Is always fixed by mass.
Explanation
The Law of Constant Proportions (also known as the Law of Definite Proportions) was formulated by Joseph Proust. It is a fundamental principle of stoichiometry which states that a given chemical compound always contains its component elements in a fixed ratio (proportion) by mass.
Detailed Analysis
- Validity of the Law: The law asserts that the composition of a pure chemical compound is invariant. This means the ratio of the masses of the constituent elements remains constant regardless of how the compound is obtained or prepared.
- Irrelevance of Source and Method: Whether a compound is obtained from a natural source or synthesized in a laboratory, and regardless of the method of preparation used, its chemical composition remains the same. For example, pure water ($H_2O$) always contains hydrogen and oxygen in a mass ratio of 1:8, whether it is collected from a river or produced by burning hydrogen gas.
- Physical State: While the physical state (solid, liquid, gas) affects physical properties like density or volume, it does not alter the chemical composition by mass defined by this law.
Therefore, the composition is always fixed by mass, making Is always fixed by mass. the correct statement.
Key Takeaway
Law of Constant Proportions: Irrespective of the source, method of preparation, or physical state, a pure chemical compound always consists of the same elements combined together in the same fixed proportion by mass.