STEM Activities: How Fish Sink and Float

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Kimberly Hobbs explains how fish achieve buoyancy through adaptations like low-density compounds, movement, reduced weight, and swim bladders. She demonstrates a simple experiment to illustrate these principles, showing how changing the volume of a gas-filled sac affects an object’s ability to float or sink in water.

Learn the answers to questions such as:

What role does the swim bladder play in fish buoyancy?
How does temperature affect the buoyancy of water and, consequently, fish?
How do marine mammals differ from fish in terms of buoyancy and swimming?

Materials Needed:

A clear glass or plastic bottle (with a wide enough neck for a deflated balloon to fit inside)
A balloon
A flexible plastic tube (like aquarium tubing)
Tape (waterproof or strong adhesive tape)
A large tub filled with water

Preparation:

1. Set Up Your Workspace:
Choose a surface that can tolerate water spills. Have towels nearby for cleanup.
2. Assemble the Balloon System:
Insert one end of the plastic tube into the neck of the balloon. Securely tape the tube to the balloon so that air does not escape during inflation.

Check the seal by inflating the balloon slightly through the tube and ensuring no air leaks.
3. Place the Balloon in the Bottle:
Carefully insert the deflated balloon into the bottle, leaving the tube sticking out through the opening.

Tape the tube to the mouth of the bottle to hold it in place and create an airtight seal.
4. Prepare the Water Tub:
Fill a large tub with enough water to fully submerge the bottle.

Instructions:

1. Initial Observation:
Place the bottle (with the deflated balloon inside) into the tub of water.

Observe and note whether the bottle floats or sinks.
Think about: Why does the bottle behave this way?
2. Inflate the Balloon Slightly:
Blow gently through the tube to partially inflate the balloon inside the bottle.

Observe the bottle’s position in the water.
Think about: How does inflating the balloon a little affect the bottle’s buoyancy?
3. Inflate the Balloon Fully:
Continue blowing into the tube to inflate the balloon until it almost fills the entire bottle.

Observe the bottle’s floating behavior as the balloon expands.
Think about: What happens as the balloon takes up more space inside the bottle?
4. Deflate the Balloon:
Let the balloon deflate gradually by releasing the air from the tube.

Observe how the bottle’s position in the water changes.
Think about: Why does deflating the balloon affect the bottle’s buoyancy?
5. Experiment Further:
Inflate and deflate the balloon multiple times to see how the bottle’s buoyancy changes.
Ask yourself: How does this activity simulate how fish use their swim bladders to control buoyancy?

Books recommended by Kimberley to learn more about fish adaptations are:

Anti-Freeze, Leaf Costumes, and Other Fabulous Fish Adaptations by Laura Perdew
Mudskippers and Other Extreme Fish Adaptations by Jody Sullivan Rake
Fish by Angela Royston
DK Eyewitness Books: Fish by Steve Parker
Fish Everywhere by Britta Teckentrup
What a Fish Knows: The Inner Lives of Our Underwater Cousins by Jonathan Balcombe

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