Which of the following is not the true about crystalline solids

Options

A. They are rigid and hard
B. They possess plane surfaces
C. They are obtained by rapid cooling of motten substances
D. They have definite geometric configuration.

Answer

To determine which statement is not true about crystalline solids, we will analyze each of the provided statements one by one.

### Step-by-Step Solution:

1. **Statement Analysis**:
- **Statement 1**: "They are rigid and hard."
- **Analysis**: This statement is true. Crystalline solids are known for their rigidity and hardness due to their orderly arrangement of particles.

2. **Statement 2**: "They possess plane surfaces."
- **Analysis**: This statement is also true. When crystalline solids are cut, they typically exhibit flat surfaces, which is a characteristic feature of their structure.

3. **Statement 3**: "They are obtained by rapid cooling of molten substances."
- **Analysis**: This statement is false. Crystalline solids are not formed by rapid cooling; instead, they are formed through slow cooling, allowing the atoms to arrange themselves in a well-ordered structure. Rapid cooling leads to the formation of amorphous solids, such as glass.

4. **Statement 4**: "They have definite geometric configuration."
- **Analysis**: This statement is true. Crystalline solids have a definite geometric arrangement of particles, which gives them a specific shape and structure.

5. **Conclusion**: Based on the analysis, the statement that is not true about crystalline solids is **Statement 3**: "They are obtained by rapid cooling of molten substances."

### Final Answer:
The statement that is not true about crystalline solids is: **They are obtained by rapid cooling of molten substances.**

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