This is a lesson that I got from another teacher, who I think got it from Science Gems. My eighth graders have to learn what chemical reactions are and how to recognize them. Our textbook describes 5 types of evidence that a chemical occurred. They are color change, temperature change, gas production, precipitation and change in properties (like smell , taste, texture, etc.). In this lesson students have to look for which types of evidence occur when calcium chloride, sodium bicarbonate, and phenol red solution are mixed together. What is really cool about this lesson is that students get to feel the reaction. The chemical reaction takes place in a sealed plastic bag. It inflates because carbon dioxide gas is produced, it gets very hot, it changes from a red color to a yellow color (the carbon dioxide makes the liquid acidic), and the smell at the end is a chalk smell (calcium carbonate). Download the student handout and view the quicktime video (or Google Video) and pictures on how the lesson works.
Tips: Emphasize to the students to feel the solid chemicals immediately after all chemicals are mixed together so they are able to sense the temperature change.