Assertion (A). A liquid crystallises into a solid and is accompanied by decrease in entropy.
Reason (R). In crystals, molecules organise in an ordered manner.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not the correct explanation of A
C. A is true but R is false
D. A is false but R is true

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that when a liquid crystallizes into a solid, there is a decrease in entropy (ΔS). Entropy is a measure of disorder or randomness in a system.

2. **Phase Transition**: When a liquid transitions to a solid (crystallization), the molecules lose energy and move closer together, forming a structured arrangement. This process is characterized by a decrease in the randomness of the molecular arrangement.

3. **Entropy in Different States of Matter**:
- In the gaseous state, molecules are far apart and move freely, resulting in high entropy due to high disorder.
- In the liquid state, molecules are still relatively free to move but are more closely packed than in gases, leading to moderate entropy.
- In the solid state, molecules are tightly packed in a fixed arrangement, resulting in low entropy due to low disorder.

4. **Conclusion on Entropy Change**: As a liquid crystallizes into a solid, the entropy decreases because the molecules become more ordered. This aligns with the assertion that crystallization is accompanied by a decrease in entropy.

5. **Understanding the Reason**: The reason states that in crystals, molecules are organized in an ordered manner. This is true, as the ordered arrangement of molecules in a crystal lattice is what leads to the decrease in entropy.

6. **Final Evaluation**: Both the assertion and the reason are true. The reason provided correctly explains why the assertion is valid.

**Final Answer**: Both assertion and reason are true, and the reason is the correct explanation of the assertion.

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