DIY · Card 29 · Engineering · Chemistry
A bottle, a straw, a spoonful of baking soda and a splash of vinegar. The fizz has only one way out, and when it rushes out the back, the boat shoots off the other way, like a tiny rocket.
01The brief
Make a boat that drives itself with a chemical reaction: no paddles, no propeller, no battery.
1.4 L
of carbon dioxide from one level teaspoon of baking soda, given enough vinegar
5 %
of ordinary vinegar is acetic acid: the rest is just water
1926
the year the first liquid-fuelled rocket flew, on the same push as your boat
Drop baking soda into vinegar and it foams up in seconds. Most people have done it once, watched the fizz and poured it away. But that fizz is a gas being made from nothing you can see, and gas that is made inside a closed bottle has to go somewhere.
Give it one way out, a straw pointing backwards, and the gas becomes a jet. The bottle is pushed forwards, exactly as a rocket is. This card is part chemistry, about counting molecules so nothing goes to waste, and part engineering, about turning that gas into the most push. It uses the same bubbles that lift the blobs in the homemade lava lamp, and the same kind of hull as the cardboard boat.
02Kit
A bottle from the recycling, a straw, and two things from the kitchen cupboard.
Tick off what you have
0 of 6 ready
03Method
Fit a straw through the cap, add vinegar and a parcel of soda, seal it, shake it and launch it.
Step 1 of 8 · Minute 0
You need a small plastic bottle with its cap, a bendy straw, a lump of modelling clay or reusable putty, baking soda, vinegar, a square of kitchen tissue, and a bath or long tray of water.
It fizzes, but the boat doesn’t move.
Why
The gas is leaking round the straw or the cap, or the straw is too wide.
Fix
Seal round the straw with more clay, tighten the cap, and try a narrower straw.
It rolls over, and the straw points the wrong way.
Why
A round bottle has nothing to stop it rolling.
Fix
Tape two pencils or lolly sticks along its sides as outriggers, or a flat piece of card underneath as a keel.
Foam squirts out of the straw.
Why
Too much soda, or too much shaking, makes foam that fills the bottle.
Fix
Use less soda, shake more gently, and point the straw up a little out of the water.
04Chemistry and physics
Baking soda is sodium bicarbonate, a mild base. Vinegar contains acetic acid. When they meet, the acid hands over a hydrogen, and the bicarbonate falls apart into water and carbon dioxide. Every molecule of soda that reacts makes exactly one molecule of gas, but only if it finds a molecule of acid to react with.
That makes it a counting problem. Chemists count molecules in moles: a mole of baking soda weighs 84 g, and a mole of any gas at room temperature fills about 24 litres. Whichever ingredient runs out first, the limiting reagent, decides how much gas you get. Mix them and count:
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂
A level teaspoon is about 5 g.
Carbon dioxide
1.00 L
Half-litre bottles of gas
2.0
Soda left over
1.5 g
Vinegar left over
0 mL
Vinegar is mostly water: only 5 % is acid. Here it runs out first, and the leftover soda sits at the bottom doing nothing. Add vinegar, not soda. Each mole of gas fills about 24 litres.
Then the gas has to do some work. Squeezed out through the straw, it leaves at speed, carrying momentum backwards. Momentum can’t appear from nowhere, so the bottle gets the same amount forwards. A narrower straw makes the same gas leave faster, and the push grows with the square of that speed. Water drag fights back, harder the faster the boat goes. Launch it:
Distance
88 cm
Top speed
13 cm/s
Strongest push
31 mN
Total push
38 mN·s
All the gas leaves within a few seconds, in one strong push. Try a narrower straw, or wrap the soda to stretch the push out.
The race runs on a simple model: gas only, made at a rate that dies away, with plenty of vinegar. Real boats also spit foam and liquid, which can add to the push, and leak a little. The trends are the real ones; test them in the bath.
05The big idea
Throw it back,
go forward.
Every action has an equal and opposite reaction. Isaac Newton wrote it down in 1687, and nothing that moves itself can escape it. A swimmer pushes water back, a car’s tyres push the road back, and a rocket throws its own exhaust back.
Your boat does it with a chemical reaction. Chemistry makes the gas, physics turns it into motion. Rocket engineers do the same two jobs on a vast scale: choosing what to burn, and shaping the nozzle so the exhaust leaves as fast as possible.
06The math
1 · How many moles
n = m ⁄ M
Mass divided by molar mass. Baking soda is 84 g per mole; acetic acid 60 g.
2 · How much gas
V = n × 24 L
One mole of carbon dioxide per mole of whichever ingredient runs out first; about 24 litres a mole at 20 °C.
3 · How much push
F = ṁ v = ρ Q² ⁄ A
Thrust is the mass of gas leaving each second times its speed. For a flow Q through a straw of area A, that is ρQ² ⁄ A.
A level teaspoon, 5 g of soda, is 5 ⁄ 84 ≈ 0.060 mol. Vinegar has 5 g of acetic acid per 100 mL, which is 5 ⁄ 60 ≈ 0.083 mol. So 100 mL of vinegar is more than enough, and the soda is the limit: 0.060 × 24 ≈ 1.4 L of carbon dioxide, weighing about 2.6 g.
