When is the law of constant proportions not obeyed ?

Explanation

### Step-by-Step Text Solution:

1. **Understanding the Law of Constant Proportions**:
The law of constant proportions, also known as 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.

2. **Identifying Exceptions**:
The law of constant proportions is not obeyed in certain situations. One significant case is when a compound contains elements that have isotopes.

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