What role does carbon dioxide play in the lime softening process?

In the lime softening process, carbon dioxide plays a crucial role in controlling the pH levels of the water being treated. When lime (calcium hydroxide) is added to the water to precipitate hardness as calcium carbonate, the pH of the water initially rises significantly. This increase in pH promotes the transformation of dissolved calcium and magnesium salts into their solid forms, effectively removing hardness.

However, to maximize the efficiency of this process and to ensure that excess lime does not lead to high pH levels—which can be counterproductive—carbon dioxide is introduced. The carbon dioxide reacts with the calcium hydroxide in the water, forming calcium carbonate and lowering the pH back to a more neutral range. This adjustment helps to stabilize the treatment process, ensuring that optimal conditions for hardness removal are maintained without creating an overly alkaline environment.

Therefore, the correct answer regarding the role of carbon dioxide in the lime softening process is that it brings pH back down, facilitating a more effective and safer water treatment process.