Which chlorination method is 100% chlorine and often used in swimming pools?

• A. Chlorine Gas
• B. Bleach
• C. High-test calcium hypochlorite
• D. Ozone

Answer: (A)

Chlorine sources for pool disinfection come in different forms, and the key difference is how pure the chlorine is. Chlorine gas is pure chlorine (Cl2), so it is 100% chlorine when released into the water. Bleach is a sodium hypochlorite solution—it's chlorine in chemical combination with sodium and is dissolved in water—so it isn’t pure chlorine. High-test calcium hypochlorite is a solid that contains chlorine in chemical form but also inert fillers and moisture; the available chlorine is less than 100% of the product weight. Ozone is not a chlorine source at all; it’s a separate oxidant used in some systems. Therefore, the form that provides 100% chlorine and is often used in pools is chlorine gas.