If hard water is softened by the ion exchange method, which one of the following will increase?

When hard water is softened using the ion exchange method, the process involves exchanging calcium and magnesium ions, which contribute to water hardness, with sodium ions. As a result, the concentration of sodium in the water increases.

This method typically uses a resin that binds the hardness ions (like calcium and magnesium) while releasing sodium ions into the water. Thus, when hard water is treated through ion exchange, sodium becomes more prevalent in the softened water.

To delve deeper into the context, it's essential to understand that the other options—such as dissolved oxygen, iron, and pH—are not directly impacted in a manner that causes a persistent increase when the hardness is removed. While dissolved oxygen levels and pH can fluctuate based on various factors in water chemistry, they do not specifically increase as a result of the ion exchange softening process. Iron levels also do not automatically increase due to this treatment, and its presence generally relies on different geological sources or contamination rather than the softening process itself.

Therefore, the increase of sodium is the defining outcome of the ion exchange method used in softening hard water.