?

Practice 14

Why can a variable leak harm reliability?

Practice 15

Why can a steady leak harm accuracy?

Practice 16

Should pupils smell gas to locate a leak?

Practice 17

A fully closed reaction makes pressure. What is the rule?

Practice 18

How should a leak explanation be worded?

Practice 19

What other causes can mimic a leak?

Practice 20

Delayed transfer: one joint bubbles underwater.

Graded independence and delayed transfer

Begin with simple diagrams containing one stopper and one tube. Add an unused hole, a second tube, a connector, a collector and an inserted thermometer one feature at a time. Each addition creates a new interface to audit. Next, solve mixed questions about leaks, blockages and wrong placement. Revisit after two days without labels.

For each error, classify whether the learner ignored a boundary, confused leak with blockage, overclaimed one cause, missed a fair-test variable or overlooked safety. Target that cause with a contrasting diagram rather than repeating an identical question.

Examination control

Use a tracing routine. Start at the reaction vessel, follow the delivery tube with a finger, pause at every connection and finish at the collector. If asked why collected volume is low, write a causal chain: gap at connection → some gas escapes → less gas reaches collector → measured volume is lower.

When the question asks whether a setup is airtight, avoid answering from one component. State that the stopper seals its opening only if it fits correctly, and all holes, tubes and joints must also be sealed while the intended route remains open.

Guidance for parents and tutors

Use printed diagrams, not improvised pressurised experiments. Ask the learner to colour the intended path and mark every interface. The goal is system reasoning and safe interpretation, not home leak testing. School laboratory instructions and teacher supervision take priority.

Challenge absolute words. Replace always, all and proves with precise conditions. “A correctly fitted stopper seals this opening” is stronger than “the stopper stops all gas.” Precision in language mirrors precision in experimental reasoning.

Frequently asked questions

What does a rubber stopper do?

It closes the opening it fits into and may support tubes or instruments through designed holes.

Does one stopper make a setup airtight?

No. Every possible unintended path must be sealed.

Can a sealed setup still have an outlet?

Yes. A controlled delivery path may be required; airtight means no unintended leakage.

How does a leak affect collected gas?

Some gas may escape before collection, reducing the amount measured.

Can repeated results still be wrong?

Yes. A consistent leak can create repeatable but systematically low readings.

How is a blockage different?

A blockage restricts the intended path, while a leak opens an unintended path.

Should learners test leaks by smell?

No. Stop and inform the teacher; follow the approved safety procedure.

What is the strongest explanation format?

Identify the failed connection, explain gas escape or obstruction, and state the effect on the observation.

Next routes

Continue with Volume 0603 on bubble-count evidence and Volume 0611 on beaker functions. Return to the complete directory for the full route.