Which of the following is not true ?

Options

A. Ordinary water is electrolysed more rapidly than `D_(2)O`.
B. Reaction between `H_(2)` and `Cl_(2)` is much faster than `D_(2)` and `Cl_(2)`.
C. `D_(2)O` freezes at lower temperature than `H_(2)O`.
D. Bond dissociation energy for `D_(2)` is greater than `H_(2)`.

Answer

To determine which statement is not true, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Option A: Ordinary water is electrolyzed more rapidly than D2O.**
- Ordinary water (H2O) contains a very small amount of heavy water (D2O). During electrolysis, H2O is converted into hydrogen (H2) and oxygen (O2) more readily than D2O. This is because D2O has a higher bond dissociation energy, making it less reactive. Therefore, this statement is true.

2. **Option B: The reaction between H2 and Cl2 is much faster than D2 and Cl2.**
- Hydrogen (H2) is indeed more reactive than deuterium (D2) due to its lower bond dissociation energy. As a result, the reaction between H2 and Cl2 occurs more rapidly than the reaction between D2 and Cl2. This statement is also true.

3. **Option C: D2O freezes at a lower temperature than H2O.**
- The freezing point of heavy water (D2O) is approximately 3.8 degrees Celsius, while the freezing point of ordinary water (H2O) is 0 degrees Celsius. This means that H2O freezes at a lower temperature than D2O, making this statement false.

4. **Option D: The bond dissociation energy of D2 is greater than that of H2.**
- The bond dissociation energy of D2 (deuterium) is indeed greater than that of H2 (hydrogen) because the presence of the extra neutron in deuterium makes the bond stronger. Thus, this statement is true.

### Conclusion:
The statement that is not true is **Option C: D2O freezes at a lower temperature than H2O.**