A student takes a newly opened tennis ball onto a court and hears a crisp bounce. Several weeks later, a heavily used ball feels dull and rebounds less energetically. I ask a Secondary 1 learner: Did the ball lose all its air through a visible hole, or could gas move slowly through the intact rubber wall itself? That question is an excellent way into polymers, gas diffusion, pressure, elasticity and the transfer of mechanical energy.
For families looking for a Secondary 1 Chemistry Tutor Hougang, the quick answer is that many standard tennis balls are pressurised: they have a rubber core holding gas at a pressure above ordinary surrounding air. Rubber is not a perfectly impermeable barrier to gas, so gas can diffuse through its wall over time, reducing internal pressure and changing rebound behaviour. Repeated impacts, changes in the rubber’s properties and wear of the felt covering also affect performance. Not all tennis balls are pressurised; pressureless models rely more strongly on the elastic construction of their core.
In Singapore, this example belongs to integrated lower-secondary Science rather than a separate national Secondary 1 Pure Chemistry examination. Depending on G1, G2 or G3 and the school’s teaching sequence, parts of the detailed polymer physics may be enrichment. The goal is to explain a familiar observation with a correct particle model, understand why product storage matters, read rebound data accurately and distinguish chemistry and materials science from sporting slogans. There is no need to puncture, cut open or deliberately over-pressurise a tennis ball to learn any of this.
1. What does a bounce measure?
A ball dropped onto a firm surface falls, deforms at contact and rebounds. The rebound height is a useful observable measure of its behaviour under a defined test. A lower rebound under identical conditions suggests different energy transfer or mechanical response.
A pupil should describe the starting drop height, rebound height and surface rather than only saying one ball feels flat. Real play involves racket impacts and spin, but controlled tests isolate part of the material’s behaviour.
2. Why a tennis ball is hollow
Many tennis balls have a hollow rubber core made from two joined hemispherical sections and a felt covering. The internal gas and rubber shell both contribute to how the ball deforms and rebounds.
The ball is not one solid piece of rubber and it is not a balloon made only of gas. A useful cross-sectional diagram labels felt, rubber wall, internal gas and the joined seam.
3. The role of the felt covering
Felt helps determine surface friction, air resistance, speed, spin and how the ball interacts with the racket and court. It also wears with use and becomes less like its original state.
Felt wear is one mechanism affecting play, but is not identical to a loss of internal gas pressure. A child should avoid using one explanation for every change in a ball’s performance.
4. What the rubber core contributes
The rubber shell deforms during impact and can store and release some mechanical energy. Its polymer structure and construction strongly influence the magnitude and timing of the rebound.
A suitable rubber core can flex and recover without being perfectly elastic or perfectly airtight. The Rubber Bands and Elasticity guide provides the underlying polymer-chain model.
5. What pressurised means
A pressurised tennis ball contains gas at an internal pressure above the external atmospheric pressure under ordinary conditions. The pressure difference contributes to the ball’s stiffness and rebound behaviour.
The precise pressure varies with ball type, manufacturer, temperature and relevant standards. For a first-year answer, it is more important to understand ‘higher than the outside’ than to memorise a number without units.
6. The difference between gauge and absolute pressure
Gauge pressure measures pressure relative to the surrounding atmosphere. Absolute pressure includes atmospheric pressure in the reference. A reading of 12 psi gauge does not mean the gas inside has an absolute pressure of only 12 psi.
This is extension material, but it prevents a misleading numerical comparison. The International Tennis Federation’s equipment resources distinguish the pressure levels of pressurised and pressureless balls.
7. What the ITF says about the gas core
The International Tennis Federation’s technical documentation describes pressurised balls as typically holding gas around 10–13 psi above atmospheric pressure, while pressureless balls have an internal pressure near the atmosphere and a different core design.
Those numbers are not a universal claim about all balls in every package or climate. They illustrate why the difference between internal and external pressure matters to how the product is built.
8. What creates gas pressure?
Gas molecules move and collide with the inside of the rubber shell. Their motion and collisions exert forces on the wall, producing pressure. The amount of gas, available volume and temperature affect the state of the system.
