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Density & Buoyancy - Raisins in 7-up (Sara Wilson)

Author

Sara Wilson

Principle(s) Illustrated

  1. Density
  2. Buoyancy
  3. Forces

Standards

  • 8.8.a - Students know that density is equal to mass per unit volume.
  • 8.8.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.
  • 8.2.b - Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

Questioning Script

Prior knowledge & experience:

Prior knowledge here can depend upon what your purpose for using the discrepant event. It can be used for introducing Life Science and the idea of what is a living thing or developing understanding of buoyancy/density.

Life Science: preconceptions of what is needed to be alive from Elementary School like movement, etc...

Density/Buoyancy: Students need to have an idea that objects can move through water, float or sink. For full understanding of the science when not used as an introduction to density it would help for students to understand that density is a ratio of mass to volume comparison.

Root question:

Life Science: Is this a living thing? What evidence do you have to support your position?

Density/Buoyancy: Based upon your observation of the raisins, are they neutrally buoyant? Is their density greater than or less than that of water? What could you do to determine scientifically the density of the raisin.

Target response:

Life Science: Movement supports the idea that it is a living thing but there is no other evidence of it being a living organism. The bubbles seem to affect the movement so they are probably not living.

Density/Buoyancy: There is no way to determine the accurate density in this type of medium because the air bubbles are changing the density of the raisins when air gets stuck under the wrinkles it changes the volume of the raisin which doesn't much change the weight causing a difference in density and a floating to the top. A way to test raisin buoyancy would be to weigh and determine the water displacement of the raisins and find the average density for the raisins. Compare it to the known density of water.

Common Misconceptions:

Life Science:

  • Its alive because it is moving.
  • It is alive because it is swimming to the top to breathe like a dolphin.
Density/Buoyancy:
  • The bubbles make it float like a floatation device in a pool. They push up the raisin.
  • The raisins are neutrally buoyant because they go up and down.
  • The bubbles make the raisin lighter and that changes the density of the raisin which helps make it less dense than water.

Photographs and Movies

Raisins in 7-up


 

References

Variations on Water fleas
  1. Dancing raisins
  2. Sewer slugs
Interactive sites/Simulations
  1. Floating lab
  2. Balloons & Buoyancy
  3. Buoyancy
  4. Buoyancy Game Show
General Information on Density/Buoyancy
  1. Understanding Density/Buoyancy
  2. 8th Grade Density Curriculum
  3. Density/Buoyancy discoveries
  4. NOVA Ship sinking Buoyancy

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