Colored Water Demo
by Chris Herald, Manhattan, KS

This demonstration involving colored water has appeared before in Level Line but it will be repeated because of an inquiry-based science idea.

Materials: 4 quart canning jars, labeled #1-4, hot and cold water, blue food coloring, index cards, plastic trays or tubs, and paper towels for cleanup (as necessary)

Fill quart canning jars #1 and #3 to the brim with HOT water. Fill jars #2 and #4 with COLD water. Add blue food coloring to jars #1 and #2. Add yellow coloring to jars #3 and #4. Using an index card over the mouth of the jar #2, invert it over jar #3. In the same way, invert jar #1 on top of #4. Have the class predict what will happen when you pull the cards out. Have them write their predictions on paper. Pull the cards out and observe the colored water. This demonstration shows convection currents and density. Hot water is less dense than cold water.

As an extension to this demonstration, one could also use a jar of colored salt water at a room temperature and a jar of tap water at the same temperature. Place the salt water on the top, since it is more dense, so that students can see it disperse in the tap water. Again, make the students predict before they observe the demonstration.

Editorial Comment:  After you do these demonstrations, ask the question,"Will an ice cube melt faster in salt water or tap water?"  Let your students hypothesize about this problem.  They may have several ideas and explanations.  Let them experiment and see what happens.  Using uniform sized and COLORED ice cubes is essential.  Using salsa cups with lids for stacking in the freezer is one possibility.  This experiment was conducted by high school students and written in the Journal of Chemical Education, September 1999 by Annis Hapkiezicz, entitled "Authentic Research within the Grasp of High School Students". Middle school students will not get so involved as high school students but they can have a lot of fun with this kind of inquiry-based science.  Do you know the answer? Find out with the aid of your students.