Question

What is the primary chemical reaction that occurs during lime softening, and what are the key precipitates formed?

Accepted Answer

The primary chemical reaction during lime softening is the precipitation of calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) through the addition of lime (calcium hydroxide, Ca(OH)2) to the water. Lime softening is a process used to reduce the hardness of water, which is primarily caused by the presence of calcium and magnesium ions. The addition of lime raises the pH of the water, converting bicarbonate alkalinity (HCO3-) to carbonate (CO3-2). The carbonate ions then react with calcium ions (Ca2+) to form calcium carbonate, which is insoluble and precipitates out of the water. The chemical equation for this reaction is: Ca2+ + CO32- → CaCO3(s). To remove magnesium, a higher pH is required, typically above 10.5. At this pH, magnesium ions (Mg2+) react with hydroxide ions (OH-) to form magnesium hydroxide, which is also insoluble and precipitates out of the water. The chemical equation for this reaction is: Mg2+ + 2OH- → Mg(OH)2(s). Therefore, the key precipitates formed during lime softening are calcium carbonate and magnesium hydroxide. These precipitates are then removed from the water through sedimentation and filtration. In some cases, soda ash (sodium carbonate, Na2CO3) is also added to remove non-carbonate hardness, which is calcium and magnesium associated with ions other than bicarbonate. The addition of soda ash promotes the formation of calcium carbonate, similar to the lime reaction. For example, if the water contains calcium sulfate (CaSO4), adding soda ash will cause the following reaction: CaSO4 + Na2CO3 → CaCO3(s) + Na2SO4.

Home → All Courses → Engineering and Technology Courses → Water Treatment Plant Operations and Maintenance → Flashcard

What is the primary chemical reaction that occurs during lime softening, and what are the key precipitates formed?