(a) Which of the following ionic species could be responsible for hardness in a water supply: \(\mathrm{Ca}^{2+}, \mathrm{K}^{+}, \mathrm{Mg}^{2+}, \mathrm{Fe}^{2+}, \mathrm{Na}^{+} ?\) (b) What properties of an ion determine whether it will contribute to water hardness?

To identify the ionic species responsible for hardness in the water supply, we need to look for ions that typically react with water to produce calcium and magnesium ions. From the given ions, \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) are the ions responsible for water hardness since they tend to combine with other chemicals in water to form calcium carbonate and magnesium carbonate. The other ions (\(\mathrm{K}^{+}, \mathrm{Fe}^{2+}, \mathrm{Na}^{+}\)) are not responsible for water hardness as they do not form the carbonates responsible for hard water and scaling. Answer: The ionic species responsible for hardness in a water supply are \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\).

Certain properties of an ion determine whether it will contribute to water hardness or not. Some of these properties are: 1. Charge: Ions with higher charge can form stronger electrostatic interactions with anions like carbonate (CO₃²⁻) or bicarbonate (HCO₃⁻), which will result in their precipitation and contribution to water hardness. In the given exercise, \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) have higher charges compared to the other ions, which make them more likely to contribute to water hardness. 2. Ionic size: Smaller ions can form stronger electrostatic interactions with anions and are more likely to contribute to water hardness. For example, \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\), being smaller in size compared to the other given ions, form stronger electrostatic interactions with carbonate anions and contribute more to water hardness. 3. Solubility of the corresponding compounds: Ions that form less soluble compounds (like calcium and magnesium carbonates) are more likely to contribute to water hardness, as they precipitate and form scale. For instance, the solubility of \(\mathrm{CaCO_3}\) and \(\mathrm{MgCO_3}\) is much lower than the solubility of \(\mathrm{FeCO_3}\), \(\mathrm{K_2CO_3}\), or \(\mathrm{Na_2CO_3}\), and therefore, calcium and magnesium ions contribute to water hardness. Answer: The properties of an ion that determine whether it will contribute to water hardness include the ion's charge, ionic size, and the solubility of the corresponding compounds.