Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

Water is often treated with chlorine to (a) kill germs (b) remove hardness (c) increase oxygen content (d) remove suspended particles

Step-by-Step Solution

Answer

Chlorine is used to kill germs in water.

Step 1: Understanding the Purpose of Chlorine in Water Treatment

Chlorine is a chemical commonly added to water supplies to improve its safety for human consumption. Historically, understanding its purpose is crucial. Chlorine mainly acts as a disinfectant to ensure the water free from harmful microorganisms.

Step 2: Identifying the Correct Function

Examine each option given: (a) kill germs - Chlorine is known for its bactericidal properties and is primarily used to eliminate bacteria and viruses in water. (b) remove hardness - This involves removing calcium and magnesium ions, which chlorine does not address. (c) increase oxygen content - Chlorine does not change the oxygen content of water. (d) remove suspended particles - This is typically done by filtration, not by chlorination.

Step 3: Choosing the Correct Answer

Given the explanation for each option, option (a) 'kill germs' directly correlates with the use of chlorine in water treatment. Its primary function is to disinfect water by eliminating germs.

Key Concepts

Water Treatment

Water treatment is an essential step in ensuring that water is safe to drink. This process involves several steps designed to remove contaminants and improve water quality. One key method of water treatment is chlorination, which uses chlorine to disinfect water by destroying harmful microorganisms. Chlorination is a widely used technique due to its effectiveness and relatively low cost.

Other methods may include:

• Filtration: Physical processes that remove particles from the water.
• Coagulation and Flocculation: Chemical processes that bind small particles into larger clumps that can be removed more easily.
• Desalination: Removing salts from seawater, which involves processes like reverse osmosis or distillation.

Each of these methods plays a critical role in delivering safe, clean water for various uses, including drinking, cooking, and hygiene.

Disinfection

Disinfection is the process of killing or deactivating pathogenic microorganisms to prevent diseases. In water treatment, disinfection is crucial as it ensures the microbiological safety of water.

Chlorination is the most common disinfectant method used worldwide. It directly targets and eliminates harmful bacteria and viruses in the water. There are different forms of chlorine used for disinfection:

• Gas Chlorine: Effective but requires careful handling due to its toxicity.
• Sodium Hypochlorite: Commonly found in liquid form and widely used in smaller water systems.
• Calcium Hypochlorite: Often used in tablet or powder form and is highly effective.

These forms of chlorine not only disinfect but also provide residual protection as water moves through pipes, ensuring that it remains safe until it reaches consumers.

Bactericidal Properties

The term "bactericidal properties" refers to the ability of a substance to kill bacteria. Chlorine is highly regarded for its bactericidal properties, making it a powerful tool in water treatment.

When chlorine is added to water, it reacts with the water to form hypochlorous acid. This acid is essential because it penetrates the cell walls of bacteria, causing them to die.

• Fast Acting: Chlorine can kill a wide range of microorganisms within minutes.
• Broad Spectrum: Effective against bacteria, viruses, and some protozoa.
• Residue Effectiveness: Continues to provide protection throughout the water distribution system.

These bactericidal properties of chlorine make it indispensable in maintaining public health by preventing waterborne diseases and ensuring a safe supply of water.