Question

Which of the following measures the hardness of water for different salts?

• A. parts per million
• B. parts per trillion
• C. parts per thousand
• D. parts per hundred

Hint:

For most water quality parameters like hardness, salinity, and dissolved solids, \textbf{ppm (parts per million)} or its equivalent \textbf{mg/L (milligrams per litre)} is the standard unit of measurement.

Answer 1

The Correct Option is A

Step 1: Understand what water hardness is. Step 1: Water hardness is primarily caused by the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. It is a measure of the concentration of these mineral salts.
Step 2: Identify the common units for measuring this concentration.
Step 2: The concentration of dissolved substances in water is typically expressed in units that are practical for the low levels found. The most common units for water hardness are:

Milligrams per litre (mg/L).
Parts per million (ppm).
For water, 1 mg/L is equivalent to 1 ppm.
Step 3: Evaluate the options.
Step 3: "Parts per million" is the standard and most frequently used unit among the choices provided. "Parts per trillion" is used for extremely low concentrations (e.g., trace pollutants), while "parts per thousand" and "hundred" are too large for this application.