Author

Chris Kipp

Principle(s) Illustrated

1. Density

2. Buoyancy

Standards

Density and Buoyancy

8. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:

8.c Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced.

Questioning Script

Materials

• fish tank

• sulfur hexafluoride gas

• aluminum sheet

• beaker

Procedure

1. Fill the tank with the sulfur hexafluoride gas. Keep a cover on the tank while filling it to keep excess gas from escaping.

2. Fold the aluminum sheet into the shape of a boat.

3. Place the boat gently into the tank. It should float on the surface of the gas.

4. Dip the beaker into the sulfur hexafluoride. Pour into the boat. It should now sink.

Prior knowledge & experience:

• Students may be unfamiliar with the gas, sulfur hexafluoride.

• Students may understand that solids are usually denser than gases.

Root question:

Pre-Demonstration

1. Is it possible for a metal to float on top of a gas?

During the Demonstration

1. Observe the boat. What makes it float?

2. Why did the boat sink? What is being poured into the boat?

Post-Demonstration

1. What can you infer about the density of the boat versus the density of the gas?

2. If the aluminum were crumpled into a ball, would it still float on the sulfur hexafluoride?

3. What possible use could you imagine for this incredibly dense gas?

Target response:

BUOYANCY

Students will be surprised to learn that a solid can float on a gas. This is counterintuitive since most students have experience of solids falling through the air and even through some liquids.

Any object feels an upward force called "buoyancy" from the fluid it sets in. Objects in water feel the upward buoyancy force from the water. You feel an upward buoyancy force from the air, as does a helium balloon. Every object also feels a downward force from gravity: every object feels its weight. When an object weighs more than the upward buoyancy, the object sinks. When weight is less than buoyancy, the object floats.

The buoyant force felt by an object is equal the weight of the fluid that it displaces. This means that if a solid can displace a large enough volume of a fluid, even a gas, it will be able to float in it.

Common Misconceptions:

Students will assume that the metal will not float on the gas, because metals are dense.

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