Which of these is a necessary condition for law of definite proportions to hold true?

Options

A. The compound under observation must not sublime
B. The compound under observation must be pure
C. The compound should not crack on heating
D. The compound should not be radioactive

Answer

To determine the necessary condition for the law of definite proportions to hold true, we need to first understand what the law states.

### Step-by-Step Solution:

1. **Understanding the Law of Definite Proportions**:
- The law of definite proportions, also known as Proust's law, states that a given chemical compound always contains its constituent elements in fixed ratios by mass, regardless of the source or method of preparation.

2. **Analyzing the Options**:
- We have four options to evaluate:
1. The compound under observation must not sublime.
2. The compound under observation must be pure.
3. The compound must not crack on heating.
4. The compound should not be radioactive.

3. **Evaluating Each Option**:
- **Option 1**: The compound under observation must not sublime.
- This is incorrect because the definition does not mention sublimation as a factor affecting the composition of the compound.

- **Option 2**: The compound under observation must be pure.
- This is correct. The law states that the elements must be in pure form, which means the compound must be pure for the law to hold true.

- **Option 3**: The compound must not crack on heating.
- This is also incorrect. The definition does not mention anything about heating or cracking affecting the composition.

- **Option 4**: The compound should not be radioactive.
- This is incorrect as well. Radioactive compounds can still adhere to the law of definite proportions, as they can form stable compounds that follow the law.

4. **Conclusion**:
- The only necessary condition for the law of definite proportions to hold true is that the compound under observation must be pure.

### Final Answer:
The necessary condition for the law of definite proportions to hold true is that **the compound under observation must be pure**.