The molecule diagram should show many tiny moving particles, not one giant bubble that grows. A first-year learner can explain how gas pushes on the wall without detailed kinetic-theory equations.
9. Why the ball compresses on impact
When the ball hits the court, it deforms. The rubber shell bends and internal gas volume can change temporarily. The gas pressure and rubber elasticity act together to resist deformation and help the ball spring back.
This is a combined materials-and-pressure mechanism. A student should not claim that the gas alone is the entire cause of bounce or that a ball with less gas cannot rebound at all.
10. Stored mechanical energy during deformation
Part of the energy from the falling or moving ball is stored temporarily in the compressed gas and deformed rubber. Some returns to motion during rebound, while other energy is dissipated as heat, sound and internal material losses.
A bounce is not a perfect cycle. The ball generally rebounds to a lower height than its starting drop under ordinary conditions because some mechanical energy is not recovered.
11. Why the energy is not perfectly returned
Rubber is viscoelastic: deformation and recovery depend on both elastic storage and time-dependent losses. Impacts can warm the material slightly and produce sound or other energy transfers.
This explains why even a brand-new ball does not ordinarily bounce back to the exact drop height in a real test. Air resistance and the court surface also influence the result.
12. What diffusion means
Diffusion is the movement of particles associated with their microscopic motion and concentration or chemical-potential differences. Gas molecules can migrate through a suitable material over time, even when no visible hole is present.
The rubber shell can act as a barrier that slows gas movement rather than stopping it completely. This is a materials example of the same broad transport principle studied in gases, liquids and membranes.
13. Why rubber is not a perfect gas seal
Many rubber compounds have some permeability to gases. Molecules may dissolve into the polymer material, diffuse through it and leave on the other side. The rate depends on polymer formulation, gas species, thickness and conditions.
The process need not involve a visible puncture. A ball can lose internal gas gradually through an intact core, changing the pressure difference over time.
14. Why pressure differences help drive transport
Gas inside a pressurised ball is at a higher partial pressure than outside for some constituents. This can create a driving force for net movement of gas through the rubber wall toward the surroundings.
The complete process depends on gas type, polymer solubility and permeability. We should not imagine every molecule moves outward in a straight line; random molecular motion produces a net transport tendency.
15. A gas molecule’s journey through polymer
A simplified model begins with gas molecules near the inner surface, their entry into the rubber, diffusion through the polymer matrix, and their departure at the outer surface.
This is different from liquid water pouring through a hole. A seemingly solid polymer can permit slow molecular transport because its chains move and free volume becomes available on microscopic scales.
16. The role of polymer chains
Rubber contains long flexible molecules whose arrangements and motion influence permeability. Small molecules may pass through suitable transient spaces as the polymer structure fluctuates.
A student must not conclude all polymers have identical gas permeability. Chemical composition, additives, temperature and cross-linking can affect the path.
17. Why gas can escape without a puncture
The absence of a visible hole does not prove that no gas molecules can cross a material. Diffusion through a polymer wall is a different transport mechanism from escaping through a tear or valve.
This is the central misconception. A ball can gradually lose pressure even when it still looks sealed and structurally intact.
18. Why the ball is sold in a pressurised can
Manufacturers often package pressurised tennis balls in sealed cans containing elevated pressure. That reduces the gas-pressure difference between inside and outside during storage and helps preserve playing performance before opening.
Once the can is opened, the surrounding gas pressure drops towards ordinary atmospheric conditions, increasing the relevant pressure gradient for gas loss. That is why packaging design matters.
19. Why a sealed can can hiss
When an appropriately pressurised package is opened, gas can move from the higher-pressure interior towards lower-pressure surrounding air, potentially producing an audible hiss.
That observation is about the can’s packaging pressure and should not be confused with every tennis ball bursting or with a new gas-producing chemical reaction.
20. Why opening starts a new storage history
Before opening, the elevated can pressure can help preserve ball pressure. After opening, balls are exposed to ordinary air pressure, and the net driving force for gas loss through their rubber walls can be greater.
The change in environment is important even if the balls are not played with immediately. A student should distinguish a ball that ages through storage from one that also experiences repeated impacts.
