Solutions and Dispersed Systems: Composition and Mass Fraction
Mixtures can look uniform or visibly heterogeneous. Their behavior depends on particle scale, interactions, and composition.
Learning goals
- Distinguish a true solution, colloid, suspension, and emulsion.
- Identify the dispersed phase and dispersion medium.
- Express composition using solute mass fraction and mass percent.
- Solve preparation, mixing, dilution, and evaporation problems.
- Connect particle scale with settling, filtration, and light scattering.
1. Mixtures at the particle level
A mixture contains two or more substances whose particles remain chemically identifiable. In a homogeneous mixture, every sampled region has the same composition at the observed scale. A solution is homogeneous: the solute is distributed as molecules or ions through the solvent. In a heterogeneous mixture, composition varies from one region to another. The component distributed as particles or droplets is the dispersed phase; the continuous component is the dispersion medium.
Scale model: solution particles are smallest; colloidal particles are intermediate; suspension particles are large enough to settle readily.
2. Types of dispersed systems
| System | Particle behavior | Typical observation | Separation |
|---|---|---|---|
| True solution | molecules or ions | no settling; no ordinary filtration | evaporation, distillation, crystallization |
| Colloid | intermediate particles | scatters light; usually slow settling | special membranes or centrifugation |
| Suspension | large solid particles | cloudy; settles with time | filtration or settling |
| Emulsion | droplets of one liquid in another | may separate into layers | settling, centrifugation, phase separation |