Alum is widely used to purify water since
(a) it forms complex with clay particles
(b) it coagulates

Q: Why doesn't the mud just settle down if we leave the water alone for a long time?

Muddy water is a colloidal dispersion where the particles carry a negative charge. This causes them to repel each other, preventing them from clumping together and settling under gravity.

Q: What exactly does alum do when added to the water?

Alum dissolves to release $\text{Al}^{3+}$ ions. These highly positive ions attract and neutralize the negatively charged mud particles.

Q: Why is alum preferred over something simple like table salt ($\text{NaCl}$)?

According to the Hardy-Schulze rule, ions with higher valency have much greater coagulating power. The $\text{Al}^{3+}$ ion from alum is far more effective than the $\text{Na}^{+}$ ion from salt.

Q: Is the water completely safe to drink right after adding alum?

Alum removes suspended colloidal impurities (like mud), making the water clear. However, it does not kill bacteria or viruses, so the water may still need boiling or chemical disinfection before drinking.

Q: What is the difference between coagulation and precipitation?

Coagulation specifically refers to the aggregation of colloidal particles into larger clumps by neutralizing their charge. Precipitation is the formation of a solid from a chemical reaction between two soluble substances.

Question

Alum is widely used to purify water since

Options:

(A) it forms complex with clay particles

(B) it coagulates the mud particles

(C) it exchanges $	ext{Ca}^{2+}$ and $	ext{Mg}^{2+}$ ions present in hard water

(D) its sulphate ion is water purifier

Sigma Percentile · Accepted Answer

The purification of muddy water using alum is a classic and beautiful application of surface chemistry in our daily lives. If you've ever seen a bucket of turbid, muddy water, you might have noticed that the dirt doesn't just settle down on its own, even if you leave it undisturbed for hours. Why is that?

### The Invisible Shield: Colloidal Stability

Muddy water is not a simple suspension; it is a **colloidal solution**. The tiny mud and clay particles dispersed in the water carry a **negative electrical charge**. Because like charges repel, these particles constantly push away from each other.

This electrostatic repulsion acts like an invisible shield, preventing the particles from coming close enough to clump together and settle under the influence of gravity. They remain in a state of stable, restless wandering.

### Enter Alum: The Chemical Hero

To break this stability, we need to neutralize that negative charge. This is where **alum** (potassium aluminum sulfate, $	ext{K}_2	ext{SO}_4 \cdot 	ext{Al}_2(	ext{SO}_4)_3 \cdot 24	ext{H}_2	ext{O}$) comes to the rescue.

When alum is dissolved in water, it dissociates to release highly charged aluminum ions, $	ext{Al}^{3+}$.

### The Hardy-Schulze Rule in Action

According to the **Hardy-Schulze rule**, the coagulating power of an ion is directly proportional to its valency. Because the aluminum ion has a high $+3$ charge, it is incredibly effective at attracting the negatively charged mud particles.

As the $	ext{Al}^{3+}$ ions swarm the mud particles, they neutralize the negative charges. Without their protective repulsive shield, the mud particles are finally able to collide and stick together.

### Coagulation and Purification

This process of particles aggregating to form larger, heavier clumps is called **coagulation**. As these clumps (or flocs) grow in size, they become too heavy to remain suspended in the water.

Gravity takes over, and the coagulated mud settles at the bottom of the container, leaving behind crystal clear, purified water at the top.

Therefore, alum is widely used to purify water because **it coagulates the mud particles**.

Animated Solution for Chemistry - Surface Chemistry: Alum is widely used to purify water since

Select Answer:

Visualized Solution

Visualizing the Problem

Nature of Mud Particles

Role of Alum

Charge Neutralization

The Coagulation Process

Final Conclusion

Food for Thought

The Sigma Insight: Colloids, Micelles and and Emulsions

The Invisible Shield

Enter Alum

The Hardy-Schulze Rule in Action

Coagulation and Purification

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