What formula represents total hardness (TH)?

Total hardness (TH) in water typically refers to the combined concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, both of which contribute to the hardness of water. The formula to represent total hardness can generally be expressed as the sum of two components: calcium hardness (CH) and non-calcium hardness (NCH).

In this context, CH represents the contribution to hardness from calcium ions, while NCH accounts for the contribution from other sources, typically magnesium ions and possibly other divalent cations. Since both components contribute to the overall hardness of water, adding these two values together yields the total hardness.

Thus, the formula listed accurately represents this relationship as TH being the sum of NCH and CH. This is fundamental in water quality assessments because monitoring total hardness involves understanding both the calcium and magnesium contributions to the overall hardness of the water.

The other options suggest different mathematical operations that do not align with the standard definition of total hardness, which reinforces why the correct choice aligns with the foundational principles of assessing water hardness.