Matter, atoms, elements, compounds, mixtures and separation
Chemistry begins with what matter is made of and how mixtures can be separated. The exam asks for particle ideas, the periodic table, the compound/mixture distinction, and a justified choice of separation technique.
Atoms, elements and the periodic table
All matter is made of particles. Atoms contain protons (positive), neutrons (neutral) and electrons (negative). Atomic number = number of protons; mass number = protons + neutrons; isotopes differ in neutrons. Elements contain one type of atom (Fe, O₂) and are arranged by atomic number. The group number gives outer electrons; the period gives the number of shells. Metals are shiny, malleable and conduct; non-metals generally do not.
Compounds and mixtures
A compound is two or more elements chemically combined in fixed proportions (H₂O, CO₂) and can be separated only by chemical means. A mixture contains substances not chemically combined, in any proportion (air, seawater), and is separated physically. Physical changes (melting, dissolving) are reversible and make no new substance; chemical changes produce new substances and are usually hard to reverse.
Separation techniques
- Magnetism - iron from a mixture
- Filtration - insoluble solid from a liquid
- Evaporation/crystallisation - dissolved solid from solution
- Simple distillation - solvent from a solution
- Fractional distillation - liquids with different boiling points (crude oil)
- Chromatography - dyes, compared using Rf = distance moved by substance ÷ distance moved by solvent
Worked example
Separate iron filings, sand and salt. Use a magnet to remove the iron; add water and stir to dissolve the salt; filter to collect sand as the residue; evaporate the filtrate to recover salt crystals. Each step exploits a different physical property: magnetism, solubility, particle size, boiling point.
Traps
- Calling air a compound.
- Using evaporation to separate two dissolved solids.
- Forgetting that dissolving is a physical change.