Which of the following is the main disadvantage associated with hard water in domestic and industrial use?

Introduction / Context:
Hard water contains elevated concentrations of calcium and magnesium ions. While not a direct health hazard, hardness creates multiple operational and domestic problems that increase costs and reduce equipment life and performance.

Given Data / Assumptions:

• Hardness primarily due to Ca²⁺ and Mg²⁺ salts.
• Applications include bathing, laundry, cooking, and industrial heating systems.

Concept / Approach:
Hardness interferes with soap (forming scum), precipitates scale in heated systems, and can promote corrosion-incrustation cycles. These issues collectively degrade efficiency, increase energy use, and impact user experience.

Step-by-Step Solution:

Verification / Alternative check:
Industrial water treatment programs (softening/conditioning) are standard precisely to prevent these problems, confirming their practical significance.

Why Other Options Are Wrong:
Each of options a–d individually states valid disadvantages; since all are true, the most complete choice is the combination, i.e., option e.

Common Pitfalls:
Confusing temporary and permanent hardness; assuming hardness is only a “soap” problem while ignoring energy penalties from scale.

Final Answer:
All of the above