Why can drops of water pile into a rounded dome on top of a coin instead of immediately running off? Surface tension helps hold a curved water–air boundary together. Water molecules attract neighbouring water molecules through intermolecular interactions, including hydrogen bonding. At the surface, the imbalance in those interactions gives the liquid an energetic tendency to reduce its exposed area. A carefully placed collection of droplets can therefore form a convex mound on a clean coin until gravity, contact-angle conditions or a disturbance causes water to spill over.
For Hougang parents comparing Secondary 1 Science tuition, Secondary 2 Chemistry tuition, small-group tutors or support for cohesive forces, adhesion, particles and fair tests, a water-on-a-coin investigation is a useful diagnosis. Can your child tell the difference between water–water cohesion and water–surface adhesion, explain why different droppers produce different counts, and identify what makes an experiment fair? This is an everyday application of particles and investigations rather than a separate named examination chapter. A three-pax tutorial should follow the actual lower-secondary Science syllabus and check the student’s independent explanation before recommending more practice.
Place an ordinary drop of water on a clean coin and it may stay surprisingly rounded. Add more gently and the water seems to rise into a tiny transparent hill. It’s a lovely reminder that a liquid has no fixed shape, but it doesn’t behave like a collection of completely independent marbles either. Molecular interactions give it a recognisable surface.
The quick distinction: cohesion and adhesion
Cohesion describes attraction between molecules of the same substance, such as water molecules attracting other water molecules. Adhesion describes attractive interactions between water and another material, such as the surface of a coin.
Both affect the drop. Cohesive interactions influence how strongly the water’s surface resists spreading and deformation; adhesive interactions influence how much the liquid wets the metal and how its edge meets the surface.
A child who says only ‘water sticks to water’ may understand part of the picture but cannot yet explain why the same volume has a different shape on metal, wax or glass.
Why water molecules attract one another
Water molecules are polar. Oxygen attracts shared electron density more strongly than hydrogen, giving the molecule an uneven distribution of electrical charge. Suitable neighbouring water molecules can form hydrogen-bonding interactions, alongside other intermolecular attractions.
These interactions continually rearrange in liquid water. Water molecules are mobile and are not joined in one rigid covalent network across the entire droplet.
Surface tension emerges from the collective behaviour of molecules at the interface. A student should not say that hydrogen bonds form a literal solid plastic shell around the droplet.
What does surface tension actually mean?
Surface tension can be understood as an energy cost per unit additional surface area, or equivalently as a force per unit length along a liquid interface under appropriate conditions. A liquid generally tends to minimise its exposed surface energy, subject to gravity and interactions with nearby solids.
For a tiny free droplet, a sphere has relatively low surface area for the amount of liquid it contains. A droplet on a coin cannot be a complete sphere because it also contacts the coin, but it can form a rounded cap.
The surface is still liquid. If the coin is tilted or the droplet becomes too large, the mound can deform or spill. Surface tension is not a promise that any quantity of water can be stacked.
Why the mound becomes convex
A convex surface curves outward and upward. When drops are added to a small region on a coin, the combined liquid may curve above the coin’s edge rather than spread immediately into a flat film.
The exact shape depends on the coin’s metal and finish, wetting characteristics, cleanliness, drop volume, surface tension and gravity. A greasy fingerprint can change the contact angle and therefore change what the mound looks like.
This is why a drawing of an ideal coin with a perfectly symmetrical dome is a learning model, not a guarantee for every real household coin.
Worked observation: ten drops, then one more
Imagine a hypothetical classroom observation. A child adds a sequence of equal-size drops to the centre of a clean, level coin. The water forms a rounded mound as the total volume rises. At some point, adding one more drop causes it to spill.
There is no universal number of drops that every coin can hold. The count depends on the dropper, drop size, coin diameter, wetting and technique. The scientific outcome is a qualitative explanation of why a dome can form and why it eventually fails.
A tutor should ask what physical condition changed as water was added: total volume and weight increased while the contact area and surface shape adjusted.
Why the number of drops isn’t a material constant
Counting drops seems wonderfully simple, but two droppers can make very different droplet volumes. One group might report 25 drops and another 40, without having discovered a new law of water.
