The Chemistry of Sidewalk Chalk


Chalk was first used in the sixteenth century for street art and is still frequently used today. It also absorbs moisture and slows tarnishing of silver, so if you need to polish some silverware put two pieces of chalk in your silverware drawer. Have a stubborn grease stain on your shirt? Rub some chalk on it before washing it again. One of the best things about chalk is that little kids can’t get enough of it! Some of my best childhood memories are from drawing on the sidewalk with my cousins and grandparents when I was little. So in a small way, chalk has had an effect on my life and the lives of small children everywhere.

Composition of ...

Sidewalk chalk consists of Plaster of Paris, water, and powdered tempera paint. As for the composition of Plaster of Paris, is consists of a powder of heated gypsum that hardens when water is added to it. Water consists of hydrogen ions (H+) and hydroxyl ions (OH-) In the simplest of terms, it is two hydrogen atoms and an oxygen atom bonded together. Lastly, powdered tempera paint consists of egg yolk, powdered pigment, water, and clove oil.

Main Chemicals, Compounds, Components

One of the two main components of sidewalk chalk is Plaster of Paris. The molecular formula of Plaster of Paris is 2CaSO4· 1/2H2O. It is commonly called Gyprock and is formed when gypsum is heated to 150⁰C and forms a powder. When water is added to that gypsum, it liquefies again, but hardens quickly. Some of the main uses of Plaster of Paris include casts for broken limbs, modeling casts, plasterboard walls, and sculptures.

The other main component of sidewalk chalk is water. The molecular formula of water, obviously, is H2O. To form water, hydrogen atoms share their electrons with oxygen atoms and form a covalent bond. About 70 percent of the human body is made of water and about 75 percent of the Earth is covered in water. Also, water is the ultimate solvent and dissolves most substances.

Chemistry's Role

Water is naturally occurring, but Plaster of Paris is man made. Plaster of Paris is created by heating up gypsum, adding water to it, and letting it harden again. Overall, sidewalk chalk is man made. What is chemistry’s role? In order for the production of chalk to be successful, all of the ingredients have to be constantly stirred when they are combined.

Background Research

To make chalk, start by mixing all of the ingredients together. For every cup of Plaster of Paris, use ¾ of a cup of water, the amount of paint does not matter. In order for chalk to properly form, stir like crazy. If desired, chalk can be poured into molds to form fun shapes. Another important step is clean up, because if Plaster of Paris is poured down the sink it will set up in the pipes.

Chalk has been used since ancient times and was used to create early cave drawings. A little later, artists began using chalk for sketches and was first formed into sticks so the artists could use it with ease. Carbon was added to the chalk to enhance black chalk and ferric acid was added to enhance red chalk. It was not until the 19th century that chalk was introduced to classrooms. In those days, chalk created a lot of dust, but now pretty much all chalk is dustless. Chalk that goes to classrooms must go through several tests in order to be considered nontoxic. Erasability is another desired quality of chalk before it goes to classrooms.


Composition, functions, and strategies for uses of chalk.

Information about Crayola sidewalk chalk.

Recipe for making homemade sidewalk chalk.

Interesing facts about Plaster of Paris.

Composition of Water.

Tempera paint ingredients.

Chemical components and uses for Plaster of Paris

How is water formed?

Interesting facts about water.

Background Information About Chalk.

Uses of Chalk

About the Author

Devon Goldhammer is a junior at Billings Senior High and is a member of the National Honor Society. She played volleyball her freshman and sophomore year. She participates in BPA (Business Professionals of America) and is in her fourth year of French. Also, she lettered in Rocket Science in the Mole Rocket Competition of Mr. Beals’ 2nd period chemistry class.