The law of definite proportions states that

Step 1: Analyze the question You are asked to identify the correct statement of the Law of Definite Proportions (also called the Law of Constant Composition).

Step 2: The concept This law was stated by Joseph Proust. It says that a pure chemical compound always contains the same elements combined in the same fixed proportion by mass, no matter where it came from or how it was prepared.

Example: Water (H₂O) always has hydrogen and oxygen in a mass ratio of about 1 : 8, whether it comes from a river, rain, or is made in the laboratory.

Step 3: Check each option

• A. Pure samples of the same compound contain the same elements combined in the same proportion by mass. This matches the law exactly.
• B. Too vague. It says "substances" rather than a compound, and does not mention the elements combined. Mixtures are also substances, and their proportions vary.
• C. This confuses the cause with the effect. A compound is not "pure" simply because it contains the same elements, and it says nothing about the mass proportions.
• D. This is the Law of Conservation of Mass, not the Law of Definite Proportions.

Step 4: Answer The correct option is A.

Tip: Link each law to a keyword:

• Definite proportions → same compound, same mass ratio
• Conservation of mass → matter is not created or destroyed
• Multiple proportions → the same elements form different compounds, with simple whole-number ratios