Which of the following chemicals is not added during the purification of raw-water for obtaining drinking water?

Appeared in: NHM MP Staff Nurse-2022

Explanation

Calcium Carbonate (CaCO₃) is the chemical compound that causes water hardness, often seen as limescale.
Water treatment processes, specifically water softening, are designed to remove calcium ions from the water.
During softening, chemicals like lime and soda ash are added to cause calcium ions to precipitate as solid calcium carbonate, which is then filtered out.
Therefore, calcium carbonate is a byproduct that is removed, not a chemical that is added for purification.

Option A: Ozone is a powerful oxidizing agent and disinfectant that is bubbled through water to kill microorganisms and remove organic contaminants.
Option B: Chlorine is the most common chemical disinfectant added to drinking water to kill pathogenic bacteria and viruses, providing a residual protective effect in the distribution system.
Option C: Soda ash (sodium carbonate) is added to water to raise the pH and to remove non-carbonate hardness by precipitating calcium and magnesium salts.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Chemicals used in water purification as background academic context rather than a clinical decision trigger.
Understanding water purification is fundamental to community health nursing and preventing outbreaks of waterborne diseases such as cholera, typhoid, and dysentery.
Nurses educate communities on safe drinking water practices, including boiling and chemical disinfection, especially in areas with unreliable water supplies.
A nurse's role includes identifying symptoms of waterborne illnesses and participating in public health responses during outbreaks, which are often linked to contaminated water sources.

How to Approach the Question

First, identify the negative keyword 'not' in the question stem. This means you are looking for the option that is an exception.
The question asks which chemical is 'not added' during water purification.
Recall the purpose of each chemical listed in the options in the context of water treatment.
Ozone and Chlorine are well-known disinfectants (they are added).
Soda ash is used for softening (it is added).
Calcium Carbonate is what makes water 'hard'. The goal is to remove it, not add it.

Concept Tested & Keywords

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E6 Parks TextBook of Preventive & Social Medicine part 2 — Subpart B (pp 233-449 of 464) pp. 32-34, 33-35, 31-33

Practise the full NHM MP Staff Nurse-2022

Attempt every question from this paper in a timed mock, then review the full solution for each one.