21. Why tennis balls lose bounce even when unused after opening
Gas diffusion can continue over time without a ball being struck. Therefore an opened but unused pressurised ball may gradually lose some rebound performance.
This is a useful prediction that tests the mechanism. If the child says only racket impacts can release pressure, they have overlooked diffusion through the intact polymer wall.
22. Why play can change performance further
Repeated impacts deform the rubber core and wear the felt. Those mechanical processes can affect the ball’s rebound, trajectory and interaction with the court or racket.
A ball’s practical decline is therefore not solely an internal pressure measurement. Performance is a property of the whole composite product in a particular test.
23. Why the felt can wear before the core is empty
Felt fibres can become abraded, flattened or altered during play while the ball still retains significant internal pressure. The resulting changes to aerodynamics and surface friction can influence playing behaviour.
The student should not assume a rough-looking ball must have lost all pressure. Visual wear and gas content require different measurements.
24. Pressurised versus pressureless balls
Pressureless tennis balls generally rely more on the elasticity of a thicker rubber core rather than a substantial pressure difference between inside and outside. They can have different rebound and feel characteristics.
The Lawn Tennis Association and ITF distinguish these categories. A ball called pressureless is not the same as a defective pressurised ball that has leaked most of its gas.
25. Why a pressureless ball can still bounce
Rubber elasticity can store and return mechanical energy even without substantial elevated internal pressure. Core wall thickness and composition are designed around that operating principle.
This is an important changed-case question. It prevents students from treating air pressure as the only possible source of restoring force in a ball.
26. A comparison with a rubber band
A rubber band stores and releases mechanical energy through its stretched network. A tennis ball’s rubber core likewise deforms and recovers, while gas pressure adds another important mechanism in pressurised models.
The rubber band article helps explain why materials can return toward their original shape without needing to create new atoms.
27. A comparison with an ordinary balloon
Balloons also contain gas behind flexible polymer walls and can lose gas gradually through permeation. But their geometry, pressure and rubber design differ from tennis balls.
A student should transfer the idea of permeability without claiming the two objects lose gas at identical rates. Diffusion depends on the materials and conditions.
28. How temperature affects a ball
Temperature changes the state of the gas and the mechanical behaviour of rubber. A ball may rebound differently in a colder or warmer environment even before substantial gas has been lost.
This is distinct from long-term diffusion. The effect of immediate temperature on pressure should not be confused with the gradual escape of gas from the ball.
29. A cold ball is not necessarily punctured
If a ball feels less lively on a cold morning, part of the difference may arise from temperature-dependent gas pressure and rubber properties rather than a new permanent hole.
A controlled test should use comparable temperatures when assessing long-term pressure loss. Otherwise the observed rebound difference includes several causes.
30. Does pressure loss mean all gas escaped?
No. A pressurised ball may lose some gas and still contain plenty of air or another gas inside. The relevant issue is the pressure difference and resulting mechanical behaviour, not complete emptiness.
The popular word flat is a practical description, not a statement that the ball’s interior is a vacuum.
31. Why the ball often keeps its shape
The rubber core and structural materials can retain a roughly spherical shape even as internal pressure declines. A used tennis ball need not visibly collapse like a very thin party balloon.
This shows why external appearance is an imperfect pressure gauge. A student should not claim that a round ball must contain its original factory pressure.
32. How polymer permeability differs from porosity in paper
A porous paper towel has connected fibre gaps that carry liquid through capillary action. Gas transport through a dense polymer wall can involve dissolution into the polymer and molecular diffusion through its structure.
The distinction is worth emphasising. A rubber core does not need an obvious open tunnel from inside to outside for gas molecules to migrate.
33. Why rubber thickness matters
For otherwise comparable material and conditions, wall thickness influences the distance gas molecules must travel and can affect permeation rate. But thickness also changes how stiffly the ball rebounds.
A designer must balance pressure retention with the required mechanical feel. Simply making the wall arbitrarily thick is not a universal solution.
34. What a barrier coating might change
A suitable coating could modify gas transport through a material, though practical tennis balls must also meet weight, feel, durability and manufacturing requirements.
