Which of the following statements are not true?

Options

A. Vacancy defect results in a decrease in the density of the substance.
B. Interstitial defects 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 some lattice sites in a crystalline solid are unoccupied. This leads to a decrease in the total mass of the solid without changing its volume significantly, thus decreasing the density.
- **Conclusion**: This statement is **true**.

2. **Statement 2: "Interstitial defect results in an increase in the density of the substance."**
- **Analysis**: An interstitial defect occurs when extra particles are present in the interstitial spaces of the crystal lattice. This increases the mass of the solid while the volume remains approximately the same, leading to an increase in density.
- **Conclusion**: This statement is **true**.

3. **Statement 3: "Impurity defect has no effect on the density of the substance."**
- **Analysis**: An impurity defect occurs when foreign atoms or ions are introduced into the crystal lattice. These impurities can have different masses and sizes compared to the original lattice atoms, which can change the overall density of the substance.
- **Conclusion**: This statement is **not true** (incorrect).

4. **Statement 4: "Frenkel defect results neither in a decrease nor increase in density of the substance."**
- **Analysis**: A Frenkel defect involves the displacement of an atom or ion from its lattice site to an interstitial site. While this defect does not change the total number of particles in the crystal, it can lead to a slight change in density depending on the size and mass of the displaced particle.
- **Conclusion**: This statement is **not true** (incorrect).

### Final Answer:
The statements that are **not true** are:
- Statement 3: "Impurity defect has no effect on the density of the substance."
- Statement 4: "Frenkel defect results neither in a decrease nor increase in density of the substance."

---