Which of the folowings statements are not true ?

Options

A. Vacancy defect results in a decrease in the density of the substance.
B. Intersitital defect results in an increase in the density of the substance .
C. Impurity defect has no effect on the density of the substance.
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 each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Vacancy defect results in a decrease in the density of the substance.**
- **Analysis**: A vacancy defect occurs when an atom or ion is missing from its lattice position. This missing atom reduces the mass of the solid without affecting its volume significantly, leading to a decrease in density (Density = Mass/Volume). Thus, this statement is **true**.

2. **Statement 2: Interstitial defects result in an increase in the density of the substance.**
- **Analysis**: An interstitial defect occurs when small atoms (like carbon or nitrogen) occupy the interstitial spaces in the lattice. This addition of mass increases the overall mass of the solid while the volume remains relatively unchanged, leading to an increase in density. Therefore, this statement is **true**.

3. **Statement 3: Impurity defect has no effect on the density of the substance.**
- **Analysis**: An impurity defect involves the introduction of foreign atoms into the lattice. Depending on the size and mass of the impurity compared to the host atoms, this can either increase or decrease the density. However, in many cases, especially when the impurity is similar in size and mass, the overall density may not change significantly. Hence, this statement can be considered **false**.

4. **Statement 4: Frankel defect results in an increase in the density of the substance.**
- **Analysis**: A Frankel defect occurs when an atom (cation or anion) leaves its lattice site and occupies an interstitial site. This does not change the overall mass of the solid, as the atom is merely relocated, and the volume remains constant. Therefore, the density remains unchanged. Thus, this statement is **false**.

### Conclusion:
The statements that are **not true** are:
- Statement 3: Impurity defect has no effect on the density of the substance.
- Statement 4: Frankel defect results in an increase in the density of the substance.