Back-to-School Sidewalk Chalk Science Activity

This activity starts with plaster of Paris, a dry powder made from a form of calcium sulfate. When water is added, more than just mixing happens — the plaster begins a chemical reaction with the water. New hydrated calcium sulfate crystals form and grow together, causing the soft mixture to become hard and solid.

At the same time, students can observe several physical changes. The food coloring spreads throughout the mixture, the wet plaster takes the shape of its mold, and excess water eventually evaporates into the air. Evaporation is a physical change because the water changes from a liquid to a gas but is still water.

The hardening of the plaster is different. Because the plaster reacts with water to form a material with a different chemical structure, setting the plaster is a chemical change. This experiment is a great way to see physical and chemical changes happening together — all while turning a powdery material into colorful sidewalk chalk you can actually use!

Back-to-school activity extension: the final step could easily become a class extension: have students use their homemade chalk to draw something that represents them, write a science question they have for the year, or create a collaborative “Science Is Everywhere” sidewalk mural.