Why is soda ash added to water in the lime softening process?

The addition of soda ash during the lime softening process primarily serves the purpose of removing noncarbonate hardness. Noncarbonate hardness is typically associated with the presence of sulfates and chlorides of calcium and magnesium in the water. When lime (calcium hydroxide) is added to water, it primarily reacts with the carbonate hardness, which is mainly due to bicarbonates. However, soda ash (sodium carbonate) is effective in addressing the noncarbonate hardness.

When soda ash is introduced, it reacts with calcium ions in the water, forming calcium carbonate, which precipitates out of solution. This process effectively reduces the overall hardness of the water, allowing for softer water that meets treatment and quality standards. The removal of noncarbonate hardness is crucial for various applications, particularly in industrial settings where hard water can lead to scaling and other operational difficulties.

Therefore, the correct answer focuses on the role of soda ash in enhancing the efficacy of the lime softening process by targeting noncarbonate hardness.