Wax, Water and Why Raincoats Work

Rub a cold candle hard across a square of cotton, then drop water on it. The water sits up in a ball and rolls off, while the untreated square beside it goes dark and limp in two seconds. That contrast is the whole of this waterproofing experiment for kids, and it leads straight to something genuinely useful: your old raincoat probably isn’t leaking. The membrane is fine. What has worn off is a wax-like coating on the outside, and once that goes, the fabric soaks up water and feels wet from the inside even though nothing came through.

Key Takeaways

• You’ll treat cotton squares with wax, oil and petroleum jelly, then compare how water behaves on each and weigh how much each absorbs.
• It teaches hydrophobic surfaces and contact angle: whether a drop spreads depends on the surface, not the water.
• Best for ages 6 to 12. No heat is needed and no wax gets melted.
• The usual failure is not rubbing hard enough, which leaves patchy coverage and a confusing result.
• The best extra test is the self-cleaning one, where a rolling droplet picks dirt off a waxed surface.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Plain cotton fabric	5 squares, about 8 × 8 cm	Old T-shirt or tea towel	Paper towel works but tears when wet. Coffee filters are a good second choice
A plain household candle	1	Supermarket	A block of paraffin or a wax crayon, rubbed hard
Cooking oil	1 tsp	Kitchen
Petroleum jelly	1 tsp	Pharmacy	Lip balm
Pipette or drinking straw	1	Medicine cupboard	A teaspoon, poured very slowly
Kitchen scale reading to 0.1 g	1	Kitchen	A 1 g scale, using three squares stacked
A waxy leaf and a soft leaf	1 each	Garden. Cabbage, kale, nasturtium, or any glossy leaf; lettuce or basil for the soft one	A supermarket cabbage leaf and a lettuce leaf
Cocoa powder	1 tsp	Baking cupboard	Fine soil, or chalk dust
Phone camera	1	Pocket	Needed for the side-on droplet photos

Nothing here is hazardous and nothing needs heating.

Time: prep 15 min, experiment 30 min, cleanup 5 min Cost: everything is probably in the house Mess level: low, as long as the water trays stay on a towel

Do the Experiment: A Waterproofing Experiment for Kids

Step 1 - Treat four squares and leave one alone

Square 1 stays untreated. Square 2 gets rubbed hard with the side of the cold candle, in two directions, until the fabric looks faintly whitened and feels slightly stiff. That means thirty seconds of real pressure, not a gentle stroke. Square 3 gets a thin smear of cooking oil, square 4 a thin smear of petroleum jelly. Square 5, if you want a reference, is a piece of plastic bag.

What you should see: the waxed square changing colour slightly and feeling papery. If it still feels exactly like the untreated one, you haven’t rubbed hard enough, and that single failure accounts for most disappointing versions of this experiment.

Step 2 - Put one drop on each and photograph it from the side

Lay all the squares flat on a tray. Put a single drop of water on each from the same height. Then get your phone down at table level and photograph each drop side-on, as close as the camera will focus.

What you should see: on the untreated cotton, the drop vanishes into the fabric almost immediately and leaves a dark patch. On the waxed square, a near-sphere that sits proud of the surface. On oil and petroleum jelly, a flattened dome somewhere in between. The side-on photos let you compare the angle where the water meets the surface, which is the actual measurement scientists use.

Step 3 - Measure absorbency by weighing

Weigh each dry square and write it down. Dunk each one in water for exactly ten seconds, lift it, let it drip for five seconds without squeezing, and weigh it again. Work out grams of water gained per gram of dry fabric.

What you should see: untreated cotton gaining its own weight in water or more. The waxed square gaining a fraction of that. The percentages are more convincing than the droplet photos because nobody can argue with a scale.

Step 4 - Compare two leaves

Drop water on a cabbage or nasturtium leaf and on a lettuce or basil leaf. Tilt each one slowly.

What you should see: on the waxy leaf, a mercury-like bead that rolls off at a very shallow tilt. On the soft leaf, a drop that spreads and clings. Plants did this first and did it better, and the waxy leaf will usually beat your waxed cotton.

Step 5 - Run the self-cleaning test

Dust the untreated square and the waxed square lightly with cocoa powder. Tilt each one at about 30 degrees and roll a large drop of water down across the dusted area.

What you should see: on the waxed square, the droplet rolls and carries cocoa with it, leaving a clean track. On the untreated square, the water soaks in and spreads the cocoa into a brown stain. This is the lotus effect in miniature, and it is why self-cleaning building glass exists.

What’s Actually Going On

Whether a drop spreads out or balls up is decided by a competition. Water molecules are strongly attracted to each other through hydrogen bonding, which is why water has an unusually high surface tension, around 72 millinewtons per metre at 20 °C, as the USGS water science pages set out. That self-attraction is called cohesion, and on its own it would pull every drop into a sphere.

