Sink or Float?
Designed by Darin Carr (BSc, DipEd)
NESA Accredited Teacher Chemistry & Physics Specialist
Creator of the LAB™ Learning System
Explore why some objects sink while others float, even when they look the same.
This experiment reveals how density determines whether something stays on top or sinks below.
7-12 yrs
Medium
min
Stage 2, Stage 3
>
Sink or Float?
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NESA Accredited Teacher
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High school chemistry & physics specialist 30+ years
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The Crazy Scientist in primary schools — 15 years
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International conference presenter on science education
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Creator of the LAB™ Learning System
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Curriculum aligned: NSW Science & Technology K–6 (2024)
A picture is worth a thousand words — check this out and see if you can spot the science hiding in plain sight.
What you will need
4–5 clear test tubes (or small clear glasses)
Water
Sugar cubes
Small potato pieces (same size if possible)
Spoon or stirring stick
Test tube rack (or something to hold them upright)
How to do it
1
Set Up Your Tubes
Fill each test tube with the same amount of water and place them in a rack.
3
Stir to Dissolve & add cubes
Stir each tube until the sugar dissolves as much as possible.
Carefully place one same-sized potato piece into each tube.
2
Add the Sugar
Tube 1 → 0 cubes
Tube 2 → 2 cubes
Tube 3 → 4 cubes
Tube 4 → 6 cubes
This is what creates different densities
4
Observe What Happens
Watch where each potato floats:
Some sink
Some float
Some stay in the middle
Did it work? Share the science! Tag @the_crazy_scientist on Instagram — we love seeing your experiments!
The Crazy Scientist Lab System™
Every experiment follows The Crazy Scientist Lab System™ — a simple way to help kids think like real scientists.
We
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LINK to what they already know,
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ACTIVATE curiosity through hands-on discovery
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BUILD understanding that actually sticks.
If you dissolved a huge amount of sugar into water, do you think something that normally sinks might start to float?
Could the same object behave completely differently just by changing the liquid it's in — not the object itself?
What do you think would happen if you kept adding more and more sugar?
Watch carefully… something strange is happening
The potatoes are the same — but they don’t behave the same
Some sink… some float… some hover in the middle
What’s changing… and what’s staying the same?
This is how the Dead Sea works — it's so salty that people float without even trying to swim. It's also why freshwater fish can't survive in the ocean (the salt concentration pulls water out of their bodies).
Engineers use the same idea to design submarines: pump water into the ballast tanks to sink, pump it out to rise — changing the density of the vessel, not its shape. If you could change the density of any liquid you wanted, what problems could you solve?
"Want the full teacher guide? The Crazy Scientist Lab includes classroom delivery tips, how to manage the WOW moment, differentiation for Stage 2 & 3, — ready to teach tomorrow."
Think Like a Scientist
Scientists don't just do ONE experiment; they change one part of the experiment (independent variable) and then see how it affects another part of the experiment
(dependent variable)
Change ONE variable and test again.
What happens if you use different materials of the same size?
What happens if you change the water (e.g. add salt)?
🧪 Try it! Change ONE thing and test again. What did you discover?
The Science Behind It
The potato didn't change. The water did. And that changes everything.
Fresh water has a certain density — a certain amount of stuff packed into every drop. A potato is just a little bit denser than fresh water, so it sinks. Not by much — but enough.
Now dissolve salt into that water. The salt packs extra mass into the same space, making the water heavier without making it much bigger.
The water's density creeps up... and at some point it overtakes the potato. Suddenly the potato floats.
Did you notice the water looks completely identical in every tube? You can't see the salt. You can't smell it. But the potato knows. It's reading the density of every liquid and responding perfectly every time.
And here's the really wild part — get the mix just right and the potato doesn't sink or float. It just hovers. Perfectly still. Suspended in the middle like it's weightless.
Scientists call that neutral buoyancy — and it's exactly how fish stay at the same depth without swimming, and how submarines dive and surface without using their engines.
A potato just taught you how a submarine works.
But how does a fish change its depth whenever it wants — without fins, without effort?
The answer is one of the cleverest things in nature. Find out in The Crazy Scientist Lab!
Know a parent or teacher who'd love this? Send it on! 👇
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