* Pure Substance: A pure substance has a fixed composition and properties throughout. Examples include elements (like gold) and compounds (like water).
* Mixture: A mixture is a combination of two or more substances where each substance retains its own properties. Mixtures can be homogeneous (uniformly distributed) or heterogeneous (non-uniform).
When potassium dichromate (K₂Cr₂O₇) dissolves in water, it forms ions:
K₂Cr₂O₇ (s) → 2K⁺ (aq) + Cr₂O₇²⁻ (aq)
The potassium dichromate molecules are no longer intact, and the solution contains water molecules, potassium ions, and dichromate ions. This is a homogeneous mixture because the ions are evenly distributed throughout the solution.
