Is Matter Pure?

• The difference between pure substances and mixtures.
• What elements and compounds are.
• Types of mixtures: homogeneous (solutions) and heterogeneous.
• Colloidal solutions — what they are and how to identify them.
• Methods to separate different types of mixtures.

Pure substances vs mixtures

Elements

• Element: a pure substance made of only one type of atom; cannot be broken down further by chemical means.
• 118 elements known; 94 occur naturally.
• Classified as metals (Na, Fe, Cu, Au), non-metals (C, O, N, S, Cl), metalloids (Si, As, B).

• Noble gases (He, Ne, Ar, Kr, Xe, Rn): non-metals; inert (do not react under normal conditions).

Compounds

• Compound: a pure substance formed when two or more elements combine chemically in a fixed ratio.
• Properties are entirely different from constituent elements.
• Can only be separated by chemical methods (not physical).

Examples:

• Law of Definite Proportions (Proust): a compound always contains the same elements in the same mass ratio (e.g., H₂O always has H:O = 1:8 by mass).

Mixtures

• Mixture: two or more substances combined without chemical reaction, in any proportion.
• Constituents retain their original properties.
• Separated by physical methods.

Types of mixtures

1. Homogeneous mixture (Solution)

• Uniform composition throughout; constituents not visually distinguishable.
• Examples: saltwater, lemonade, air, alloys (brass = Cu + Zn), alcohol + water.
• Components: solute (dissolved) + solvent (dissolving medium).
• Concentration = amount of solute per unit volume/mass of solution.
• Saturated solution: maximum solute dissolved at a given temperature.

2. Heterogeneous mixture

• Non-uniform composition; constituents visually distinguishable or settles on standing.
• Examples: soil, sand in water, oil in water, smoke, granite.
• Sub-types:
  • Suspension: large particles (> 1000 nm), settle on standing, opaque. Example: muddy water, chalk in water.
  • Colloid: medium particles (1–1000 nm), do not settle, scatter light. Example: milk, fog, smoke, blood, jelly.

Colloidal solutions (Colloids)

• Particles between 1 nm and 1000 nm — too small to see but large enough to scatter light.
• Tyndall effect: scattering of light by colloidal particles — visible as a beam of light through the colloid.
  • Sky is blue because of Tyndall scattering of sunlight by air particles.
  • Headlights visible in fog = Tyndall effect.
  • Milk appears white = Tyndall scattering of fat droplets.

Dispersed phase = substance spread out; Dispersion medium = substance in which it is spread.

Separation of mixtures

• Chromatography: separates coloured components (dyes, chlorophyll) — components travel at different rates on paper; used in forensics, food testing, drug detection.
• Fractional distillation of air: liquefies air → warm up → N₂ boils first (−196°C), then O₂ (−183°C), then Ar.

Elements, compounds and mixtures — summary

MATTER
├── Pure substances
│   ├── Elements (one type of atom)
│   └── Compounds (two+ elements, fixed ratio, chemical bond)
└── Mixtures
    ├── Homogeneous (solutions, alloys)
    └── Heterogeneous
        ├── Colloids (Tyndall effect)
        └── Suspensions (settle on standing)

• What is the difference between a compound and a mixture?
• Name the law that governs the composition of compounds.
• What is the Tyndall effect? Give two examples from daily life.
• How would you separate: (a) oil and water (b) two miscible liquids (c) dyes in ink?
• Classify: milk, air, saltwater, soil, blood — as solution, colloid, or suspension.

Open the Practice tab for graded questions on Is Matter Pure.