Matter: States, Properties, and Why It All Changes
Solid, liquid, gas — and the difference between a physical and chemical change. Mass conservation, mixtures vs pure substances, and the 5 major reaction types every CBE student must recognize on sight.
Matter is anything that takes up space and has mass
That seemingly trivial definition does heavy work. Light is not matter (no mass, no volume in the everyday sense). A magnetic field is not matter. But everything chemistry studies — atoms, molecules, ions, solutions, gases, the air you breathe — is matter.
Matter exists in states. The three you'll see on the CBE:
- Solid — definite shape, definite volume. Particles vibrate in fixed positions.
- Liquid — indefinite shape (takes the container's shape), definite volume. Particles slide past each other.
- Gas — indefinite shape AND volume (fills the container). Particles move freely, far apart.
(Plasma — ionized gas at extreme temperatures — is a fourth state, less commonly tested.)
Two ways to describe a property
Intensive properties don't change with sample size: density, temperature, color, melting point, refractive index. A spoonful of mercury and an Olympic pool of mercury both have density 13.6 g/cm³.
Extensive properties DO change with size: mass, volume, total heat content, length. The pool has vastly more mass than the spoonful.
Then separately:
- Physical property — can be observed without changing the substance's identity (color, melting point, density).
- Chemical property — describes how the substance REACTS (flammability, reactivity with acid).
Physical change vs chemical change
A physical change alters form but not identity. Ice melting into water is still H₂O — the molecules are unchanged, just rearranged. Tearing paper, dissolving sugar in water, boiling water — all physical changes.
A chemical change forms new substances with new properties. Iron rusting (Fe + O₂ → Fe₂O₃), wood burning (combustion), baking soda + vinegar fizzing (acid-base). The original substance is gone; new substances exist.
Five indicators of chemical change to memorize:
- Color change (and not from physical mixing)
- Gas production (bubbles without boiling)
- Precipitate formation (insoluble solid from clear liquids)
- Heat/light emission (or sustained temperature change)
- Difficult to reverse (you can't unburn the wood)
Pure substance vs mixture
- Pure substance — one type only. Either an ELEMENT (a single atom type: Au, O₂, Fe) or a COMPOUND (atoms chemically bonded in a fixed ratio: NaCl, H₂O, CO₂).
- Mixture — two or more substances physically combined, separable by physical means.
- HOMOGENEOUS = uniform throughout (salt water, air, brass)
- HETEROGENEOUS = non-uniform, distinct regions (oil + water, sand + water, granola)
Conservation of mass — the master rule
In any chemical reaction at the macroscopic scale, mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products. (Antoine Lavoisier proved this in 1789 with sealed-flask experiments.)
Why this matters: every chemical equation must BALANCE — same atoms in equal counts on both sides. If you start with 2 mol H₂ and 1 mol O₂, you cannot end with 1 mol H₂O — there are 4 H atoms on the left and only 2 on the right. The balanced equation is 2 H₂ + O₂ → 2 H₂O.
Five reaction types you must recognize on sight
- Synthesis (combination): A + B → AB. Two or more reactants combine into ONE product. Example: 2 H₂ + O₂ → 2 H₂O.
- Decomposition: AB → A + B. One compound splits into simpler substances. Example: 2 KClO₃ → 2 KCl + 3 O₂.
- Single replacement: A + BC → AC + B. A more reactive element displaces another. Example: Zn + CuSO₄ → ZnSO₄ + Cu.
- Double replacement: AB + CD → AD + CB. Two compounds swap partners, often producing a precipitate, gas, or water. Example: AgNO₃ + NaCl → AgCl(s) + NaNO₃.
- Combustion: Fuel + O₂ → CO₂ + H₂O + energy. Burning hydrocarbons in air. Example: CH₄ + 2 O₂ → CO₂ + 2 H₂O.