The reaction between chalk and vinegar is a neutralization reaction between calcium carbonate and acetic acid to produce water, carbon dioxide and calcium acetate. Carbon dioxide is a gas that causes the reaction to bubble.
The following equation represents the reaction: 2 CH3COOH + CaCO3 = H2O + CO2 + Ca(CH3COO)2. Environmental websites recommend this experiment for showing children the effects of acid rain. The weak acid dissolves calcium carbonate, also known as limestone or chalk. There are reports of the experiment failing. The problem with the experiment is found in the type of chalk used. High-grade chalk developed for use in the classroom is created by extruding a slurry of calcium carbonate. Manufacturers sell this chalk with the dustless label. Crayola also sells a less expensive molded chalk for use on children's blackboards and as sidewalk chalk made of plaster of paris, which is calcium sulfate. Vinegar does not react with calcium sulfate to create the carbon dioxide gas, and it does not cause dissolve this form of chalk. A similar children's experiment involves use of an egg and vinegar. After several days, the vinegar dissolves the calcium carbonate from the egg shell, using the same reaction, and leaves the rubbery membrane surrounding the remainder of the egg.