Alum purifies muddy water by

Options

A. dialysis
B. absorption
C. coagulation
D. ultrafiltration

Answer

### Step-by-Step Solution:

1. **Understanding the Problem**: The question asks how alum purifies muddy water. We need to identify the process by which alum acts to clarify water.

2. **Composition of Alum**: Alum, specifically potash alum (KAl(SO₄)₂·12H₂O), dissociates in water to release potassium ions (K⁺) and aluminum ions (Al³⁺).

3. **Coagulation Process**: The aluminum ions (Al³⁺) have a higher charge compared to potassium ions (K⁺). According to Hardy-Schulze rule, ions with higher charges have a greater coagulating power. This means Al³⁺ is more effective in neutralizing the negative charges present in muddy water.

4. **Nature of Muddy Water**: Muddy water contains negatively charged particles (sols). When alum is added, the positively charged Al³⁺ ions interact with these negatively charged particles.

5. **Neutralization and Coagulation**: The Al³⁺ ions neutralize the negative charges of the muddy water particles, leading to coagulation. This process causes the particles to clump together and settle down, thus purifying the water.

6. **Conclusion**: Therefore, alum purifies muddy water primarily by the process of coagulation.

### Final Answer:
Alum purifies muddy water by **coagulation**.