How does a pure substance differ from a homogeneous mixture?

A pure substance is made up of only one kind of particle—it has a fixed or definite composition throughout. Examples include distilled water (H₂O), oxygen gas (O₂), and pure gold (Au). No matter where you take a sample from, the composition and properties remain the same. A pure substance cannot be separated into simpler materials by physical means such as filtration or evaporation; it can only be changed chemically.

A homogeneous mixture, on the other hand, is made up of two or more substances that are evenly mixed so they appear uniform throughout. Examples include salt water, air, or alloys like brass. Though homogeneous mixtures look like a single substance, they contain multiple components that can still be physically separated based on their physical properties. For instance, water can be separated from salt water through distillation.

In short, the difference lies in composition and separability:

• Pure substance: one kind of particle, fixed composition, cannot be separated physically.
• Homogeneous mixture: more than one kind of particle, uniform appearance, separable by physical methods.

Thus, while both appear uniform, only a pure substance has a constant structure at the molecular level.