Which of the following is NOT a sign of having hard water?

• A. Reduced effectiveness of detergents
• B. Formation of soap scum when washing with soap
• C. Corrosion of metal pipes
• D. Scale buildup in kettles and boilers

Answer: (C)

Verified step by step guidance

1. Step 1: Understand what hard water is. Hard water contains high concentrations of dissolved calcium (Ca^{2+}) and magnesium (Mg^{2+}) ions.
2. Step 2: Identify common signs of hard water. These include reduced effectiveness of detergents, formation of soap scum when washing with soap, and scale buildup in kettles and boilers due to precipitation of calcium and magnesium salts.
3. Step 3: Recognize that corrosion of metal pipes is generally not caused by hard water. In fact, hard water can sometimes form a protective scale layer that reduces corrosion.
4. Step 4: Compare each option to the known effects of hard water. Reduced detergent effectiveness, soap scum formation, and scale buildup are typical signs, while corrosion of metal pipes is not.
5. Step 5: Conclude that the option 'Corrosion of metal pipes' is NOT a sign of hard water.