Floating Solid

Description

Students often operate with the misconception that solids sink when placed in liquids. When a hole is drilled in a paraffin disc, students unfamiliar with the properties of paraffin are surprised that the disc does not sink when placed in a container of water.

Go to Top

Hot paraffin is flammable.

Go to Top

Use a flameless heat source and a double-boiler of some type to melt the paraffin. Use caution when removing the molten paraffin from the double boiler. Have solid NaHCO₃ available to cover any burning paraffin.

Go to Top

1. Cut about 2 cm of paraffin from a paraffin block and place it in a cup to be heated. Place the cup in a pan of hot water. Heat using a flameless heat source
2.  Wait until melted. Pour the liquid paraffin into a watch glass. The watch glass serves as a mold. Allow the paraffin to cool to room temperature.
3.  Place a cooled, hardened paraffin disc in a 600-mL beaker half filled with water. Attempt to submerge it.
4. Use a small screw to drill a hole in the paraffin.
5. Attempt to submerge the paraffin disc.
6. Observe.

Go to Top

Name ___________________________ Class _______

Teacher __________________________

DoChem 003 Floating Solid

Answer these questions after viewing the movie up to the point where the paraffin has been poured.

1. Will the solid paraffin float?
2. Will the solid paraffin float if there is a hole in it?

Answer these questions after you view the entire demonstration.

1. Can steel float? Explain
2. What property of a substance is important in determining whether a substance floats?
3. Would the liquid make a difference?
4. How could this property be used to separate mixtures of compounds?

Go to Top

Purpose

To illustrate the buoyancy of some solids on liquids.

Go to Top

• 20 g paraffin
• hot plate
• 600-mL beaker (or larger) filled with water
• paper towels
• pan and styrofoam cup for melting paraffin
• hot pad
• watch glass
• 1 inch long wood screw (or screw eye)
• dull knife

Go to Top

You may want to cast several pieces of paraffin before the students come to class. Storing the paraffin castings in a refrigerator for several hours makes them harder but more brittle than cooling at room temperature.

Go to Top

Teacher preparation: 20 minutes

Class presentation: 10 minutes

Go to Top

The solid paraffin may be disposed of with ordinary solid wastes, or remelted in a suitable storage vessel and kept for reuse.

Go to Top

• A common misconception is that solids cannot float unless they have a shape specifically designed for floating, as in a boat. The experiment is intended to deal with this misconception. It is effective with students who are not familiar with the properties of paraffin.
• Begin by asking if the solid will float. After the hole is drilled, but before you try to float the paraffin, ask whether the paraffin with a hole will float. At this point, the misconception is likely to become voiced by several students.

Go to Top

Presentation Question:

• Can steel float?
  • The criterion for floating is based upon the density of the system, which may involve averaging the densities of many subcomponents such as steel and air. A steel boat can float. (A steel needle can float due to surface tension of the liquid.) According to Archimedes principle, an object immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid.

Go to Top

The paraffin does not sink because, regardless of its shape, it is still less dense than water.

The upward force of the water on the paraffin is called a buoyant force. When the paraffin is placed in the water, some of the water is displaced by the paraffin. The volume of water displaced is equal to the volume of paraffin submerged. An immersed body is buoyed up by a force equal to the weight of the liquid it displaces. This relationship between buoyancy and displaced liquid is known as Archimedes' Principle.

Go to Top

• float
• sink
• discrepant event
• misconception
• density
• buoyancy
• buoyant force
• Archimedes' Principle
• water displacement

Go to Top