Which chemical is incorrectly attributed as the main coagulant when compared to alum?

The focus of this question is on identifying a chemical that is not primarily categorized as a coagulant in comparison to alum, which is a commonly used coagulant in water treatment processes.

Alum, or aluminum sulfate, works effectively in coagulating suspended particles in water by neutralizing their charges, allowing them to clump together and settle out, facilitating better water clarity. Ferric sulfate is also a recognized coagulant, used similarly to alum, and is effective in treating water, particularly when dealing with phosphates and color removal.

Calcium hydroxide, primarily known for its role in adjusting pH and as a softening agent, can have coagulating properties in certain contexts but is not a primary coagulant as alum and ferric sulfate are. Sodium sulfite, while useful in reducing chlorine in treated water, does not serve as a coagulant at all.

Therefore, the distinction here lies in the classification and common usage of these chemicals in water treatment processes. Ferric sulfate is a legitimate coagulant, while sodium sulfite does not play a coagulant role, making it the chemical that is incorrectly attributed as the main coagulant when compared to alum.