**Correct Option: C. the use of an ion-exchange resin** ### Detailed Explanation: **Understanding Hard Water:** Hard water contains high concentrations of dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These minerals can cause various problems, such as scale buildup in pipes and appliances, reduced effectiveness of soaps and detergents, and can affect the taste of water. **What is Soft Water?** Soft water is water that has low concentrations of calcium and magnesium ions. It is more desirable for household use because it lathers easily with soap and does not leave mineral deposits. **Ion-Exchange Resin Method:** The ion-exchange resin method is a widely used technique to convert hard water into soft water. Here’s how it works: 1. **Ion-Exchange Process:** - The ion-exchange resin is typically made of small beads that are charged with sodium ions (Na⁺). - When hard water passes through the resin, the calcium (Ca²⁺) and magnesium (Mg²⁺) ions in the water are attracted to the resin beads and replace the sodium ions. 2. **Chemical Reaction:** - The process can be summarized as follows: \[ \text{Ca}^{2+} + 2 \text{Na-R} \rightarrow \text{Ca-R}_2 + 2 \text{Na}^+ \] \[ \text{Mg}^{2+} + 2 \text{Na-R} \rightarrow \text{Mg-R}_2 + 2 \text{Na}^+ \] - Here, R represents the resin. The calcium and magnesium ions are effectively removed from the water, and sodium ions are released into the water. 3. **Regeneration of Resin:** - After a certain period, the resin becomes saturated with calcium and magnesium ions and needs to be regenerated. This is done by flushing the resin with a concentrated sodium chloride (salt) solution, which replaces the calcium and magnesium ions back with sodium ions. ### Why Other Options Are Incorrect: **A. Chlorination:** - Chlorination is a method used primarily for disinfection purposes, to kill bacteria and pathogens in water. It does not remove hardness-causing minerals (calcium and magnesium) from water. Therefore, it does not convert hard water to soft water. **B. Passage Over Activated Charcoal:** - Activated charcoal is effective for removing impurities, odors, and some organic compounds from water, but it does not remove dissolved minerals like calcium and magnesium. Thus, it does not soften water. **D. Aeration:** - Aeration involves introducing air into water to remove dissolved gases (like carbon dioxide) and volatile compounds. While it can improve water quality, it does not affect the hardness of water, as it does not remove calcium or magnesium ions. ### Summary of Key Points: - Hard water contains high levels of calcium and magnesium ions. - Soft water is desirable for household use due to its low mineral content. - The ion-exchange resin method effectively removes hardness by replacing Ca²⁺ and Mg²⁺ ions with Na⁺ ions. - Other methods like chlorination, activated charcoal, and aeration do not address the hardness of water. ### Revision Summary: - Hard water contains calcium and magnesium ions; soft water does not. - Ion-exchange resin is the effective method for softening water. - Other methods (chlorination, activated charcoal, aeration) do not remove hardness. - Regular maintenance of ion-exchange systems is necessary for continued effectiveness.