Atoms, elements, compounds and mixtures
All chemistry rests on a single big idea: everything is made of atoms, and the way atoms combine determines the properties of every substance you'll meet at GCSE.
Atoms, elements and compounds — the definitions
- Atom — the smallest particle of an element that retains its properties. Diameter ≈ 0.1 nm (10⁻¹⁰ m).
- Element — a substance made of only one type of atom. There are about 100 elements; the periodic table lists them all.
- Compound — a substance made of two or more elements chemically combined in fixed proportions (e.g. H₂O, CO₂, NaCl).
- Molecule — two or more atoms chemically bonded together. A molecule of an element has identical atoms (O₂); a molecule of a compound has different atoms (H₂O).
A chemical formula tells you how many of each atom: H₂SO₄ contains 2 hydrogen, 1 sulfur and 4 oxygen atoms.
Mixtures
A mixture consists of two or more substances not chemically combined. The components keep their own properties and can usually be separated by physical means.
Examples: air (mixture of gases), salty water (NaCl + H₂O), bronze (Cu + Sn), crude oil.
The key difference: in a compound atoms are joined by chemical bonds, requiring a chemical reaction to break them apart; in a mixture they are simply mixed and can be separated physically.
Chemical equations
A chemical reaction rearranges atoms but doesn't create or destroy them (conservation of mass). Equations must balance — same number of each atom on both sides.
Word equation: hydrogen + oxygen → water. Symbol equation (balanced): 2H₂ + O₂ → 2H₂O.
Use state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Separating mixtures — five methods
1. Filtration
Separates an insoluble solid from a liquid using filter paper. Sand from water.
2. Crystallisation / evaporation
Recovers a dissolved solid from a solution by gently evaporating the water (often using a water bath, then leaving to cool to form crystals). Used to obtain salt from salty water.
3. Simple distillation
Separates a liquid from a solution (e.g. pure water from salty water). Heat the solution; the liquid evaporates and rises through the apparatus, then condenses on a Liebig condenser back into a clean flask. The dissolved solid stays behind.
4. Fractional distillation
Separates two or more miscible liquids with different boiling points. Used to separate ethanol (78 °C) from water (100 °C), and to separate the components of crude oil. The fractionating column has a temperature gradient, and substances condense at different heights.
5. Chromatography
Separates substances dissolved in a solvent based on how strongly each is attracted to the stationary phase (paper) versus the mobile phase (solvent). Calculates R_f values:
R_f = distance moved by spot ÷ distance moved by solvent front
R_f is always between 0 and 1. Compounds with the same R_f under identical conditions are likely the same.
Required practical: chromatography
Procedure: pencil baseline (pencil doesn't dissolve), spot small amounts of mixtures and known dyes, place in solvent (water or ethanol) below the line, allow solvent to rise. Mark solvent front before drying. Calculate R_f and compare.
⚠Common mistakes
- Confusing atom with molecule. An atom is a single particle; a molecule is two or more atoms bonded.
- Treating air as a compound. Air is a mixture — its gases aren't chemically bonded.
- Drawing the chromatography baseline in pen. Use pencil — ink dissolves and contaminates the experiment.
- Saying mixtures are "always uniform". Some are (solutions); others are obviously not (sand and gravel).
Links
Sets up C1.2 (atomic structure) and C2 (bonding). Separation techniques crop up again in C7 (crude oil) and C10 (potable water).
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