Assertion : Melting and boiling points of `D_(2)O` are higher than those of ordinary `H_(2)O`.
Reason : `D_(2)O` has lesser degree of association and lower molecular mass than `H_(2)O`.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the assertion-reason question, we will analyze both the assertion and the reason step by step.

### Step 1: Understand the Assertion
The assertion states that the melting and boiling points of D₂O (heavy water) are higher than those of H₂O (ordinary water). This is a known fact in chemistry.

**Hint for Step 1:** Recall the properties of heavy water compared to ordinary water.

### Step 2: Understand the Reason
The reason given is that D₂O has a lesser degree of association and lower molecular mass than H₂O. We need to evaluate this statement.

1. **Molecular Mass Comparison**:
- H₂O has a molecular mass of approximately 18 g/mol (2 for hydrogen and 16 for oxygen).
- D₂O has a molecular mass of approximately 20 g/mol (4 for deuterium and 16 for oxygen).
- Therefore, D₂O has a greater molecular mass than H₂O.

**Hint for Step 2:** Compare the molecular masses of H₂O and D₂O to understand their differences.

### Step 3: Degree of Association
The degree of association refers to how molecules interact with each other. In the case of water, hydrogen bonding plays a significant role in the properties of both H₂O and D₂O.

- D₂O has stronger hydrogen bonds compared to H₂O due to its greater mass, which leads to a higher degree of association.
- This means that D₂O molecules are more strongly attracted to each other than H₂O molecules.

**Hint for Step 3:** Consider how hydrogen bonding affects the physical properties of water and heavy water.

### Step 4: Conclusion
Now we can conclude:
- The assertion is true: D₂O does have higher melting and boiling points than H₂O.
- The reason is false: D₂O has a greater degree of association and a greater molecular mass than H₂O.

Thus, the correct answer is that the assertion is true, but the reason is false.

**Final Answer:** Assertion is true; Reason is false.