Assertion(A): The micelle formed by sodiumm stearate in water has -COO groups at the surface.
Reason(R): Surface tension of water is reduced by addition of stearate.

Options

A. If both (A) and (R) are correct, and (R) is the correct explanation of (A)
B. If both (A) and (R) are correct, but (R) is the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct. S

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

**Assertion (A):** The micelle formed by sodium stearate in water has -COO groups at the surface.

**Reason (R):** Surface tension of water is reduced by the addition of stearate.

### Step-by-Step Solution:

1. **Understanding Micelles:**
- Micelles are formed when amphiphilic molecules (like sodium stearate) are added to water. Sodium stearate has a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail.
- In aqueous solutions, these molecules arrange themselves such that the hydrophilic heads face the water while the hydrophobic tails are shielded from it, forming a spherical structure.

2. **Structure of Sodium Stearate:**
- Sodium stearate (C17H35COONa) consists of a long hydrocarbon chain (C17H35) and a carboxylate group (-COO⁻).
- When sodium stearate forms micelles in water, the -COO⁻ groups (carboxylate) will indeed be oriented towards the water, forming the surface of the micelle.

3. **Effect on Surface Tension:**
- The addition of sodium stearate to water reduces the surface tension. This is because the hydrophilic heads of the stearate molecules disrupt the hydrogen bonding network of water, allowing the water molecules to move more freely.
- As the surface tension decreases, it indicates that the water molecules are less tightly bound to each other at the surface, which is a direct result of the presence of the surfactant (sodium stearate).

4. **Conclusion:**
- Both the assertion and the reason are true. The micelle formed by sodium stearate does have -COO groups at the surface, and the addition of stearate does reduce the surface tension of water.
- Therefore, the assertion (A) is true, and the reason (R) is also true, and R correctly explains A.

### Final Answer:
Both assertion (A) and reason (R) are true, and R is the correct explanation of A.

---