I found this activity on a science forum. The teachers were all talking about how much they loved it. It is a great way to show students the informal part of the scientific method. The scientific method isn't always done in the same way. It is still systematic and precision is important, but sometimes it is not in a formal way by starting with a hypothesis.
The premise is to teach the kids how to discover what substances cause what parts of the reaction. They first see the complete reaction with all 4 substances. This will produce heat, foaming, gas production, and a color change. Then they combine two substances at a time to see what they do together to discover their role in the reaction. Such a great way to teach about chemical reactions. All of this is done in a zip lock bag, so clean up is easy. Performing the reaction in a bag allows them to see the gas production and feel the temperature change. Some combination will do nothing. Others will be exothermic or endothermic reactions. Some will even produce a color change.
Here is the link for the specific directions and worksheets. It does a fabulous job explaining the science that I won't repeat it here.
Changes to the experiment.
I made same changes to make it more cost effective and use materials I already had.
Calcium Chloride: I bought Damp Rid at my local superstore. I use the unscented, which is pure calcium chloride.
Red cabbage produced a different change than the phenyl red. Both are indicators, but produce different color changes. Red cabbage is an acid/base indicator. In the reaction with all 4 ingredients it produced a purple color. Most likely because the substance were neutralizing. In the presence of a base it will turn more green. In the presence of an acid it will turn more pink.
When students began testing two items together they quickly noticed that sodium bicarbonate and red cabbage juice turned green. Sodium bicarbonate is a base. This really interested them because it did not happen when all 4 substances were mixed. They were also surprised because the liquid gets cool. When all 4 are mixed the students feel the heat from calcium chloride and water reacting.
If you have a group of 5th-8th graders I strongly recommend this activity. It is memorable and really applies higher order thinking skills. Click the link above for the detailed directions.