No one coating is guaranteed to preserve pressure forever. Material compatibility and the rules of the sport matter for real equipment.
35. Does gas pressure create energy?
Internal compressed gas can store mechanical energy associated with its state, but the ball’s impact or initial pressurisation supplies that energy. The bounce does not create energy from nothing.
During a collision, energy is redistributed and some is dissipated. A lower rebound is consistent with imperfect mechanical energy recovery.
36. What rebound height tells us
In a defined drop test, rebound height gives information about the combined properties of ball, surface and environmental conditions. Higher rebound under the same method usually means more motion energy recovered vertically.
The measurement does not uniquely tell us the exact internal gas pressure. Rubber stiffness and other factors can produce similar rebound heights.
37. An idealised energy calculation
If an object of mass m rebounds vertically to height h, its gravitational potential energy at that peak is approximately mgh relative to the impact surface, using a simple model.
This is later-level physics enrichment. The student should learn that rebound height provides a meaningful proxy for vertical mechanical energy under comparable conditions, not a full molecular analysis.
38. Coefficient of restitution
A coefficient of restitution compares relative speeds after and before a collision in an idealised setting. For a simple vertical drop onto a fixed rigid surface without major complications, it can be related to the square root of rebound height divided by drop height.
This is advanced context, not a requirement for every first-year pupil. The concept reminds students that bounce is measured quantitatively rather than by a casual impression of liveliness.
39. A fictional rebound-height table
Imagine identical drop conditions with a new ball rebounding 140 cm, a used ball 122 cm and an older ball 105 cm. The measured rebound heights decline across these samples.
These are fictional values used for reasoning, not a claim that every tennis ball will follow this decline or fail a particular sport standard. The measurement alone cannot isolate pressure from rubber wear.
40. Compute a drop in height
A ball rebounds 138 cm when new and 115 cm later under comparable conditions. The decrease is 23 cm. Relative to the original rebound, this is about 16.7%.
That is a relative change in measured rebound height, not a direct measurement of the percentage of gas molecules lost. The numerical concepts must remain separate.
41. Compare pressurised ball categories with reliable specifications
Official tennis-ball testing involves defined conditions and measures including size, mass, deformation and rebound. Different approved ball types and introductory balls have different intended performance ranges.
Students should not invent an exact official standard from one home bounce. Use published ITF procedures and appropriate laboratory equipment for formal comparisons.
42. Why the court matters
A tennis ball rebounding from a rigid test surface may behave differently from one rebounding on grass, clay or an elastic floor. The surface also deforms, absorbs energy and affects friction.
A fair test should use the same court or test surface. A lower bounce on a softer surface does not necessarily mean the ball has lost more internal pressure.
43. Why a racket strike is not the same as a vertical drop
A racket collision includes string deformation, ball deformation, spin and motion at varying angles. A vertical drop test isolates a more limited measure of rebound.
The student should avoid claiming a single rebound height predicts every aspect of real play. Different tests answer different performance questions.
44. A safe investigation uses measurements, not puncturing
Do not puncture tennis balls, build improvised pressure vessels, heat pressurised cans or attempt uncontrolled repressurisation. Those procedures can create injury risks and are unnecessary for this learning task.
A teacher-supplied bounce dataset, cross-sectional diagram and ITF manufacturing explanation allow students to study pressure and diffusion safely.
45. Why gas permeability is a materials property
Permeability is influenced by how readily a gas dissolves in the material and how quickly it diffuses through it. Different polymer formulations can have different gas barriers.
A pupil should not treat all rubber products as identical. A tennis ball is manufactured to balance permeability, elasticity and other requirements.
46. The importance of storage
Packaging a pressurised ball at elevated surrounding pressure helps reduce the driving force for pressure loss while unopened. After opening, storage environment becomes an important part of the ball’s history.
There are commercial storage solutions designed to manage pressure, but the educational conclusion should not claim that any improvised sealed container can safely restore a ball’s original performance.
47. The difference between gas diffusion and a leak
A hole or damaged seam can create a much faster direct escape path than slow molecular permeation. Both can reduce internal pressure, but their physical mechanisms differ.
