Sidewalk chalk is used outdoors and washes away with water. Chalkboard chalk is used in instructional settings and removed with a special eraser. There are a few key differences between sidewalk chalk and chalkboard chalk, despite their similar ingredient.
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Sidewalk chalk and chalkboard chalk were once made of calcium carbonate. Calcium carbonate is a type of soft limestone that forms underwater. Sidewalk chalk and chalkboard chalk are now made from the mineral calcium sulfate, or gypsum, explains the Earth Science Picture of the Day website.
Chalkboard chalk is approximately .35 of an inch in diameter and 3.15 inches long. Sidewalk chalk, on the other hand, is typically larger in length, thicker and comes in an array of colors.
Sidewalk chalk and chalkboard chalk have different functions. Sidewalk chalk is chiefly used in artistic applications with asphalt and cement being the canvas of choice. Chalkboard chalk is primarily found on chalkboards to convey text.