In water treatment, what is often used to precipitate calcium and magnesium?

In water treatment processes, lime is commonly used to precipitate calcium and magnesium ions. Lime, which is calcium hydroxide (Ca(OH)₂), reacts with calcium and magnesium in water to form insoluble compounds that can then be removed from the water. The addition of lime raises the pH of the water, which facilitates the precipitation of these hardness-causing minerals, allowing them to be filtered out or settled at the bottom of the treatment system.

This process not only helps in reducing water hardness but also enhances the overall quality of the water being treated. The use of lime is particularly effective in treating hard water, improving performance in water softening applications. This is a critical step in water treatment, as high levels of calcium and magnesium can lead to scaling and other issues in plumbing and appliances.

The other options do not serve the same purpose effectively. Chlorine is primarily used for disinfection rather than precipitation, baking soda (sodium bicarbonate) can sometimes be used to adjust pH but does not effectively precipitate calcium and magnesium, and calcium sulfate does not effectively aid in the precipitation process in the same way that lime does. Thus, lime stands out as the correct choice for precipitating these minerals in water treatment.