DIY Sidewalk Chalk

Materials

· 2 Tongue Depressors

· 4 tbsp (2 containers) Tempera Paint

· 2 Plastic Disposable Cups

· 2 (1/3c) bags Plaster of Paris

· 2 Silicone molds

· Chilled water

Procedure

1. In each plastic cup dump in the tempera paint.

2. Add 1/3c chilled water into each cup and gently stir. (color blobs will smooth out in the next step)

3. Dump 1 bag of Plaster of Paris into each cup with paint/water mixture and stir things together until combined.

4. Carefully pour mixtures into silicone molds. You can mix the colors, layer them, swirl them or keep them separate!

5. Let chalk firm up for 1-2 hours then carefully remove from molds and let dry for 24-48 hours so the inside can completely dry!

6. PLAY!

Notes: Plaster of Paris should NOT be put down any drains, it will harden and can cause pipe damage. This is why we are using disposable cups and tongue depressors for this activity.

The Science Behind it:

Plaster of Paris is made from gypsum, a soft mineral that contains sulfur (CaSO4·2H2O) that has been heated to about 300 degrees. This heating causes some of the water in the gypsum to evaporate, which changes it into anhydrite. Adding water to the anhydrite (or plaster powder) reforms the gypsum into a soft calcium solid. Plaster of Paris is commonly used in building (drywall) and making crafts.

Plaster of Paris heats up as it hardens. In chemistry, it is called an exothermic reaction because it transfers energy to its surroundings.