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Whelmer #49: Potato Float


Picture of three glasses of water with a potato wedge floating in each: one at the top of the water, one in the middle, and one at the bottom of the glass.Description:
A slice of potato mysteriously floats in the exact center of a glass of water.

Science process skills:

  1. observation
  2. space/time relations
  3. prediction
Complex reasoning strategies:
  1. comparison
  2. induction
Standards:
K-4:
  • Use appropriate tools and techniques to gather, analyze, and interpret data (Standard A.1.3).
  • Use data to construct a reasonable explanation (Standard A.1.4).
  • Objects have many observable properties (Standard B.1.1).
5-8:
  • A substance has characteristic properties, such as density, which are independent of the amount of the sample (Standard B.1.1).
9-12:
  • No standard.
Above Standards from the National Science Education Standards.

Content topics:
  1. solubility
  2. density
You will need:
  1. three tall beakers or glass tumblers
  2. water
  3. sugar
  4. spoon
  5. knife
  6. potato
Instructions:
Cut several 1" wedges from the potato. Try to cut the pieces to the same size. This is a density activity. Students might be misled if they see that the pieces are not all the same size.

Make a strong sugar solution by dissolving sugar in water. Dissolve sugar until a piece of potato will float in the solution. If the solution appears cloudy, allow it to stand for a few minutes until it clears.

Fill one glass with the clear sugar solution. Fill a second glass with pure water. Fill the third glass half full of sugar solution. Very carefully and very slowly pour pure water on top of the sugar solution in the second glass. Pour the water down the side of the glass or use a spoon or glass rod to direct the stream against the side of the glass. If you are careful and avoid too much mixing, the less dense water will "float" on the sugar water. The sugar water should be clear enough to prevent any indication that the second glass is filled with anything but water.

Make certain that all three glasses are filled to the same level. Carefully place a wedge of potato in each glass. The potato floats at the top of the liquid in the first glass. The potato sinks to the bottom of the second glass, and it sinks to the middle of the third glass.

Presentation:
Before your presentation, fill the three glasses with the proper solutions. Do not allow students to see how you made the sugar solution, or how you created the layered solution in the third glass.

Place a piece of potato in the first glass. The piece of potato floats. Ask students which is more dense; the potato or the liquid? (the liquid)

Place a piece of potato in the second glass. The piece of potato sinks to the bottom. Ask students which is more dense, the potato or the liquid? (the potato)

Inform students how you made the solutions in the first two glasses. (The first is sugar-water, the second is pure water.)

Place a piece of potato into the third glass. The potato sinks, stopping at the middle of the glass. Inform them that you used only sugar and water to make the solution in the third glass, but do not tell them how you did it. Ask students to share their thoughts concerning the third glass. How do they think you did it? Challenge them to use sugar and water to create the effect themselves.

Content:
The density of water is 1 gram/milliliter. The density of saturated sugar-water (common household sugar) is ~1.83 grams/milliliter. Sugar-water is almost twice as dense as water. Most varieties of potatoes have a density of ~1.6 g/ml.

Assessment:
Type: class or small group.
Content/Process: density.
Age/Level: all.

Use Whelmer #64: Liquid Rainbow as an assessment for this activity. The directions are given following the Whelmer #64 activity.



Notes:





  
     
 

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