Chemical Reactions are a must at Halloween. A great way to introduce students to chemical reactions is to use effervescent tablets, also known by the name brand: Alka-Seltzer.
Here is a great activity where the students take the lead on investigating chemical reactions. Instead of having the investigation procedures prepared the students come up with the procedures. This is a great way to introduce them to writing their own investigations. Having students design their own investigation on effervescent tablets helps them to understand how they work.
My students loved it. Make sure you leave enough time to plan their experiments. You also will need a variety of supplies to choose from. The students can get very creative.
To make it Halloween themed have things like glow powder, pumpkin shaped containers, Halloween candy, Halloween sugar toppings, food coloring, etc.