In basic water treatment, if drinking water develops a bad smell, which substance is most effective for removing the unpleasant odour?

Introduction / Context:
Safe drinking water should be clear, colourless and free from unpleasant odour. However, in real world conditions, water stored in tanks or supplied through contaminated sources can develop bad smell due to organic impurities, algae or other dissolved substances. This question checks your knowledge of basic water treatment methods and which agent is preferred specifically for removing odour by adsorption rather than simply disinfecting or clarifying the water.

Given Data / Assumptions:

• The water has developed an unpleasant smell or odour.
• We need a treatment that targets odour removal, not just disinfection or coagulation.
• Options include alum, bleach, activated carbon and a clearly incorrect term deactivated nitrogen.

Concept / Approach:
Different substances are used in water treatment for different purposes. Alum is mainly a coagulant that helps suspended particles settle. Bleach or chlorine compounds are primarily disinfectants that kill microorganisms. Activated carbon, on the other hand, is a highly porous material that adsorbs dissolved organic compounds, many of which are responsible for bad tastes and odours. Therefore, when the main issue is smell, activated carbon is the standard choice because of its strong adsorption capacity for odour causing molecules.

Step-by-Step Solution:
Step 1: Identify that the key problem mentioned in the question is bad smell in water. Step 2: Recall the primary uses of alum, bleach and activated carbon in water treatment. Step 3: Alum is mainly used as a coagulant to remove turbidity by helping fine particles clump together and settle. Step 4: Bleach acts as a disinfectant, killing germs, but it can sometimes add its own smell if overused. Step 5: Activated carbon has a very large surface area and is widely used to adsorb odour and taste causing organic substances. Step 6: Deactivated nitrogen has no standard role in simple water purification and is not a recognised treatment chemical for odour control. Step 7: Therefore, activated carbon is the most appropriate choice for removing bad smell from water.

Verification / Alternative check:
Many household water purifiers use carbon filters as one of the key stages. These filters are often advertised as removing bad taste and odour, which reinforces that activated carbon is the standard material for this purpose. If you recall product descriptions of domestic filters or municipal treatment processes, activated carbon is always associated with improving taste and removing smells rather than simply clarifying water or disinfecting it.

Why Other Options Are Wrong:
Alum: It is mainly a coagulant that helps remove suspended particles and turbidity, not specifically odour causing dissolved compounds, so it is not the best answer for smell removal.
Bleach: While bleach can disinfect water, it can leave a strong chlorine smell itself if not dosed carefully. It is not primarily used for odour removal by adsorption.
Deactivated nitrogen: This is not a standard term in basic water treatment and has no recognised role in simple odour control, so it is clearly incorrect.

Common Pitfalls:
Students often choose alum or bleach because they are familiar names from school experiments and municipal water treatment discussions. However, they sometimes overlook the specific function asked in the question, which is odour removal. Always pay attention to whether a question is about turbidity, germs, hardness or smell, because the correct chemical choice may change accordingly.

Final Answer:
The bad smell in drinking water can be most effectively removed by using activated carbon to adsorb the odour causing substances.