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 can follow these steps:

### Step 1: Understand the Law of Definite Proportions
The Law of Definite Proportions states that a chemical compound always contains its component elements in fixed ratio by mass, regardless of the source or method of preparation.

### Step 2: Identify the Conditions
To apply this law, certain conditions must be met. The most crucial condition is that the compound being analyzed must be pure.

### Step 3: Analyze the Given Statements
1. **The compound under observation must not sublime**: This is incorrect. Sublimation does not affect the law.
2. **The compound under observation must be pure**: This is correct. A pure compound ensures that the mass ratio of the elements can be accurately determined.
3. **The compound should not crack on heating**: This is incorrect. Cracking does not influence the law.
4. **The compound should not be radioactive**: This is also incorrect. Radioactivity does not affect the mass ratio of the elements in a compound.

### Step 4: Conclusion
The necessary condition for the Law of Definite Proportions to hold true is that **the compound under observation must be pure**.

### Final Answer
The correct option is: **The compound under observation must be pure.**

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