What does the lime-soda softening process primarily precipitate?

The lime-soda softening process is specifically designed to address water hardness caused by the presence of dissolved calcium and magnesium salts. When lime (calcium hydroxide) and soda ash (sodium carbonate) are added to hard water, a series of chemical reactions occur. The calcium and magnesium ions in the water react with the hydroxide ions from calcium hydroxide and the carbonate ions from sodium carbonate. This results in the formation of insoluble precipitates, primarily calcium carbonate and magnesium hydroxide, which can then be removed from the water.

This process is a key method used in water treatment to reduce hardness, thus making the water more suitable for various applications, including boiler feed water and industrial processes, where hard water can cause scaling and other issues. The precipitates formed effectively reduce the concentration of calcium and magnesium in the water, transforming hard water into softer water.

The other options do not pertain to the primary outcome of the lime-soda softening process. Iron and aluminum salts are not the focus of this treatment method, nor are sodium and silica compounds. Additionally, carbon dioxide and sulfur oxides are byproducts of different chemical reactions and do not relate directly to the precipitation of hardness-causing salts in the lime-soda process.