The experiments might simply have used different droppers, hand positions or coin surfaces. The number of drops is not equivalent to surface tension unless the technique and geometry are accounted for.
A fair-test answer must explain the difference between a measured drop count and the material property being investigated.
A worked fair-test plan
- Question: Does adding a small amount of detergent change the maximum number of equal-size water drops that can remain on a coin?
- Independent variable: the specified presence or concentration of detergent.
- Dependent measure: the defined number of drops added before the liquid spills.
- Controls: coin type, coin cleanliness, dropper, approximate drop size, coin orientation, temperature and handling.
- Repeat trials: use several comparable trials and discuss variation.
- Limitation: drop count is an indirect and imperfect proxy for the resulting interfacial behaviour.
The observation can be discussed with ordinary water and classroom-safe materials. Coins should be clean and hands washed afterwards; the learning should never require adding hazardous chemical solutions.
Why washing-up liquid often changes the result
Many detergent molecules contain both water-compatible groups and hydrocarbon groups. They can accumulate at water–air and water–solid interfaces, altering the surface’s energetic behaviour and commonly lowering water’s surface tension.
Under suitable conditions, that can make a droplet spread differently across a coin and alter how much liquid can be held before spilling. The number of drops does not have to change by the same amount in every experiment because drop size and wetting change too.
The student should say that the detergent changes the interfacial properties, not simply that it ‘destroys’ the water molecules.
Does soap always make a bubble weaker?
A surfactant can lower surface tension yet also help stabilise thin films by affecting how their surfaces respond to changes in thickness. That is why soap can both alter the dome on a coin and help ordinary soap bubbles persist.
These statements are not contradictory: the behaviour of a drop on a solid coin and the stability of a free-standing liquid film depend on more than the numerical magnitude of surface tension alone.
Our Hougang explanation of why soap bubbles pop owns that separate thin-film problem. Here the focus is a liquid drop’s surface and wetting a solid.
What makes droplets stick to a surface?
When water contacts metal, glass or plastic, molecular and surface interactions influence whether it wets the surface readily. A highly wetting surface encourages water to spread into a flatter shape. A poorly wetting surface may let the liquid bead into a more rounded shape.
This behaviour is often described using a contact angle. A high contact angle typically indicates poor wetting, while a smaller contact angle indicates more spreading under the relevant conditions.
A learner does not need to calculate contact angles at Secondary 1, but the distinction helps explain why identical water drops can look different on a coin, a waxy leaf and clean glass.
Why the surface must be clean
A small amount of cooking oil, hand cream or other residue can change how a droplet wets a metal coin. That changes its shape and can alter the number of drops the coin appears to hold.
In an experiment comparing liquids, an uncleaned coin is therefore a possible confounding variable. The student should explain the effect in terms of changed surface chemistry and adhesion rather than simply write ‘human error’.
This connects ordinary cleanliness to scientific control of variables, a useful practical lesson in its own right.
Why the coin being level matters
Gravity acts on the water’s mass. If the coin tilts, the weight of the water can draw the mound towards one edge and cause a spill earlier than on a level surface.
An equal amount of water may then produce a different shape because the forces no longer act symmetrically with respect to the coin.
This is another reason a good experiment reports conditions rather than a supposedly universal maximum drop count.
How a paper clip can sometimes rest on water
A carefully placed small object can sometimes be supported at the water surface through the interaction of surface tension, the contact line and buoyancy. That does not mean the object is less dense than water.
The curved liquid surface can exert upward force components, allowing a small item to be supported under suitable conditions. If the surface is disturbed or its interfacial properties change, it may sink.
A child can learn that surface tension and ordinary density-based buoyancy are related to forces but are not interchangeable explanations.
A comparison with an ice cube
An ordinary ice cube floats primarily because its average density is lower than that of surrounding liquid water. Its buoyant support depends on displaced water and the cube’s weight.
The mound on a coin, by contrast, is explained through liquid surface energy, surface tension, wetting and gravity. A tutor should not use ‘less dense’ to explain why a droplet sits as a dome.
