Which of the following are not the characteristics of crystalline solids ?

Options

A. They are isotropic
B. They exhibit polymorphism
C. After melting, they become non - crystalline
D. They donot have thermodynamic defects

Answer

To determine which of the following statements are not characteristics of crystalline solids, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Analyze Statement 1: "They are isotropic."**
- Crystalline solids are characterized by anisotropy, meaning their physical properties vary with direction. Therefore, this statement is incorrect.
- **Conclusion:** This statement is NOT a characteristic of crystalline solids.

2. **Analyze Statement 2: "They exhibit polymorphism."**
- Polymorphism refers to the ability of a substance to exist in more than one crystalline form. Crystalline solids can exhibit polymorphism, making this statement correct.
- **Conclusion:** This statement IS a characteristic of crystalline solids.

3. **Analyze Statement 3: "After melting, they become non-crystalline."**
- When crystalline solids melt, they lose their ordered structure and become non-crystalline (amorphous). This statement is also correct.
- **Conclusion:** This statement IS a characteristic of crystalline solids.

4. **Analyze Statement 4: "They do not have thermodynamic defects."**
- Crystalline solids can have thermodynamic defects. According to the third law of thermodynamics, perfect crystals exist at absolute zero, but as temperature increases, defects can occur. Thus, this statement is incorrect.
- **Conclusion:** This statement is NOT a characteristic of crystalline solids.

### Final Answer:
The statements that are NOT characteristics of crystalline solids are:
- Statement 1: "They are isotropic."
- Statement 4: "They do not have thermodynamic defects."