Hard water is caused mainly by salts of:

**The reasoning** Hard water forms when water passes through rocks and soil containing calcium and magnesium compounds — mainly calcium carbonate (limestone), calcium sulfate, and magnesium carbonate. These dissolve partially, releasing Ca²⁺ and Mg²⁺ ions into the water. When you use hard water with soap, these ions react with soap to form **scum** (that greyish residue). The principle: **divalent cations (2+ charge) bind strongly to soap molecules**, preventing lather formation. This is the defining characteristic of hard water. **Why the wrong options tempt you** - **Sodium (A)** and **Potassium (B)** also form salts, but they're *monovalent* (1+ charge). They actually make water "soft" because they don't precipitate with soap. Common table salt (NaCl) doesn't cause hardness! - **Iron (D)** can be in water, but it causes *discoloration* (rust-brown water), not the soap-scum problem that defines hardness. **Quick takeaway** Remember: **"Hard water = Hard ions (Ca²⁺, Mg²⁺) = Hard to lather"** — it's the double positive charge that makes water chemically "hard" to work with!