A homogeneous mixture has a uniform composition throughout and appears as a single phase. A heterogeneous mixture has a non-uniform composition, with two or more distinguishable regions or phases.
This distinction applies at both SL and HL within Structure 1.1, Introduction to the particulate nature of matter.
The Reasoning
A mixture contains two or more substances that are physically combined rather than chemically bonded. Its components retain their chemical properties and can be separated using physical techniques.
In a homogeneous mixture, particles are distributed evenly on the macroscopic scale. For example, salt solution contains hydrated sodium and chloride ions spread uniformly through the water. Any small sample taken from the solution should have the same composition as another sample.
In a heterogeneous mixture, the particles are not distributed uniformly. For example, sand in water forms visibly different solid and liquid regions, so samples taken from different positions may have different compositions.
| Feature | Homogeneous mixture | Heterogeneous mixture |
|---|---|---|
| Composition | Uniform throughout | Non-uniform throughout |
| Number of visible phases | One | Two or more |
| Typical appearance | Components are not distinguishable | Components or regions may be distinguishable |
| Example | Air, salt solution, brass | Soil, oil and water, sand in water |
| Possible separation | Distillation, evaporation, chromatography | Filtration, decanting, separating funnel |
A phase is a region with uniform physical and chemical properties. Oil and water therefore form a heterogeneous mixture even if vigorous shaking temporarily disperses small droplets of one liquid through the other.
A common misconception is that every transparent mixture is homogeneous. Appearance alone is not always sufficient: the key criterion is whether the composition and properties are uniform throughout the sample.