Element, compound, mixture defined
An element is made of only ONE type of atom (cannot be broken down chemically). A compound is two or more elements chemically combined in fixed proportions — separating it needs a chemical reaction. A mixture is two or more substances NOT chemically combined; its components keep their own properties and separate physically. Examples: is an element; water is a compound; air is a mixture of gases.
Pure = sharp m.p./b.p.; mixture = range
A pure substance is a single element or compound with a fixed (sharp) melting and boiling point. A mixture melts or boils over a range of temperatures. This tests purity: a sample melting sharply at one temperature is pure; one melting over a range is impure. Pure ice melts sharply at , but salty water melts below and over a range. In chemistry "pure" means a single substance, not "natural".
Match separation technique to property
Match technique to property. Filtration separates an insoluble solid from a liquid (solid = residue, liquid = filtrate). Crystallisation gets a soluble solid from solution: evaporate to saturate, then cool to form crystals (not to dryness). Simple distillation gets a solvent from a dissolved solid. Fractional distillation separates miscible liquids by boiling point. Chromatography separates soluble coloured substances.
Drawn from real examiner reports.
Compound vs mixture: chemically combined
The most-tested definition trap here. A compound is two or more elements chemically combined (fixed proportions) — "joined" or "mixed" do not score. A mixture is two or more substances NOT chemically combined — "different things together" is too vague. By the bond: a compound needs a reaction to separate, a mixture separates physically.
June 2024 Paper 1CR examiner report: the report repeatedly notes lost marks on standard definitions and advises candidates to "learn the standard definitions as they are often tested" — vague wording that omits "chemically combined" is the recurring cause.
Insoluble solid: keep the residue
To obtain a dry insoluble solid from a soluble-salt solution: filter (the solid is the residue), wash the residue with distilled water, then dry it. A common error is to evaporate the filtrate and crystallise — but that gives the SOLUBLE salt, not the insoluble solid wanted. Forgetting to wash the residue also loses a mark. Check: residue or filtrate?
June 2024 Paper 2C examiner report (separating insoluble lead(II) bromide from a soluble salt): "a significant number of candidates thought that they needed to heat the mixture to evaporate some of the liquid... on cooling crystals will form. This limited them to a maximum of one mark"; others lost the second mark "for not washing the residue".
Solvent front, not end line
Chromatography labelling is examined precisely. The line the solvent reaches at the end is the solvent front — not the "end line". The starting pencil line is the base line (origin). The mobile liquid is the solvent, not the "solute". The solvent front is one of the least well-answered terms, so learn each one exactly.
June 2024 Paper 1CR examiner report: "solvent front was the least well known answer... end line is not the solvent front"; candidates also wrote "solute" in place of "solvent".
Rf = spot ÷ solvent (0 to 1)
The ratio is written upside down surprisingly often. The correct formula is . Writing solvent ÷ spot gives a value greater than 1, which is impossible — every value lies between 0 and 1. Always show your working and round sensibly (usually to 2 decimal places).
June 2024 Paper 1CR examiner report: "some lost the second mark for not rounding correctly or the Rf values were upside down".
Simple vs fractional distillation
Simple distillation separates a liquid from a dissolved solid (pure water from salt water) — collect the condensed liquid, not the residue. Fractional distillation separates two or more miscible liquids by their different boiling points. Match the method to whether a dissolved solid or a second liquid is present — the wrong choice loses marks.
(common Edexcel chemistry error)
Do not evaporate crystals to dryness
When crystallising a soluble salt from solution, heat to evaporate some solvent to a saturated solution, then leave to cool so crystals form; filter off and pat dry. Do NOT evaporate to dryness — that traps impurities and gives poor crystals. Test for saturation by dipping a cold glass rod: crystals forming on it show the solution is ready to be left to cool.
(common Edexcel chemistry error)
Describe the process, not the name
For "describe a separation", give ordered steps. Fractional distillation of ethanol/water: heat it; ethanol (b.p. 78 °C) evaporates first, rises up the column and condenses; water (100 °C) stays behind. Give four linked points — "splits into fractions" scores nothing.
Rf: measure both from the base line
In an calculation, measure both distances from the same base line. Put spot on top, solvent front on the bottom, and show your substitution. Check the answer is between 0 and 1 — above 1 means the ratio is upside down. Round as asked (usually 2 d.p.).
Identify what you are separating first
Decide what you want before choosing a technique: insoluble solid → filter (keep the residue); soluble solid → crystallise; solvent from a dissolved solid → simple distillation; two liquids → fractional distillation; dyes → chromatography. Then check: residue or filtrate?
| Term | Mark-scheme definition (two elements) |
|---|---|
| Element | A substance made of only ONE type of atom |
| Compound | Two or more elements chemically combined in fixed proportions |
| Mixture | Two or more substances NOT chemically combined |
| Pure substance | A single element or compound, with a fixed (sharp) melting and boiling point |
| Mixture (purity) | Melts or boils over a range of temperatures (no fixed m.p./b.p.) |
A compound's elements can only be separated by a chemical reaction; a mixture's components keep their own properties and are separated by physical methods. Testing purity: a pure substance melts sharply at one temperature; an impure substance melts over a range and usually at a lower temperature than the pure substance.
Define an element.
Classify each of the following as an element, a compound or a mixture, and give a reason for each:
(a) nitrogen gas, (b) carbon dioxide, (c) sea water
