Acceptable hardness range for potable water (as CaCO3):\r Within which range (in parts per million, ppm) is total hardness typically prescribed for safe drinking water quality?

Introduction / Context:
Total hardness in potable water mainly arises from calcium and magnesium salts expressed as mg/L (ppm) of calcium carbonate equivalent. While hardness does not pose a direct health risk within moderate limits, it affects aesthetic acceptability, scaling, and soap consumption. This question tests awareness of a commonly prescribed acceptable band for hardness in drinking water.

Given Data / Assumptions:

• Hardness is expressed as CaCO3 equivalent in ppm.
• The ranges shown mirror typical textbook guidance for acceptable palatability and scale control.
• Regulatory limits can vary; we focus on commonly cited teaching ranges.

Concept / Approach:
Very soft water (< 50 ppm) can be corrosive and tastes flat; very hard water (> 300 ppm) causes significant scale and soap wastage. Many standard references present 75 to 115 ppm as a comfortable range, offering acceptable taste with limited scaling in household use. Therefore, this middle range is often singled out in exam-style questions as “prescribed”.

Step-by-Step Solution:
Compare each option range to a commonly acceptable zone.Recognize that 75 to 115 ppm aligns with standard guidance in teaching texts.Select 75 to 115 ppm as the correct range.

Verification / Alternative check:
Utilities commonly target hardness near 100 ppm for balanced operation where softening is used, supporting the educational range provided here for general acceptability.

Why Other Options Are Wrong:

• 50 to 75 ppm is on the softer side and may increase corrosion potential.
• 100 to 150 ppm and 150 to 200 ppm are acceptable to many consumers but are usually above the “prescribed” teaching band noted in many syllabi.
• “None of these” is incorrect as a reasonable range exists.

Common Pitfalls:

• Confusing aesthetic hardness guidance with strict legal limits, which vary by jurisdiction.
• Not distinguishing between carbonate (temporary) and non-carbonate (permanent) hardness when diagnosing scaling problems.

Final Answer:
75 to 115 ppm