Working against it is adhesion: the attraction between the water and whatever it is sitting on. Cotton is cellulose, and cellulose is covered in hydroxyl groups that hydrogen-bond to water very happily. So on cotton, adhesion beats cohesion, the drop spreads, and then capillary action pulls it along the gaps between fibres and the whole square goes dark. Candle wax is a long-chain hydrocarbon with essentially no polar groups at all, so there is nothing for water to hydrogen-bond to. Adhesion is weak, cohesion wins, and the drop holds itself in a ball.

The angle at the edge where water, air and surface all meet is the contact angle, and it is set by the balance of those interfacial energies, a relationship first written down by Thomas Young in 1805. Below about 90 degrees we call a surface hydrophilic, above it hydrophobic. Flat wax gets you to roughly 100 to 110 degrees, which is enough to bead but not enough to be dramatic.

Roughness is what makes it dramatic, and this is the part most explanations leave out. Cassie and Baxter showed in 1944, in Transactions of the Faraday Society, that on a rough or porous surface a droplet can sit on a composite of solid and trapped air. Because the drop is partly resting on air, and air is as non-wetting as it gets, the apparent contact angle climbs far higher than the flat material could manage, sometimes past 150 degrees. Barthlott and Neinhuis described the biological champion of this in Planta in 1997: the lotus leaf has microscopic wax crystals on a bumpy epidermis, and the combination gives contact angles so high that droplets roll rather than slide, picking up dirt particles as they go. That is the self-cleaning effect you reproduce in Step 5, and it is why the field is called biomimetics.

Now the useful correction. Most rain jackets are not waterproof barriers in the sense of a plastic bag. They are a breathable membrane, which lets water vapour out but blocks liquid water, laminated behind a face fabric treated with a durable water repellent coating. That coating is doing exactly what your candle wax does: keeping the contact angle high so rain beads and rolls. When it wears off through abrasion and washing, the face fabric wets out, saturates, goes cold and heavy, and stops water vapour escaping, so your sweat condenses inside. It feels precisely like a leak. The membrane may be perfectly intact. The fix is to re-treat the outside, which is why wash-in and spray-on repellent products exist, and knowing that saves a jacket.

How to Explain It to Your Kid

Ages 5–8: water holds hands with itself

Say it as a story about friendship. Water molecules hold hands with each other very tightly. Cotton is covered in tiny hooks that water also likes to hold, so the drop lets go of itself and grabs the cotton instead, and spreads out. Wax has nothing to hold, so the water just holds its own hands and stays in a ball. Let them do the two leaves from Step 4, because a drop rolling off a cabbage leaf is one of the better things in a garden.

Ages 9–12: contact angle and a percentage

Introduce hydrophobic and hydrophilic, and show them their own side-on photos. You can measure the angle well enough by printing a photo and laying a ruler along the surface and along the edge of the drop. Then do the weighing arithmetic: if the plain square gained 1.4 g and the waxed one gained 0.3 g, the coating cut absorption by nearly 80 percent. Two numbers, one percentage, a real result.

Ages 13+: Young’s equation, Cassie-Baxter and wetting out

Give them Young’s equation as a balance of three interfacial tensions, and the Cassie-Baxter picture of a drop sitting on a solid-air composite. Then the investigation: take one fabric and vary only the roughness, by comparing a smooth tightly woven cotton, a looser weave, and a brushed or fleecy fabric, all waxed the same way. Measure contact angle from photos and absorbency by weight. Finish on the raincoat question and ask them to find out what a DWR treatment actually is and why the chemistry of it has been changing.

The question to ask: “If we waxed the inside of a drinking glass, would water still fill it? Predict what would change and what wouldn’t.”

Why Teaching This Early Pays Off

This one pays off in an unusual way: it replaces a word with a mechanism. “Waterproof” is a label children see on jackets, phones, watches and paint, and it hides a whole spectrum of behaviours from a sealed barrier to a surface treatment to a rating for how deep you can take a watch. A kid who has waxed a square of cotton understands that repellency is a surface property and can be lost, which is more than most adults know.

The curriculum connections are real but spread out. Surface tension, intermolecular forces and polarity are chemistry. Capillary action and plant surfaces are biology. Biomimetics, materials selection and coatings are design and technology, and the same cohesive force turns up again in our guide to kitchen chemistry done safely. And contact angle is a genuine measurement, with real instruments called goniometers, approximated here with a phone and a ruler to an accuracy that is good enough to rank four samples.

A note on the evidence for hands-on teaching, since it is easy to overclaim. Freeman and colleagues’ 2014 analysis in PNAS found that active learning improved performance in undergraduate science courses compared with lecturing, but that was measured on university students in structured courses, and it does not translate directly into claims about kitchen-table afternoons. What the broader literature does support is narrower and still useful: predicting before observing, and explaining afterwards, beats watching. The National Research Council’s Framework for K-12 Science Education lists “constructing explanations” as a practice, and asking your child why the cabbage leaf beat the waxed cotton is that practice.

