What is the primary function of lime and soda ash in precipitation softening?

Lime and soda ash play a crucial role in the process of precipitation softening by specifically targeting the removal of calcium and magnesium, which are the primary contributors to water hardness. The addition of lime (calcium hydroxide) raises the pH of the water, leading to the precipitation of calcium carbonate and other sulfates, which effectively removes the calcium ions from the water. Soda ash (sodium carbonate) complements this process by reacting with magnesium ions to form magnesium carbonate, which also precipitates out of the solution.

Through this chemical reaction, the hard water is softened because the unwanted hardness ions are transformed into insoluble compounds that can be removed from the system. This process is essential in water treatment plants to ensure the treated water is suitable for consumption and other uses, as hard water can lead to scaling and inefficiencies in plumbing systems and water appliances.

The other options presented do not align with the primary function of lime and soda ash in this context. Reducing water temperature is not related to softening but rather pertains to physical methods. The removal of heavy metals is associated with different chemical treatments, and disinfection is typically achieved through processes like chlorination or ultraviolet light, rather than precipitation softening involving lime and soda ash.