Drinking-water quality (India, BIS): What is the maximum desirable limit for total hardness, expressed as CaCO3, in potable water supplied for human consumption?\r (Choose the closest standard value used for routine compliance checks.)

Introduction:
Total hardness in drinking water is primarily due to calcium and magnesium salts and is conventionally expressed as milligrams per liter (mg/L) of calcium carbonate (CaCO3), often abbreviated as ppm. Regulatory bodies publish guideline values to protect plumbing, taste, and household equipment while also considering health and practicality. This question checks recognition of the typical Indian Bureau of Standards (BIS) desirable limit used in many water-supply assessments.

Given Data / Assumptions:

• Total hardness is reported as CaCO3 equivalent.
• We are asked for the commonly cited maximum desirable limit used in routine monitoring.
• Units: mg/L is numerically equal to ppm in dilute aqueous solutions.

Concept / Approach:
Hardness affects soap consumption, scaling in pipes and heaters, and user acceptability. Standards typically present two tiers: an ideal or desirable limit for consistent aesthetic/operational quality, and a higher permissible limit in the absence of an alternative source. In many exam and practice contexts, the desirable limit is represented as 300 mg/L as CaCO3, with a higher permissible ceiling for challenging supply situations.

Step-by-Step Solution:
Identify the requested tier: “maximum desirable limit.”Recall the widely taught value associated with Indian potable water guidelines.Match to options: 300 ppm (mg/L) is the standard desirable level frequently cited.

Verification / Alternative check:
Practical field guides for municipal water operations commonly set routine targets at the desirable level, with operational allowances up to a higher permissible limit when supply constraints exist. This is consistent with consumer acceptability and infrastructure longevity considerations.

Why Other Options Are Wrong:

• 600 ppm: Often referenced as a higher permissible value, not the desirable target.
• 500 ppm and 1000 ppm: Exceed typical aesthetic and operational recommendations.
• 200 ppm: A stricter target than the commonly taught desirable value in many syllabi; not the expected answer here.

Common Pitfalls:
Confusing “desirable” with “permissible” limits; always check which tier the question asks for. Also, ensure units are correctly recognized as mg/L or ppm in dilute water.

Final Answer:
300 ppm