Before You Start: What Parents Should Know

Best age	6 to 12. Younger children can do the leaves and the droplet test; weighing and percentages land from about 9
Supervision	Light. An adult should do any warming if you choose to do it, and manage the water trays
Real hazards	None beyond normal kitchen care. The whole experiment runs cold. Oil and petroleum jelly make floors and worktops slippery, so work on a towel, and nothing gets tasted. If you decide to fix the wax in place with heat, use a hairdryer on low or an iron over baking paper with an adult holding it, never a flame
Don’t	Don’t melt wax in a pan to dip the fabric. Molten wax burns badly, it can ignite if it gets too hot, and the dipping version of this activity is no more informative than rubbing a cold candle. Also don’t use a scented candle if anyone is sensitive to fragrance

When It Doesn’t Work

• The waxed square soaked up water anyway. Not enough wax. Rub hard, in two directions, for a full thirty seconds, and hold the fabric against a hard flat surface while you do it. Patchy coverage behaves like no coverage.
• Every square beaded water. Your fabric has a factory finish on it. New tea towels and some cottons are treated. Wash the squares in hot soapy water, rinse and dry them before you start.
• The side-on photos are useless. Phone cameras will not focus closer than about 7 cm unless they have a macro mode. Back off to 10 cm and crop in, and put a sheet of white paper behind the drop for contrast.
• The oiled square performed as well as the waxed one. It often does for the droplet test and then loses badly on the self-cleaning test, because oil is sticky and traps the cocoa. Run Step 5 before concluding they are equivalent.
• The leaf didn’t repel water. Not every glossy leaf is waxy, and supermarket vegetables are often washed with detergent. Try an outdoor leaf: nasturtium, cabbage, lupin and many grasses work well.

Take It Further

Test real repellent treatments head to head. Five identical squares treated with candle wax, petroleum jelly, cooking oil, hairspray and a commercial fabric protector spray. Measure absorbency by weight for each, then abrade each square with twenty rubs of a dry cloth and measure again. The second measurement is the one that matters, because durability is what fails on a real jacket.

Change the roughness, not the chemistry. Wax a smooth cotton, a loose-weave cotton and a fleecy fabric identically, then compare droplet behaviour. The rougher surfaces should repel better despite identical coating, which is Cassie and Baxter’s result reproduced at home. It also connects neatly to the way surface tension holds a needle on water, since both experiments are measuring the same cohesive force from different angles.

What to Watch For Over the Next 3 Months

• Week 4: they start noticing beading on things. A car bonnet after polish, a freshly waxed table, a duck, a lupin leaf after rain.
• Month 2: the word “hydrophobic” appears somewhere unexpected, often about a non-stick pan or a phone screen that has stopped repelling fingerprints.
• Month 3: worth encouraging: the question of how a breathable waterproof membrane can block liquid water and pass water vapour at the same time. The answer is pore size versus molecule size, and it is a good step up in sophistication.

Frequently Asked Questions

Is wax actually how raincoats are made?

Historically, almost exactly. Waxed cotton jackets were standard outdoor wear for a century and are still sold. Modern jackets use a breathable membrane plus a thin durable water repellent coating on the outer fabric, which does the same job as wax but far thinner so the fabric stays flexible and breathable.

My child’s coat is leaking at the shoulders. Is it broken?

Often not. Shoulders and forearms abrade first, under rucksack straps and sleeves, so the repellent coating wears off there first. The fabric then wets out, the jacket stops breathing, and moisture condenses inside. Washing it properly and re-applying a repellent treatment fixes most cases, and it’s worth trying before replacing the coat.

Can we make the wax permanent?

Somewhat. Warming the waxed fabric gently, with a hairdryer on low or an iron over baking paper, melts the wax into the fibres and improves durability noticeably. An adult should hold the iron, and it should be on a low setting. It will still wash out, which is the point: all repellent coatings are temporary.

Why is a lotus leaf better than my waxed cotton?

Because of roughness at the right scale. A lotus leaf has microscopic wax crystals on a bumpy surface, so a droplet rests partly on trapped air and barely touches the leaf at all. Your cotton has the right chemistry but nothing like that structure, which is why the leaf wins.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Barthlott, W., & Neinhuis, C. (1997). “Purity of the sacred lotus, or escape from contamination in biological surfaces.” Planta, 202(1), 1–8. https://doi.org/10.1007/s004250050096
2. Cassie, A. B. D., & Baxter, S. (1944). “Wettability of porous surfaces.” Transactions of the Faraday Society, 40, 546–551. https://doi.org/10.1039/TF9444000546
3. US Geological Survey. “Surface Tension and Water.” Water Science School. https://www.usgs.gov/special-topics/water-science-school/science/surface-tension-and-water
4. Freeman, S., Eddy, S. L., McDonough, M., Smith, M. K., Okoroafor, N., Jordt, H., & Wenderoth, M. P. (2014). “Active learning increases student performance in science, engineering, and mathematics.” PNAS, 111(23), 8410–8415. https://doi.org/10.1073/pnas.1319030111
5. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165
6. National Institute of Standards and Technology. “NIST Chemistry WebBook: physical property data.” https://webbook.nist.gov/chemistry/