Classification of Matter: Elements, Compounds, and Mixtures

Key Ideas

• Element — a pure substance made of only one type of atom that cannot be broken into simpler substances by chemical means (e.g., oxygen, O; gold, Au; carbon, C).
• Compound — a pure substance of two or more elements chemically combined in a fixed ratio (e.g., water, H₂O; sodium chloride, NaCl). It can be decomposed by chemical means, but not by physical means.
• Homogeneous mixture (solution) — uniform composition throughout; a single phase with no visible boundaries (e.g., salt water, air, brass).
• Heterogeneous mixture — non-uniform; visibly distinct regions or phases (e.g., granite, salad dressing, sandy water).
• Physical property — observed without changing the substance's identity (color, density, melting point, boiling point, hardness, electrical conductivity).
• Chemical property — observed when the substance changes into a different substance (flammability, reactivity with acid, tendency to rust).
• Physical change — identity is preserved; only appearance or state changes (melting, boiling, dissolving, grinding).
• Chemical change — a new substance forms (burning, rusting, digesting food, electrolysis of water).
• Intensive property — independent of how much matter is present (density, temperature, boiling point, color, concentration).
• Extensive property — depends on the amount (mass, volume, length, total energy).

Equations and Variables

This topic is largely qualitative, but one equation is worth stating because it doubles as the classic example of an intensive property. Density is mass per unit volume:

d = m / V

• d = density (commonly g/cm³ or g/mL for solids/liquids; g/L for gases)
• m = mass (g)
• V = volume (cm³ or mL)

Density is intensive: a single drop of water and a full swimming pool both have d ≈ 1.00 g/mL at 4 °C. Mass and volume are extensive: doubling the amount doubles both m and V, so their ratio (density) stays constant.

How It Works or Problem-Solving Method

Use a decision tree to classify matter:

1. Is the sample uniform throughout (same appearance and composition everywhere)?
   • No → heterogeneous mixture.
   • Yes → continue.
2. *Can the sample be separated into simpler substances by physical* means** (filtration, evaporation, distillation, a magnet, decanting)?
   • Yes → homogeneous mixture (solution).
   • No → pure substance; continue.
3. *Can the pure substance be decomposed into simpler substances by chemical* means** (heat, electrolysis, reaction with acid)?
   • No → element.
   • Yes → compound.

For properties, ask one question: does observing it require changing the substance into something new? Yes → chemical property; no → physical property. For changes, the same test applies to the outcome: did the identity change? For intensive vs. extensive, ask: does the value change if I take twice as much? No → intensive; yes → extensive.

Worked Example

Classify each sample and each observation.

Property classification:

• Density of ethanol = 0.789 g/mL → intensive, physical.
• A 100 mL sample of ethanol has a mass of 78.9 g → extensive, physical (the mass scales with volume).
• Iron turns to reddish rust in moist air → chemical change (a new substance, iron(III) oxide, forms).
• Water boils at 100 °C (at 1 atm) → physical change and intensive property (steam is still H₂O).

Common Confusions

• "Dissolving salt in water is a chemical change." Wrong — the NaCl can be recovered unchanged by evaporation, so identity is preserved; it is a physical change producing a solution.
• "A homogeneous mixture is a pure substance." Wrong — a solution has variable composition (you can make weak or strong salt water); pure substances have a fixed, constant composition.
• "Mass is intensive because every object has some mass." Wrong — "intensive" means the value does not change with amount; mass clearly doubles when you double the sample, so it is extensive. Density is the intensive quantity.