What is formed when alum or iron compounds react with alkalinity in the water?

When alum or iron salts react with alkalinity in water, they lead to the formation of insoluble hydroxides. This reaction typically occurs during the coagulation and flocculation processes in water treatment.

In these processes, alum (aluminum sulfate) and certain iron compounds (like ferric chloride) dissociate in water and release aluminum or iron ions. When these ions come into contact with hydroxide ions present due to alkalinity, a precipitation reaction occurs, resulting in the formation of insoluble aluminum hydroxide or iron hydroxide. These insoluble compounds can then effectively remove suspended particles and impurities from the water by agglomerating them into larger flocs that can be easily removed through sedimentation or filtration.

The formation of insoluble hydroxides is pivotal in water treatment systems targeting particulate matter and enhancing the overall water quality, which illustrates why this option is the correct choice.