A ball that loses pressure gradually without visible damage can be consistent with diffusion. An unusually rapid loss might require other investigation, but a student should not diagnose a specific defect from one observation.
48. A fair test of ball ageing
Imagine a teacher provides data for identical balls stored for different periods after opening, tested at the same temperature on the same surface and from the same drop height.
The independent factor might be storage duration. The dependent measurement could be rebound height. Relevant controls include ball type, temperature, drop procedure, surface and prior use.
49. Why a control group helps
A comparable new ball tested under the same conditions provides a reference. Without that reference, a single low rebound number is difficult to interpret because different ball types have different designs.
The control should not secretly differ in temperature or construction. Good experimental design isolates the intended comparison rather than decorating a table with two labels.
50. Reading a fictional gas-pressure curve
A teacher supplies a graph where internal gauge pressure gradually decreases over several weeks. The trend is consistent with loss of the original pressure difference under the stated storage conditions.
The curve does not prove exactly how the gas moved unless the ball’s construction and environment are known. Pressure readings alone cannot show the precise diffusion path through the polymer.
51. G1: particles and bounce
For a G1 learner, begin with gas particles inside a rubber shell and ask how the ball responds when squeezed or bounced. Explain that some gas can slowly escape through the material and change the rebound.
Use the MOE G1 lower-secondary Science syllabus and actual school lessons to determine the required detail.
52. G2: polymer permeability and a fair test
A G2 learner can explain diffusion through rubber, distinguish pressure loss from temperature effects and read a simple rebound-height graph.
Refer to the MOE G2/G3 lower-secondary Science syllabus. Formal gas laws may be a later-level extension.
53. G3: pressure, material ageing and evidence
A secure G3 learner can compare pressurised and pressureless balls, discuss pressure gradients and polymer permeability, and explain why rebound height does not uniquely determine gas loss.
A changed case involving an unused but opened ball helps test whether the learner understands diffusion rather than assuming all performance changes come from repeated impacts.
54. A 90-minute small-group tutorial
Minutes 0–10: Ask how a ball can become less lively without a hole. 10–25: Label felt, rubber and internal gas. 25–40: Explain diffusion and pressure. 40–55: Compare pressurised and pressureless models.
55–70: Interpret fictional drop-test data. 70–80: Correct misconceptions about pressure, temperature and wear. 80–90: Require each student to solve a changed case independently.
55. Parent diagnostic: locate the missing link
One child may understand pressure but believe rubber is perfectly impermeable. Another may know diffusion yet confuse ball condition with court effects. A third may make a percentage-calculation error.
These require different teaching responses. A useful tutor repairs the first unstable concept, then checks an unfamiliar situation. Hougang and Kovan parents should verify actual lesson location, group size, syllabus match and availability.
56. The one-week practice route
Day 1: Draw the ball’s layers. Day 3: Explain gas diffusion without a visible hole. Day 5: Compare pressurised and pressureless balls. Day 7: Read a fresh rebound-height table and identify an unsupported claim.
The 20-minute Science Revision Plan keeps the repetition focused. The aim is independent reasoning, not memorising product marketing.
57. Frequently asked questions
Why do tennis balls lose bounce? Gas can diffuse out of pressurised cores, and use also changes rubber and felt. Does a ball need a hole to lose pressure? No. Why sealed cans? They help maintain pressure before opening.
Are all balls pressurised? No. Can a pressureless ball bounce? Yes, by suitable core design. Can an old ball’s condition be judged from colour alone? No.
58. Authoritative tennis sources
The International Tennis Federation’s Ball Manufacture guide describes pressurised rubber cores, gas permeability and sealed storage cans. The Lawn Tennis Association’s ball guide compares pressurised and pressureless categories.
The Tennis Industry Association’s science of tennis balls describes mechanical deformation and rebound testing. These are evidence sources, not instructions to alter sporting equipment.
59. Follow the Hougang Chemistry route
Connect Why Rubber Bands Stretch and Spring Back, Diffusion in Liquids and Gases and Why a Balloon Shrinks in the Fridge for related but different mechanisms.
