Final answer:
The maximum excess hardness that can be left in water depends on the solubility of the compounds present in the water. Excess hardness refers to the concentration of ions such as calcium and magnesium that exceed the solubility limit.
Step-by-step explanation:
The maximum excess hardness that can be left in water depends on the solubility of the compounds present in the water. Excess hardness refers to the concentration of ions such as calcium (Ca²+) and magnesium (Mg²+) that exceed the solubility limit.
For example, in the case of calcium carbonate (CaCO₃), the maximum solubility is about 7.1 mg/L or 7.1 ppm. Therefore, any concentration of Ca²+ and Mg²+ ions that exceeds this limit would be considered excess hardness.