What substance can produce a positive Langelier index when added?

A positive Langelier index indicates that water is oversaturated with calcium carbonate, which can lead to scaling and precipitation of calcium carbonate in pipes and surfaces. When considering the effect of different substances:

Adding caustic soda (sodium hydroxide) to water increases the water's pH and can enhance the alkalinity. This increase in alkalinity can shift the saturation state of calcium carbonate by promoting precipitation as the carbonate equilibria are altered. Higher pH values can lead to calcium carbonate being less soluble, causing an increase in calcium carbonate saturation—hence, a positive Langelier index.

In contrast, caustic acid (sodium hydroxide) would lower pH and not typically contribute to a positive Langelier index. Sodium bicarbonate is generally used to buffer or stabilize pH, and while it can impact bicarbonate levels, it does not inherently lead to conditions that would create a positive Langelier index. Calcium carbonate itself would not create a positive index when added to water—rather, it serves as a benchmark for the index itself.

Therefore, the introduction of caustic soda is significant as it raises the pH of the water, potentially leading to conditions that result in a positive Langelier index, making