The white powder commonly used to mark lines on baseball, football, and soccer fields is calcium carbonate (CaCO3). It is produced through a chemical reaction between calcium oxide (CaO), commonly known as quicklime, and water (H2O). This process, known as slaking or hydration, results in the formation of calcium hydroxide [Ca(OH)2] and the liberation of heat in the form of steam.

The reaction can be represented as:

CaO(s) + H2O(l) → Ca(OH)2(s)

However, upon further exposure to carbon dioxide (CO2) present in the atmosphere, calcium hydroxide undergoes carbonation, where it reacts with CO2 to form calcium carbonate:

Ca(OH)2(s) + CO2(g) → CaCO3(s) + H2O(l)

Calcium carbonate is a white, insoluble compound that is commonly known as chalk or limestone. Due to its natural abundance, cost-effectiveness, and high visibility, calcium carbonate is widely used in various sports to mark field boundaries, lines, and other markings on athletic fields, including baseball, football, and soccer.

In summary, the white powder used to mark lines on sports fields is calcium carbonate (CaCO3), which is produced through the reaction between calcium oxide (CaO) and water (H2O), followed by carbonation with atmospheric carbon dioxide (CO2).