Which method is used for purifying water that involves calcium?

The method used for purifying water that involves calcium is calcium hypochlorite. This compound is an effective chlorinating agent, commonly employed in water treatment processes. It serves as a source of chlorine, which effectively disinfects water by killing harmful pathogens, including bacteria and viruses.

Calcium hypochlorite is preferred in many water purification scenarios because it is stable, easy to handle, and has a high chlorine content compared to other chlorinating agents. When calcium hypochlorite is dissolved in water, it releases chlorine, which governs the disinfection process.

In contrast, the other options—chlorination, filtration, and iodine tablets—do not specifically involve calcium in their primary disinfection or purification mechanism. Chlorination generally refers to the process of adding chlorine, which can derive from various sources, while filtration and iodine tablets utilize different methods of purification that do not depend on calcium compounds. Therefore, calcium hypochlorite distinctly links the purification process to calcium, making it the correct choice in this context.