Density & Bouancy-Ice in Alcohol (Paul DeCunzo)
Author
Paul DeCunzo
Principle(s) Illustrated
Density
Buoyancy
Solubility
Standards
8a. Students know density is mass per unit volume.
8b. Students know how to calculate the density of substances (regular and irregular
solids and liquids) from measurements of mass and volume.
8c. 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.
8d. Students know how to predict whether an object will float or sink.
Questioning Script
Prior knowledge & experience:
Ice floats in water. Is less dense.
Root question:
What will happen to the ice cubes if they're placed in each solution? Sink or Float?
Target response:
Density: "It will float."Materials that are less dense than water (or have a density lower than 1.00 g/cm3) will float in water, while materials that are more dense than water (or have a density greater than 1.00 g/cm3) sink. The same is true of any liquid, such as ethyl alcohol with a density of 0.79 g/cm3. Materials less dense than ethyl alcohol will float in it, while materials more dense will sink. Therefore as observed in this demonstration, ice with a density of 0.92 g/cm3 will float in water but will sink in ethyl alcohol.
Bouyancy: the weight of water displaced is greater than the weight of the ice cube. The weight of the alcohol displaced is less than the ice cube so the alcohol cannot support the ice and the ice sinks
Soubility: Food coloring is soluble in water but not the alcohol.
Common Misconceptions:
That all clear, colorless liquids are water and/or have the same properties of water.
Ice cube would float in both solutions.
Photographs and Movies
Extension Demonstrations
The ice will float between layers and after melting will drip to the bottom.
References
Reference 1:
Flinn Scientific Demo Directions
Reference2:
Ethyl alcohol's density=0.79 g/cm3
Water's density=1.00 g/cm3