In water treatment and public health, which element is commonly used to disinfect the water in swimming pools by killing harmful microorganisms?

Introduction / Context:
This question connects basic chemistry with everyday public health practice. Swimming pool water must be kept free from harmful bacteria, viruses and algae to protect swimmers. To achieve this, disinfecting agents are added to the water. Among the halogens and other elements listed, one particular element is widely used worldwide for disinfecting drinking water supplies and swimming pools. Recognising this element and understanding its role is important in both chemistry and environmental science.

Given Data / Assumptions:

• We are dealing with disinfection of swimming pool water.
• The options are fluorine, chlorine, bromine, sulfur and iodine.
• The chosen element should be effective at killing microorganisms at low concentrations.
• It should be practical and economical for large scale use.
• Standard water treatment practices are assumed.

Concept / Approach:
Chlorine is the most commonly used element for disinfecting water in swimming pools and municipal supplies. It can be added as chlorine gas, sodium hypochlorite solution or other chlorine releasing compounds. In water, chlorine forms hypochlorous acid and hypochlorite ions, which are strong oxidising agents that can destroy cell walls and vital enzymes in microorganisms. While other halogens like bromine and iodine also have disinfectant properties, chlorine is the most widely used because it is effective, relatively inexpensive and well studied. Fluorine is too reactive and dangerous for such use, and sulfur does not serve as a primary disinfectant in pools.

Step-by-Step Solution:
Step 1: List the candidates that are known disinfectants: chlorine, bromine and iodine are all halogens with antimicrobial properties. Step 2: Recall that chlorine is the standard chemical used for chlorination of drinking water and swimming pool water. Step 3: In water, chlorine forms hypochlorous acid (HOCl), which penetrates microbial cells and oxidises essential components, killing them. Step 4: Bromine is sometimes used in hot tubs and smaller systems, but it is less common and more expensive than chlorine for large pools. Step 5: Iodine is used in small scale water purification, such as emergency tablets, but is not standard for swimming pools due to cost and other concerns. Step 6: Fluorine is extremely reactive and toxic and is not added directly to pools; fluoride compounds are used in very low concentrations in drinking water for dental health, not for pool disinfection. Step 7: Sulfur has uses in agriculture and industry but not as the main swimming pool disinfectant. Therefore, chlorine is the correct answer.

Verification / Alternative check:
Observation of real life practice supports this conclusion. Swimming pools often have a characteristic smell associated with chlorinated water, and pool maintenance instructions frequently mention maintaining free chlorine levels within a specific range to ensure proper disinfection. Public health guidelines discuss chlorination standards rather than bromination or iodination for large municipal systems. In chemistry, chlorine based compounds like bleaching powder (calcium oxychloride) and sodium hypochlorite are standard disinfectants and bleaching agents, further confirming that chlorine is the primary element used in such applications.

Why Other Options Are Wrong:
Fluorine is highly toxic and reactive, and elemental fluorine gas is far too dangerous to handle for routine pool treatment. Bromine is used in some specialised pool and spa systems but is not the most common or standard choice, especially in general exam level discussions. Iodine is effective against microbes but is used mainly for small scale disinfection, and prolonged exposure can have health and taste issues. Sulfur in elemental or common compound forms is not used as a primary disinfectant for swimming pools. Hence, these options do not match standard practice.

Common Pitfalls:
Some students may confuse fluoride addition to drinking water, which is done to help prevent tooth decay, with disinfection. Fluoride is not the primary disinfectant; chlorine usually is. Another mistake is to think that any halogen could be used interchangeably and therefore choose bromine or iodine. Although they can act as disinfectants, their cost, stability and side effects mean they are not the common choice for large public pools. To answer such questions correctly, it is helpful to relate them to real life observations and widely known public health practices.

Final Answer:
The element commonly used to disinfect swimming pool water is chlorine.