Which mineral causes noncarbonate magnesium hardness that can be precipitated out of water by lime?

The mineral that causes noncarbonate magnesium hardness, which can indeed be precipitated out of water using lime, is magnesium hydroxide. In water treatment, hardness is typically caused by the presence of various mineral ions, including both carbonate and noncarbonate species. Noncarbonate hardness is primarily associated with elements such as magnesium and calcium in forms that do not involve carbonate ions.

When lime (calcium hydroxide) is added to water, it increases the pH and can react with magnesium ions to form magnesium hydroxide. The reaction facilitates the precipitation of magnesium from the water, effectively reducing hardness. This process is particularly significant in water treatment, as it helps in achieving desired water quality standards.

In contrast, calcium carbonate is primarily responsible for carbonate hardness and would not precipitate in the same manner under these conditions. Calcium sulfate and sodium carbonate do not directly lead to noncarbonate magnesium hardness; rather, they contribute to other forms of hardness or alkalinity in water. Thus, magnesium hydroxide is the key mineral that aligns with the conditions and process described.