What must be true about a pure compound regarding its mass composition?

A pure compound is defined as a substance that is made up of only one type of molecule and has specific chemical properties. The defining characteristic of a pure compound is that it consists of elements that are chemically combined in a fixed ratio by mass. This means that regardless of the quantity of the substance or how it is prepared, the elements involved will always be in the same proportion to one another.

For example, water (H₂O) is a pure compound made up of hydrogen and oxygen in a ratio of 2:1 by atoms, which corresponds to a consistent mass ratio of approximately 11.2% hydrogen and 88.8% oxygen. This fixed composition is fundamental to the identity and properties of the compound.

The other options suggest variability, which is not true for pure compounds. For instance, the mass composition does not change with different separation methods, exposure to heat, or environmental factors, as these would not alter the inherent ratio of elements in a pure compound.