To reduce non-carbonate hardness in water, soda ash must be added to form:

• A. Magnesium chloride
• B. Calcium carbonate
• C. Barium sulfate
• D. Sodium sulfate

Answer: (B)

To reduce non-carbonate hardness in water, adding soda ash (sodium carbonate) facilitates the precipitation of calcium ions from the water by converting them into calcium carbonate. This process occurs as the soda ash reacts with the dissolved calcium in the water, leading to the formation of insoluble calcium carbonate, which can then be effectively removed from the water system through sedimentation or filtration.

The formation of calcium carbonate is significant because non-carbonate hardness primarily arises from the presence of calcium and magnesium ions that cannot be removed just by a simple water treatment process. By precipitating these ions into a solid form, the hardness can be significantly reduced, improving water quality.

In contrast, the other compounds mentioned do not directly result from adding soda ash in the context of controlling non-carbonate hardness. Magnesium chloride, barium sulfate, and sodium sulfate do not play a role in precipitating hardness as effectively as calcium carbonate does when soda ash is used. Hence, the formation of calcium carbonate is the desired and correct outcome of using soda ash in this scenario.