The process that requires the use of hardwater in its operation is

Analysis

The question asks us to identify an industrial or domestic process that specifically requires the use of hard water. Hard water contains dissolved calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions.

Concept Explanation

• Hard Water: Water that contains high concentrations of dissolved minerals, specifically calcium and magnesium salts.
• The Processes: • Laundry: Hard water is generally avoided in laundry because the calcium and magnesium ions react with soap to form "scum" (insoluble calcium/magnesium stearate), which wastes soap and makes cleaning difficult. • Bottling: Bottled water companies usually treat water to remove hardness and impurities to ensure a consistent taste and prevent mineral deposits in pipes. • Dyeing: Hard water is often problematic in dyeing because the mineral ions can react with the dye molecules, leading to uneven coloring or "spotting" on fabrics. • Tanning: In the leather industry, the presence of certain minerals (hardness) can be beneficial. Specifically, in the tanning of leather, calcium salts are often used to help in the "liming" process, which helps remove hair and prepare the hides for tanning.

Step-by-Step Solution

1. Evaluate Laundry: Hard water reacts with soap to form scum. This is undesirable, so soft water is preferred.
2. Evaluate Bottling: Bottling plants prioritize water purity and softness to prevent scaling in machinery and ensure quality.
3. Evaluate Dyeing: Hard water interferes with the chemical bonding of dyes to fibers, leading to poor results.
4. Evaluate Tanning: The tanning process involves chemical treatments where calcium compounds are intentionally used to modify the hide structure. Therefore, the presence of hardness (calcium ions) is a functional requirement in this specific industrial process.

Conclusion

Among the options provided, Tanning is the process that utilizes the properties of hard water (specifically the calcium content) to facilitate the treatment of animal hides.

Exam Tip: When you see questions about hard water, always remember the "Soap Rule": If a process involves soap (like laundry), hard water is the enemy! If it involves heavy industrial chemical processing (like tanning), hard water might actually be a requirement.