Teacher Support Materials:

Convection-Diffusion Demonstration

Overview:

This demonstration of convection and diffusion uses three systems of two bottles containing water. In one system, a cold bottle is inverted above a warm bottle. In another system, a warm bottle is placed above a cold bottle. In the third system, both bottles are of equal temperature.

Food coloring is added to the bottle on the bottom of each system and a thin barrier separates the two bottles of each system. When the barrier is removed, students will observe convection and diffusion through the movements of the colored water.

Teacher and Student Objectives:

·  Demonstrate convection in liquids.

·  Demonstrate diffusion in liquids.

·  Differentiate between diffusion and convection.

·  Predict how liquids will behave when a barrier separating them has been removed.

Who’s Being Taught?

This demonstration can be used with nearly any age group. The attached worksheet is designed for an 11th – 12th grade Earth science class. The class should already be familiar with convection and diffusion.

Strengths of Exercise:

·  Visual representation of convection and diffusion.

·  Clarifying the differences of convection and diffusion.

·  By having a diffusion system it makes it apparent that the convection of the warm water on top is counteracting diffusion of the lower cold water.

·  The diffusion system also acts as a control group.

How to Assess:

·  Ask for volunteers to give predictions and explanations.

·  Compare the explanations given by the students after the demonstration to the answers written for the first question on the worksheet.

What to Look Out For:

·  Make sure the bottles of water are big enough and positioned so that the entire class can see them.

·  Be sure to have a pan or tub under the bottles to catch any spilled water.

·  When you invert the bottles, you should have the tub and the bottom bottles where you want them. You can’t move the tub after the bottles are on top.

·  You should set up only a few minutes before the demonstration. You do not want the top bottles to fall off when you are not paying attention.

·  Make sure to use something thin between the bottles to minimize the effect of pulling it out.

Technical Information:

·  Convection is the internal movement of currents within liquids and gases. This movement is caused by a difference in the movement of molecules (heat) of two or more liquids or gasses which in turn causes a difference in density. This difference in temperature will cause the hotter fluid to be less dense than the cooler fluid. The fluid with the lower density will rise while the fluid with the higher density will sink. Convection cannot occur in solids due to the particles not being able to flow freely.

·  Diffusion is the spontaneous movement of particles, heat, or light from a higher concentrated area to a lower concentrated are. Diffusion is driven by entropy.

Materials:

·  4 bottles (sports drink bottles work well because of their wide mouths)

·  Food coloring

·  Something thin to go between the bottles. (I used a transparency cut into squares)

·  Tub or pan to catch spilled water

Procedure:

Before Class:

  1. Prepare bottles with appropriate temperature water. You should have two cold, two hot, and two warm bottles. The greater the temperature difference between the hot and cold bottles, the more dramatic the convection effect.
  1. Add food coloring to the bottles that will be on the bottom. Color doesn’t matter, but if you use the accompanying work sheet you should use the following color configuration:

  1. Place colored bottom bottles into a tub with low sides. The tub will catch most of the water you spill inverting the bottles. The tub sides should not be so large as to block the students’ view of the bottom bottles. The tub should be in a position that maximizes the view for the class.
  1. Hold square of transparency over the mouths of the bottles that will go on top as you invert them and place them on the colored bottles in the tub.

During The Demonstration:

  1. Hand out Convection and Diffusion worksheet. Indicate the temperature of each bottle in the demonstration to the class. Instruct the students to answer the first question before you pull out the barrier.
  1. Pull out the barrier of each system carefully and watch the results!

Follow up:

·  After the initial excitement of the demonstration has worn off, ask for volunteers to explain what happened in each of the three systems.

·  If the class has trouble with explaining the demonstration, be sure to go over the ideas that they are having trouble with.

·  Have the students finish the worksheet.

Name:______

Hour:______

Convection and Diffusion

  1. What do you think will happen in each system? Why? (3 pts)

Blue-

Green-

Red-

  1. Explain what you observed for each system (3 pts)

Blue-

Green-

Red-

  1. In which system did diffusion take place? (1 pt)

A. Blue B. Green C. Red D. All three

  1. In which system did convection take place? (1 pt)

A. Blue B. Green C. Red D. All three

  1. Which has the lowest density? (1 pt)

A. Hot Water B. Cold Water C. Warm Water

  1. Which molecules have the higher temperature? (1 pt)