Density & Buoyancy - Raisins in 7-up (Rana Khan)

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.

Why is it happening?

Raisins are denser than the liquid in the soda, so initially they sink to the bottom of the glass. The carbonated soft drink releases carbon dioxide bubbles. When these bubbles stick to the rough surface of a raisin, the raisin is lifted because of the increase in buoyancy. When the raisin reaches the surface, the bubbles pop, and the carbon dioxide gas escapes into the air. This causes the raisin to lose buoyancy and sink. This rising and sinking of the raisins continues until most of the carbon dioxide has escaped, and the soda goes flat. Furthermore, with time the raisin gets soggy and becomes too heavy to rise to the surface.

We might want to try other objects to see if they exhibit this behavior. Any object whose density is just slightly greater than water’s and has a rough surface to which the gas bubbles can attach should be able to dance in the carbonated water. Some of the more common dancing substances are mothballs and pieces of uncooked pasta. Try putting other objects in the carbonated water. Can you find other substances that dance?

Carbonated beverages are prepared by putting the beverage into a can under high pressure of carbon dioxide gas. This high pressure causes the carbon dioxide gas to dissolve in the liquid. When you open a can of soda, the noise you hear is produced by the carbon dioxide gas as it rushes out of the can. When the can is opened, the decreased pressure allows some of the carbon dioxide gas dissolved in the liquid to escape. This is what makes the bubbles in a soft drink.

Alternative Method:

Another way to do this experiment is to generate the carbon dioxide gas using the reaction of baking soda and vinegar. Fill your glass about 1/2 full with water. Add one teaspoon of baking soda and stir until it is dissolved in the water. Add 6 or 7 raisins to the glass. SLOWLY pour in vinegar until the glass is about 3/4 full. The vinegar and baking soda react to form carbon dioxide bubbles, and the raisins will dance just as in the soft drink!

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