Floating Coin Surface Tension Assembly Demo
A student-led science demonstration on surface tension using a coin and water for Indian school morning assemblies.
- Keep all water away from electrical microphone cords on the assembly stage.
- Mop up any spilled water immediately so students do not slip in line.
Can a metal coin float on top of plain water?
क्या एक भारी सिक्का पानी की सतह पर तैर सकता है? Kya ek bhaari sikka paani ki satah par tair sakta hai?
Floating Coin Surface Tension Demo- Runs
- 3:00
- Materials
- 4, each with a stand-in
- Steps
- 8 · 3:00 on the bench
- The moment
- at step 5
What to bring, and what works instead
Run it once with the clock before you run it for the room
-
1
Respected Principal, teachers, and dear friends, good morning.
Stand tall behind the assembly bench facing the lines.
-
2
Watch what happens when I drop this coin directly into water.
Drop the coin straight into the water bowl.
-
3
It sinks because metal is heavier than water. Now let us try a scientific trick.
Retrieve the coin and wipe it completely dry with a cloth.
-
4
I place a small piece of tissue on the surface, and put the coin gently on top.
Float tissue paper flat on water and place the dry coin on it.
-
5
Now watch closely as I gently push the tissue down with a pencil tip.
Slowly push the edges of tissue down until it sinks, leaving the coin resting on the water surface.
The momentThe paper tissue sinks away while the solid metal coin floats on top of the clear water surface.
Say nothing for 6 seconds. Let it land. -
6
Look! The metal coin is floating on top of the water!
Hold up the bowl gently so the front lines can see the floating coin.
-
7
Watch what happens when I touch the surface with one drop of soap.
Touch a pencil tip coated in dish soap to the water near the coin.
-
8
Thank you, and have a wonderful day ahead!
Bow head slightly and step back into your house line.
Then: Jai Hind, and have a curious day ahead!
Why it works, at two levels
Water molecules like to stick together tightly. At the top surface, water molecules pull on each other to form a thin, elastic skin. This skin is strong enough to hold up lightweight objects like a dry coin or paperclip if you place them down very gently.
Water molecules experience strong cohesive forces, pulling toward each other in all directions. At the air-water interface, molecules have no water above them, so they bind more strongly to neighboring surface molecules. This creates surface tension, acting like a stretched elastic membrane. Soap acts as a surfactant, breaking these hydrogen bonds and collapsing the surface skin instantly.
- the coin sinks along with the tissue paper: make sure the coin is completely dry and use a smaller piece of dry tissue.
- the water already has soap residue: rinse the plastic bowl thoroughly and refill with fresh tap water.
🧹 Wipe the bench with a towel, dump the water down the sink, and dry all coins for future use.
Questions for the room, and the words
The invisible skin on top of water created by water particles pulling tightly together.
The attraction that holds molecules of the same substance together.
So the whole room sees it
- Stand to the side of the demonstration table so the entire assembly line can see the bowl.
- Keep silent for 6 seconds during the reveal moment while holding the bowl steady.
- Stand to the side of your demonstration table so students lined up in front have a clear view.
- Hold the bowl high or place it on a raised stool if you are presenting on an outdoor assembly ground.
- Speak clearly into the microphone and pause after the coin floats to let the audience react.
- Keep your hands steady when lowering the coin so you do not break the top layer of water.
Worth knowing
- Surface tension creates a invisible skin on top of water that can hold lightweight objects like coins or paperclips.
- Using a bent paperclip or fork lets you lower the coin gently onto the water without breaking the surface skin.
- Liquid dish soap breaks surface tension instantly, causing the floating coin to sink to the bottom.
- The demonstration uses simple kitchen items with no dangerous chemicals, fire, or heat.
- Morning assemblies in Indian primary schools provide an ideal setting to share quick science tricks during Science Week or National Science Day.
- Presenting clearly on stage requires standing to the side of the bowl so all students in line can see.
Where it is done, and why
Performing a science demonstration during your school morning assembly is a great way to share a surprising experiment with your classmates. This guide gives you a complete, step by step plan to demonstrate surface tension using a light coin, clean water, and paper tissue. You will learn how to set up your demonstration table, what to say into the microphone, and exactly when to pause so everyone standing in line can see the result.
You can use the step by step timer to practice your timing at home before standing on stage in front of your teachers and peers. Every phase of the demonstration includes clear physical actions, spoken lines, and essential safety rules to keep your presentation clean and smooth. You will also find both simple and deeper scientific explanations to help you answer questions from your audience after the assembly ends.
About this piece
- Morning assemblies are a daily 20-minute routine in Indian schools, often held outdoors with students arranged in class lines.
- Science demonstrations frequently feature during morning assemblies, especially during Science Week or National Science Day celebrations.
- Demonstrations rely on easily available kitchen items to illustrate fundamental physical principles like capillary action and surface tension.
- Surface tension occurs because water molecules attract each other tightly, creating a film-like surface layer.
- Adding soap to water introduces a surfactant that breaks hydrogen bonds, instantly destroying surface tension.
- Student presenters practice standing to the side of their display table so that children across all assembly lines have an unobstructed view.
How it is done
- In India, students gather in disciplined lines for morning prayers, pledges, news readings, and short educational presentations before starting classes.