Squeeze that through a straw 5 mm across and, if it all comes out in a few seconds, it leaves at tens of metres a second. The thrust is small, a few hundredths of a newton, the weight of a couple of grams. But a floating bottle has very little to fight, so even that is enough to send it gliding down the bath.
07Real life
From rockets to squid to the cake in the oven, gas made on the spot does real work.
Rockets
A rocket carries everything it pushes against. Burning fuel makes hot gas that rushes out of the nozzle, and the rocket is pushed the other way. It works in space because it doesn’t push on air at all, only on its own exhaust, exactly like your bottle.
Squid
A squid fills the cavity inside its mantle with water, then squeezes it out through a narrow funnel. The jet shoots it backwards. Squid can aim the funnel to steer, and some species jet so hard they leap clear of the sea and glide.
Water jets
Jet skis and some fast ferries have no propeller showing. A pump sucks water in under the hull and fires it out of a nozzle at the back. Fire boats and garden hoses kick back for the same reason: anything that throws fluid one way is pushed the other.
Soda-acid extinguishers
For most of the twentieth century, many fire extinguishers held a solution of baking soda with a small bottle of acid inside. Turning the extinguisher upside down mixed them, and the carbon dioxide pushed a jet of water out of the hose, the same chemistry driving a jet.
Baking
Baking soda’s day job is making cakes rise. Mixed with something acidic in the batter, like buttermilk, yoghurt or the acid in baking powder, it makes carbon dioxide, and the bubbles swell in the oven’s heat until the batter sets round them.
Airbags
A car airbag fills in a few hundredths of a second because a chemical reaction makes a burst of gas almost instantly. Engineers choose the chemicals to make exactly the right amount of gas, the same mole counting you do with soda and vinegar.
1926
On 16 March 1926, on his aunt’s farm in Auburn, Massachusetts, the physicist Robert Goddard lit a rocket of thin pipes burning petrol and liquid oxygen. It flew for two and a half seconds, climbed about 12 metres, and landed 56 metres away in a cabbage patch.
It was the first liquid-fuelled rocket ever to fly, and it worked on exactly the principle of your bottle: a chemical reaction makes hot gas, a nozzle squeezes it into a fast jet, and the jet pushes the rocket the other way. Newspapers had mocked Goddard for claiming a rocket could work in a vacuum. Forty-three years later, rockets descended from his carried people to the Moon.
08Is and isn’t
The gas pushes against the water behind the boat.
It doesn’t need anything to push against. The gas is pushed out of the bottle, and the bottle is pushed the other way. That is why rockets work in empty space.
More baking soda always means more gas.
Only while there is acid to react with it. Vinegar is just 5 % acid, so a big spoonful of soda in a little vinegar leaves soda sitting unused at the bottom. Balance the two.
The fizz is the baking soda dissolving.
Dissolving makes no gas. The bubbles are a new substance, carbon dioxide, made when the soda and the acid swap atoms. The liquid left behind is sodium acetate solution, not vinegar.
Mixing them makes a dangerous explosion.
It makes a harmless gas. The only danger is pressure: in a sealed bottle with no way out, the gas can pop the cap off hard. The straw is the safety valve, so never block it for long.
09Go further
A
Find the balance
Keep the vinegar at 100 mL and try 2, 4, 6, 8 and 10 g of soda. Measure how far the boat goes each time. Where does adding more stop helping, and why?
B
Straw trial
Make three caps with a thin, a normal and a wide straw. Which pushes the boat furthest? Does the thinnest really win?
C
Catch the gas
Instead of a straw, stretch a balloon over the bottle’s neck and add soda. Measure the balloon round the middle and work out how much gas you made.
D
Other acids
Try lemon juice, or citric acid powder dissolved in water, in place of vinegar. Which fizzes hardest? Which pushes the boat furthest?
E
Make it steer
Bend the straw a little to one side. Can you make the boat turn in a circle? How do squid and rockets steer the same way?
10Questions
Push a bendy straw through a hole in the cap of a small plastic bottle and seal round it with clay. Pour about 100 mL of vinegar into the bottle, drop in a teaspoon of baking soda wrapped in tissue, cap it, and shake gently with a thumb over the straw. Lay it in water with the straw pointing backwards and let go.
Baking soda and vinegar react to make carbon dioxide gas. The gas can only escape through the straw, so it rushes out backwards. Pushing gas backwards pushes the boat forwards, by Newton’s third law: the same principle that drives rockets and jet engines.
Sodium bicarbonate (baking soda) reacts with acetic acid (in vinegar) to make sodium acetate, water and carbon dioxide: NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. It is an acid–base reaction, and each molecule of soda that reacts releases one molecule of gas.
A level teaspoon, about 5 g, is 0.06 moles of sodium bicarbonate. With enough vinegar, about 70 mL of the ordinary 5 % kind, it makes 0.06 moles of carbon dioxide, which fills roughly 1.4 litres at room temperature: nearly three times the volume of a half-litre bottle.
To give yourself time. Loose powder reacts the moment it touches vinegar, so the gas escapes before the cap is on. A tissue parcel holds the vinegar back for a few seconds, long enough to seal the bottle, and it stretches the push out over a longer time.
Safety · A grown-up should make the hole in the cap. Never seal the bottle without the straw, keep the straw pointed away from faces, and wear glasses or goggles: vinegar stings eyes. Rinse with clean water if any splashes.
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