### Correct Option: C. Calcium and Magnesium #### Explanation of Why the Answer is Correct: Hard water is defined as water that has a high mineral content, primarily due to the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are typically derived from the dissolution of minerals in the earth, such as limestone (which contains calcium carbonate) and dolomite (which contains both calcium and magnesium carbonates). 1. **Calcium Ions (Ca²⁺)**: - Calcium is one of the most common ions found in hard water. It originates from the weathering of rocks and minerals, particularly limestone and gypsum. - When water percolates through these geological formations, it dissolves calcium salts, leading to an increase in calcium ion concentration in the water. 2. **Magnesium Ions (Mg²⁺)**: - Magnesium is another prevalent ion in hard water, often found alongside calcium. It is sourced from minerals such as dolomite and magnesite. - Similar to calcium, magnesium ions enter the water supply through the dissolution of these minerals. The presence of these ions in water can lead to various issues, such as the formation of scale in pipes and appliances, reduced effectiveness of soaps and detergents, and other complications in water heating systems. #### Why the Other Options are Wrong or Weaker: - **Option A: Copper and Zinc** - Copper (Cu²⁺) and zinc (Zn²⁺) are not typically associated with hard water. While they can be present in water due to plumbing materials or industrial processes, they do not contribute to the hardness of water. Hardness is specifically related to calcium and magnesium ions. - **Option B: Magnesium and Silver** - While magnesium is correctly identified as a contributor to hard water, silver (Ag⁺) is not associated with water hardness. Silver is a noble metal and does not form salts that would contribute to hardness. It is more commonly known for its antibacterial properties and is not a significant component of hard water. - **Option D: Potassium and Tin** - Potassium (K⁺) is not a contributor to water hardness. It is a soluble ion that does not precipitate or form scale like calcium and magnesium. Tin (Sn²⁺) is also not relevant to hard water; it is a metal that does not typically dissolve in water to form ions that would contribute to hardness. ### Summary of Key Points: - Hard water is characterized by high concentrations of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. - These ions come from the dissolution of minerals in the earth, such as limestone and dolomite. - Other options (copper, zinc, silver, potassium, and tin) do not contribute to water hardness. - Understanding the composition of hard water is essential for addressing its effects on plumbing and cleaning efficiency. This detailed explanation should help clarify why calcium and magnesium are the correct answers regarding the metallic ions associated with hard water.