For the separate density reasoning, see Why Ice Floats on Water — Hougang Science.
Why water doesn’t become a solid skin
Water’s surface can resist small deformations, but molecules there remain part of the liquid and move continuously. There is no solid membrane made by normal water molecules merely because a surface appears rounded.
The phrase ‘skin of water’ is an analogy describing interfacial forces. It is not an exact chemical description.
A good Chemistry answer recognises when a model is helpful and states where the analogy becomes inaccurate.
What can a student infer from a bigger mound?
A larger mound might indicate that more water has been added, that the coin has a different wetting condition, or that the drops are smaller. It is not automatically proof that this water has ‘stronger hydrogen bonds’.
To compare liquids meaningfully, the investigator must control surface conditions and measurement method.
The same principle appears in many exam questions: an observation may be consistent with a hypothesis without uniquely proving it.
The four errors that look like correct explanations
- ‘Water molecules are held together by one giant covalent bond.’ Hydrogen bonding in liquid water is intermolecular and dynamic.
- ‘The coin is magnetic, so the water sits in a dome.’ Ordinary magnetic attraction is not the explanation for the standard water-on-a-coin observation.
- ‘Density alone lets water stack beyond the edge.’ Surface tension and wetting are central.
- ‘Soap destroys water molecules.’ A surfactant changes interfacial interactions; it does not decompose H₂O in the ordinary experiment.
- ‘Every coin holds exactly thirty drops.’ Drop count depends on method and conditions.
A three-pax tutor can use one observation to distinguish misconceptions about bonding, surface forces and fair tests.
What Singapore lower-secondary students actually need
Lower-secondary Science covers matter, particle reasoning and the design and evaluation of scientific investigations at the appropriate G-level depth. A coin-and-water surface-tension investigation is a useful enquiry context, not a separately named compulsory Chemistry assessment task.
The more advanced molecular explanation, involving hydrogen bonding and interfacial energy, can be introduced when it helps the learner connect observed behaviour with particle-level reasoning.
For the official curriculum pathway, start with the MOE Full Subject-Based Banding overview and eduKate Secondary Science Shelf.
How a three-student Science tutorial can be different
Imagine one learner saying the coin has a magnetic pull, a second describing a literal solid water skin, and a third explaining surface tension but using a different dropper for every trial.
The tutor should not simply announce the correct word and move on. The first learner needs a force explanation, the second a molecular-model correction, and the third a fair-test improvement.
The final check is a changed example, such as a water droplet on a waxy surface, answered without the tutor’s original wording.
A five-minute parent question set
- Why can water form a curved dome above the coin’s surface?
- What is the difference between cohesion and adhesion?
- What changes when detergent is added?
- Why do two different droppers invalidate a simple drop-count comparison?
- Does a rounded water surface mean that water has become solid?
- Would the same explanation apply to ice floating in a drink?
If the learner can compare a coin, a glass surface and an ice cube using the correct mechanisms, the Science has become transferable.
Questions Hougang parents often ask
Does hydrogen bonding make a water droplet a solid ball?
No. It contributes to intermolecular attraction in a liquid whose molecules remain mobile.
Does detergent always reduce the number of drops?
It often alters the outcome through surface tension and wetting, but the observed count also depends on drop size, coin condition and experimental technique.
Does the same drop count prove two liquids have the same surface tension?
No. Identical counts can arise despite differences in measurement and wetting conditions.
Should a Secondary 1 child memorise surface-tension equations?
Not automatically. Begin with particle-level reasoning and a fair investigation, then follow the actual school Science requirements.
The better answer is a physical explanation, not a magic number
A small water dome teaches cohesion, adhesion, surface tension and the discipline of a fair test. Once a child knows what the surface is doing and what the experimental count can really tell us, the phenomenon stops being a trick and becomes usable Science.
Continue with Why Raincoats Make Water Bead Up, Hougang Science Learning Hub and Hougang Chemistry Tuition Hub. For diagnosis-led three-pax Science support, confirm the actual teaching venue, group and session availability with eduKate Singapore.