Which of the following statements are not true ?

Options

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

Answer

To determine which statements are not true regarding defects in solids, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Vacancy Defect:**
- A vacancy defect occurs when an atom is missing from its lattice site in a crystal structure.
- **Effect on Density:** When an atom is removed, the mass of the crystal decreases, but the volume remains the same. Therefore, the density (density = mass/volume) decreases.
- **Conclusion:** The statement "Vacancy defect results in a decrease in the density of the substance" is **true**.

2. **Understanding Interstitial Defect:**
- An interstitial defect occurs when extra atoms are inserted into the crystal lattice at interstitial sites (spaces between the regular lattice points).
- **Effect on Density:** The addition of extra mass (due to the extra atoms) increases the overall mass of the crystal without significantly changing its volume. Thus, the density increases.
- **Conclusion:** The statement "Interstitial defects result in an increase in the density of the substance" is **true**.

3. **Understanding Impurity Defect:**
- An impurity defect occurs when foreign atoms are introduced into the crystal lattice.
- **Effect on Density:** For example, if a heavier ion is introduced, it can either increase or decrease the density depending on the type of impurity and the number of ions removed to maintain charge neutrality. In many cases, the introduction of impurities can lead to a decrease in density.
- **Conclusion:** The statement "Impurity defect has no effect on the density of the substance" is **not true**.

4. **Understanding Frenkel Defect:**
- A Frenkel defect occurs when an atom leaves its lattice site and occupies an interstitial site.
- **Effect on Density:** In this case, the total mass of the crystal remains unchanged because an atom is merely relocated, and the volume also remains unchanged. Therefore, the density does not change.
- **Conclusion:** The statement "Frenkel defect results in an increase in the density of the substance" is **not true**.

### Final Answer:
The statements that are **not true** are:
- **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.