The importance of sodium aluminate (III) in the treatment of water is to

Analysis

The question asks for the specific role of sodium aluminate (III), also known as sodium aluminate ($NaAlO_2$), when it is added to water during the treatment process.

Concept Explanation

In water treatment, the process of coagulation is used to remove suspended impurities (like dirt, clay, and organic matter) that are too small to be removed by simple filtration.

Coagulants are chemicals that neutralize the negative charges on these suspended particles, allowing them to clump together into larger particles called flocs. Once these particles become heavy enough, they settle at the bottom of the tank (sedimentation) or can be easily filtered out. Sodium aluminate is a common chemical used for this purpose, especially in water that is slightly acidic or when a rapid coagulation process is required.

Step-by-Step Solution

1. Identify the chemical: Sodium aluminate ($NaAlO_2$) is an alkaline salt.
2. Understand the reaction: When added to water, it reacts to form aluminum hydroxide, $Al(OH)_3$, which is a gelatinous (jelly-like) precipitate.
3. Mechanism: This gelatinous precipitate acts as a "sticky" agent. It traps suspended particles and impurities, causing them to aggregate (coagulate) into larger masses.
4. Evaluate the options: • A (Cause coagulation): This aligns with the chemical action of sodium aluminate described above. • B (Neutralize acidity): While sodium aluminate is alkaline, its primary industrial purpose in water treatment is coagulation, not just pH adjustment. • C (Prevent goitre and tooth decay): This is the role of iodine and fluoride additives, respectively, not sodium aluminate. • D (Kill germs): This is the role of disinfectants like chlorine or ozone.

Conclusion

Sodium aluminate is primarily used as a coagulant to clear suspended matter from water. Therefore, the correct option is A.

Exam Tip: Whenever you see questions about water treatment, remember the "Big Three" roles:

• Coagulants (like Alum or Sodium Aluminate) for removing suspended particles.
• Disinfectants (like Chlorine) for killing germs/bacteria.
• Fluoridation (adding Fluoride) for preventing tooth decay.