A “soda bottle submarine” can mean a Cartesian diver that sinks when you squeeze its sealed bottle, or a rubber-band-powered model that uses a propeller to travel forward. Choose the diver to demonstrate pressure and buoyancy with simple materials; choose the propelled model to explore motion, balance, and stability. They are different projects, so the steps and explanations below keep them separate.
Which soda bottle submarine should you make?
| Build | What it demonstrates | What to expect | Best fit |
|---|---|---|---|
| Cartesian diver | Pressure, gas volume, density, and buoyancy | A small diver sinks when you squeeze the sealed bottle and rises when you release it | A quick demonstration of sinking and floating |
| Rubber-band-powered model | Buoyancy, propulsion, and stability | A twisted rubber band turns a propeller to move the bottle through water | A longer build focused on forward travel and design changes |
The American Chemical Society lists its condiment-packet activity for ages 5–12 and 10–15 minutes; Maker Camp describes its propelled project for ages 8 and up and 1–3 hours. Those are descriptions of those specific activities, not guarantees for every child, group, or build. [American Chemical Society; Maker Camp]
Build a Cartesian diver: the squeeze-and-release version
This is the simplest choice if the goal is to make an object sink and float without propelling a bottle across the water. You can use a condiment packet, a pipette diver, or a homemade pen-cap diver. In each case, adjust the object so it barely floats before you seal it into the bottle.
Option 1: Condiment packet
- Partially fill a clear one-liter bottle with water and add a condiment packet that just barely floats. The American Chemical Society notes that duck-sauce packets work well.
- Top off the bottle with water, then close the cap tightly.
- Squeeze the sides of the bottle to make the packet sink. Release your grip to let it rise.
Follow the American Chemical Society’s Cartesian Diver activity for its complete directions and safety guidance.
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Option 2: Pipette diver
- Adjust the air and water in a pipette diver in a bowl until it barely floats.
- Fill a one-liter plastic bottle with room-temperature water, place the diver inside, and close the cap securely.
- Squeeze the bottle and observe the diver sink; release it and observe it rise.
TeachEngineering describes this approach and notes that a diver may be made or purchased. See its Soda Bottle Cartesian Diver activity.
Option 3: Pen-cap diver
- Seal the hole at the top of a pen cap with clay.
- Add clay to the bottom until the cap barely floats when pushed below the surface of a bowl of water.
- Put it in a completely water-filled 20-ounce soda bottle, close the bottle, and squeeze its sides.
These directions are from the Virginia Tech Department of Physics Cartesian Divers Project.
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Why squeezing makes the diver sink
The bottle is sealed, so squeezing raises the pressure on the water and the diver’s trapped air. The air bubble compresses, and water occupies more of the diver. That increases the diver’s average density, so it sinks. When you release the squeeze, the air expands and the diver rises again. The American Chemical Society puts it this way: “When you squeeze the bottle, the air bubble inside of the diver is forced into a smaller space, making the gas more dense, and causes the diver to sink.” [American Chemical Society]
To investigate the activity question—how the air bubble changes as the object sinks and floats—watch the diver closely: the bubble gets smaller under pressure as the diver sinks, then expands as pressure is released and the diver rises. This is a simplified demonstration of pressure and buoyancy, not a model of all the systems a full-size submarine uses.
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Build a rubber-band-powered bottle model
This is the project to choose if you want the bottle to move forward. Maker Camp’s design uses two plastic bottles, a six-inch ruler, three rubber bands, two chopsticks, two paper clips, needle-nose pliers, and scissors. Its instructions use a paper clip and rubber band inside the bottle, attach the ruler as a stabilizer, and form a propeller from the base of the second bottle. The rubber band is twisted through the propeller; place the model in water and release it to test the motion. Follow the Maker Camp build instructions for the complete construction sequence.
The twisted rubber band stores energy and turns the propeller. The propeller pushes water, moving the model forward. As it turns, the body tends to rotate in the opposite direction; the stabilizing fin helps resist that counter-rotation so the propeller can turn while the model travels.
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Explore buoyancy and stability
- Change the air-and-water balance: More air tends to make the model more buoyant; more water tends to make it sink. Science Buddies suggests marking measured water volumes on the bottle and recording whether it sinks, rises, or stays level.
- Vary propeller shape: Compare how different shapes affect movement, without assuming one shape will always work best.
- Check the stabilizer: Observe whether the model spins or travels straighter when the fin is attached.
For measured-volume testing and further build details, use Science Buddies’ Bottled-up Buoyancy project. Its testing suggestions are experiments to try, not guaranteed performance results.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the U.S. Navy activity includes
The U.S. Navy National Museum of the U.S. Navy presents a separate bottle-submarine lesson about how buoyancy allows submarines to dive and ascend. Its listed materials include an empty 16- or 20-ounce plastic soda bottle, a cap with a hole large enough for a flexible straw, three wide rubber bands, 24 pennies, aluminum foil, adhesive tape, a flexible straw, and a large tub of water. The page points to an activity sheet for the procedure, so use that sheet for the build directions rather than assuming it is the same as Maker Camp’s propeller model. [U.S. Navy National Museum of the U.S. Navy]
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Safety and troubleshooting
Work safely
- For the American Chemical Society activity, follow its directions and warning labels, work with an adult, use suitable protective equipment, secure loose hair and clothing, clean up properly, and wash hands before and after. Do not drink the activity water. [American Chemical Society]
- Some propelled-model builds require cutting or drilling. Science Buddies advises adult help for difficult razor or knife cuts and for pool testing. Keep sharp tools with an adult, and do not take a model into a pool without adult supervision. [Science Buddies]
If the Cartesian diver does not behave as expected
- It does not sink when squeezed: Check that the diver barely floats before it goes in the bottle, the bottle is sealed, and the sides are being squeezed firmly enough.
- It stays down after release: Recheck the diver’s air-and-water balance; it may be too heavy to float back up.
- It floats too high or is inconsistent: Adjust the packet, pipette, or clay in small amounts until it barely floats, then try again.
If the propelled model does not travel well
- The bottle spins instead of moving forward: Check that the stabilizing fin is attached and positioned as the build instructions show.
- Water enters the bottle during testing: Find and seal openings before continuing; water entering changes the air-and-water balance and affects the experiment.
- It sinks, rises, or stays level unexpectedly: Record the water volume and outcome, then change the air/water balance in measured increments. This makes the result easier to compare than changing several parts at once.
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