What should the hardness level of water be when leaving the ion exchange vessel?

The hardness level of water when leaving the ion exchange vessel should ideally be zero. Ion exchange is a process used predominantly for the removal of hardness-causing minerals, specifically calcium and magnesium ions, from water. During this process, these ions are exchanged with sodium ions on the resin within the ion exchange vessel.

When the water exits the vessel, the goal is to have removed as many hardness ions as possible, resulting in softened water. This means that the total hardness should be as close to zero as achievable. Achieving a zero hardness level ensures that the water is suitable for applications that require low mineral content, such as in boilers or in processes where scaling can be problematic.

In contrast, having low, moderate, or high levels of hardness would indicate that some hardness ions remain in the water, which could lead to scaling and other water quality issues in subsequent uses. Therefore, the expected outcome of an effective ion exchange process is water with zero hardness, making it the correct answer.