The Hougang Chemistry Tutor Library maps the full Science route. The question I want the student to answer is ‘How can a solid rubber wall allow slow gas movement while still remaining elastic and looking intact?’
28 Worked Questions and Rebound Challenges
Q1. The core structure
What lies inside a common pressurised tennis ball? Answer: Gas held within a rubber core.
The felt is on the outside.
Q2. The pressure difference
What does pressurised mean? Answer: Internal pressure above external atmospheric pressure.
Name the reference pressure.
Q3. Why gas escapes
Can gas move through intact rubber? Answer: Yes, by permeation or diffusion over time.
A visible hole is not required.
Q4. What diffusion means
What is diffusion? Answer: Net particle movement resulting from molecular motion and relevant driving gradients.
Rubber permeability determines the rate.
Q5. Rubber permeability
Do all rubber compounds admit gas at the same rate? Answer: No.
Polymer chemistry and thickness matter.
Q6. Why the sealed can?
Why store pressurised balls in an elevated-pressure can? Answer: To help preserve their internal pressure before opening.
The pressure gradient during storage is reduced.
Q7. The can hiss
What can cause a hiss as a pressurised can opens? Answer: Gas escaping toward lower outside pressure.
This is not new gas forming by a reaction.
Q8. A ball kept unused
Can an opened unused pressurised ball lose pressure? Answer: Yes.
Diffusion can continue without impacts.
Q9. A pressureless ball
Must a pressureless ball be completely unable to bounce? Answer: No.
Its thicker rubber core provides more of the elastic response.
Q10. The rubber wall
Why does rubber influence bounce? Answer: It deforms and recovers, storing and returning some mechanical energy.
Not every joule is recovered.
Q11. Felt
Does felt wear change performance? Answer: Yes.
It influences friction, spin and air resistance.
Q12. Energy loss
Why is rebound usually lower than drop height? Answer: Some mechanical energy dissipates.
Sound, heat, deformation and surface effects matter.
Q13. Rebound height
Drop height 200 cm, rebound 120 cm. What fraction of height? Answer: 120/200 = 0.6.
The fraction is not identical to the percentage of gas retained.
Q14. Relative rebound
New rebound 150 cm, used rebound 120 cm. Difference? Answer: 30 cm.
The reduction is 20% of the initial 150 cm.
Q15. Temperature
Can temperature affect ball performance? Answer: Yes.
Gas pressure and rubber properties respond.
Q16. Cold versus leak
A cold ball bounces less. Must gas have escaped? Answer: No.
The immediate thermal state can matter.
Q17. A hole
Would a puncture differ from slow diffusion? Answer: Yes.
A direct leak can be much faster.
Q18. A round old ball
Does a round appearance prove original pressure? Answer: No.
The shell maintains form even as pressure changes.
Q19. Which variable?
If varying storage time, what is independent? Answer: Time after opening in the stated design.
The dependent quantity might be measured rebound.
Q20. A fair drop
Which things should be controlled? Answer: Ball type, temperature, surface, drop height and method.
Uncontrolled conditions weaken the inference.
Q21. A lower reading
Does a lower bounce prove only gas loss? Answer: No.
Rubber wear and other factors can contribute.
Q22. A metal comparison
Does a solid metal wall have identical gas permeability to rubber? Answer: No.
Material structure matters.
Q23. G1 model
Draw a ball with gas inside and a rubber wall.
Explain that some molecules slowly migrate.
Q24. G2 model
Contrast a new pressurised ball with an opened stored ball.
Name the pressure and diffusion change.
Q25. G3 model
Compare pressurised and pressureless cores.
Identify different roles of gas and rubber elasticity.
Q26. Safety
Should a child puncture or pressurise tennis balls as a home experiment? Answer: No.
Use supervised data and diagrams.
Q27. The parent test
Explain why a ball can age while sitting unused.
A correct response needs gas permeability, not repeated impacts.
Q28. Final transfer
A different inflated rubber object becomes less firm without a hole. Could diffusion be involved? Answer: Yes, depending on its material and conditions.
The specific product requires its own evidence.