A compound is indeed considered a pure substance. This is because a compound consists of two or more elements that are chemically bonded together in a fixed ratio. Unlike mixtures, which can vary in composition and do not have a consistent chemical structure, compounds have a specific and uniform composition. For example, water (H2O) is a compound made of hydrogen and oxygen in a 2:1 ratio. Regardless of how much water you have, it will always have the same chemical formula and properties.
Furthermore, compounds have distinct characteristics that differ from the individual elements that make them up. When elements combine to form a compound, they lose their individual properties and gain new characteristics. This transformation is a clear indication that a compound is a pure substance, as it has a defined set of properties and behavior under specific conditions.
In summary, a compound qualifies as a pure substance because of its consistent composition and unique set of properties, differing from mixtures that lack such uniformity.