After water has been softened, it is close to pH 11; thus it needs to be stabilized by a process known as recarbonation. What small amount of hardness chemical is formed and what is precipitated in this process?

After water has been softened, it is close to pH 11; thus it needs to be stabilized by a process known as recarbonation. What small amount of hardness chemical is formed and what is precipitated in this process?

• A. Calcium Carbonate, Magnesium Hydroxide
• B. Calcium Bicarbonate, Magnesium Carbonate
• C. Calcium Hydroxide, Calcium Carbonate
• D. Magnesium Hydroxide, Calcium Carbonate

Answer: (B)

Recarbonation stabilizes softened water by reintroducing carbon dioxide to lower the pH and shift the carbonate balance so that some hardness remains as dissolved bicarbonate while other hardness precipitates as carbonate. As CO2 dissolves, calcium tends to form soluble calcium bicarbonate in the water, while magnesium tends to form insoluble magnesium carbonate and precipitates. So the small amount of hardness that remains in solution is calcium bicarbonate, and the portion that is removed from the water as a solid is magnesium carbonate.

Recarbonation stabilizes softened water by reintroducing carbon dioxide to lower the pH and shift the carbonate balance so that some hardness remains as dissolved bicarbonate while other hardness precipitates as carbonate. As CO2 dissolves, calcium tends to form soluble calcium bicarbonate in the water, while magnesium tends to form insoluble magnesium carbonate and precipitates.

So the small amount of hardness that remains in solution is calcium bicarbonate, and the portion that is removed from the water as a solid is magnesium carbonate.