Problem 7 - Chemistry and Chemical Reactivity

Question

Various reagents are added to water supplies to kill pathogens and make the water safe to drink. Among the substances listed below, which does not serve that function? (a) \(\mathrm{Cl}_{2}\) (b) \(\mathrm{O}_{3}\) (c) \(\mathrm{Ca}(\mathrm{ClO})_{2}\) (d) \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)

Step-by-Step Solution

Answer

(d) \( \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \) does not kill pathogens.

Step 1: Analyze Each Reagent

Let's take a closer look at what each chemical listed does when added to water.- (a) \( \mathrm{Cl}_{2} \) or chlorine is commonly used as a disinfectant to kill bacteria and other pathogens in water supplies.- (b) \( \mathrm{O}_{3} \) or ozone is another disinfectant used for killing bacteria, viruses, and other microorganisms in water.- (c) \( \mathrm{Ca}(\mathrm{ClO})_{2} \) or calcium hypochlorite is used as a bleaching agent and a disinfectant similar to chlorine, helping to eliminate pathogens.- (d) \( \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \) or aluminum sulfate is commonly used as a coagulating agent to remove impurities from water, not as a disinfectant.

Step 2: Identify the Non-Disinfectant

The problem asks for the reagent that does not serve the function of killing pathogens. In Step 1, we identified the roles of each reagent:- \( \mathrm{Cl}_{2} \), \( \mathrm{O}_{3} \), and \( \mathrm{Ca}(\mathrm{ClO})_{2} \) are all disinfectants that kill pathogens.- \( \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \) is used primarily as a coagulating agent and does not serve the purpose of killing pathogens.Thus, \( \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \) is the reagent that does not kill pathogens.

Key Concepts

Disinfectants

Disinfectants are critical in making water safe for consumption by killing harmful microorganisms and pathogens. These include viruses, bacteria, and protozoa that can cause diseases if ingested by humans.
Disinfectants work by applying chemicals that are toxic to these microorganisms, rendering the water safe. Commonly used disinfectants in water treatment include chlorine (\(\mathrm{Cl}_{2}\)), ozone (\(\mathrm{O}_{3}\)), and calcium hypochlorite (\(\mathrm{Ca(ClO)_{2}}\)). Each of these has unique properties and applications that make them suitable for different scenarios in water treatment.

• Chlorine: A widely used disinfectant due to its cost-effectiveness and ability to kill a wide range of pathogens. It's typically added to large water supplies to ensure that the water remains safe over long distances in the distribution system.
• Ozone: A powerful disinfectant effective against viruses and bacteria. It acts quickly and decomposes into oxygen, leaving no harmful residues but requires more expensive equipment.
• Calcium Hypochlorite: Used in smaller water systems and swimming pools for controlling bacterial growth. It is similar to chlorine in action but is more stable in storage.

Pathogen Removal

Ensuring the removal of pathogens from water supplies is a key goal of water treatment. Pathogens are harmful organisms like bacteria, viruses, and other microorganisms that can cause diseases such as cholera, typhoid, and dysentery if not removed from drinking water.
Pathogen removal involves several stages, including filtration, chemical disinfection, and biological treatments to effectively purify the water. While disinfectants are directly aimed at killing pathogens, other methods like filtration can physically remove them.

• Filtration: Physically removes a large amount of debris and microorganisms from water by passing it through filters of various sizes.
• Coagulation and Flocculation: Pre-treatment processes that help remove larger particles and microorganisms, making subsequent filtration and disinfection more effective.

Coagulation

Coagulation is a critical step in water treatment that assists in removing suspended particles that cloud water and may harbor pathogens. It involves adding a coagulant, typically aluminum sulfate (\(\mathrm{Al}_{2}(\mathrm{SO}_{4})_{3}\)), to raw water supplies.
The purpose of coagulation is not to kill pathogens but to make them easier to remove. The coagulant helps small particles come together to form larger aggregates known as flocs. These flocs can then be removed via sedimentation or filtration. This step is crucial in ensuring that water is clean before any disinfection processes are applied.

• Flocculation: After the addition of the coagulant, the water is gently stirred to help the flocs grow in size, allowing them to settle out of the water more efficiently.
• Particle Removal: With improved water clarity from coagulation, subsequent processes like filtration perform more effectively.

Water Supply Safety

Ensuring water supply safety is the main goal of water treatment processes, and it's critical for protecting public health. This involves multiple stages and methods to ensure that the water delivered is free from harmful pathogens and impurities.
The safety of water supply encompasses several key points:

• Continuous Monitoring: Regular testing of water at different stages of the supply process to ensure that it meets safety standards.
• Use of Reagents: Purposeful addition of chemicals like disinfectants to eliminate pathogens.
• Infrastructure Maintenance: Regular maintenance of pipes and distribution networks to prevent contamination.

The efficiency of water treatment processes, including pathogen removal, coagulation, and the cautious use of disinfectants, together ensure a potable water supply that meets health standards and protects communities.