Which of the following statements are not true ?

Options

A. Vacancy defect results in a decrease in the density of the substances
B. Interstitial defect results in an increase in the density of the substances
C. Impurity defect has no effect on the density of the substances
D. Frenkel defect results in an increase in the density of the substance

Explanation

To determine which statements are not true regarding defects in solids, we will analyze the statements provided in the video transcript step by step.

### Step 1: Analyze the first statement
The first statement mentions that "Vacancy defect results in the decrease in the density of the substances."

- **Explanation**: A vacancy defect occurs when some lattice sites in a crystal structure are unoccupied. This means that there are fewer particles in the same volume, which typically leads to a decrease in density because density is defined as mass per unit volume. Therefore, this statement is **true**.

### Step 2: Analyze the second statement
The second statement discusses "impurity defect can change the mass but not the volume."

- **Explanation**: An impurity defect occurs when foreign atoms are introduced into the crystal lattice. This can change the overall mass of the solid because the foreign atoms add mass. However, if the volume of the solid remains unchanged (as the foreign atoms occupy the lattice sites without significantly altering the structure), then the density may change. Therefore, the statement that impurity defect can change the mass but not the volume is **not entirely accurate**. The volume may change depending on the size and nature of the impurity.

### Conclusion
Based on the analysis:
1. The first statement is **true**.
2. The second statement is **not true**.

### Final Answer
The statement that is not true is the one regarding impurity defects changing the mass but not the volume.

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