Final answer:
A substance with varying composition is considered a mixture. It can be either a homogeneous or heterogeneous mixture, depending on its uniformity or lack thereof. For example, air is a homogeneous mixture of gases.
Step-by-step explanation:
A substance with varying composition is considered a mixture. In chemistry, there are two types of mixtures: homogeneous and heterogeneous. A homogeneous mixture has uniform composition throughout, like a solution, while a heterogeneous mixture has different regions with different compositions, like a salad dressing.
For example, air is a mixture of gases including nitrogen, oxygen, and carbon dioxide. It has a varying composition, but it is still considered a homogeneous mixture because it appears the same throughout.