Element, compound, mixture, pure substance — the definitions
Element: only one type of atom, not chemically breakable into simpler substances. Compound: two or more different elements chemically combined (bonded) in fixed proportions, separable only by a chemical reaction. Mixture: two or more substances not chemically combined, separable by physical methods. Pure substance: a single element or compound, nothing else mixed in. Each definition scores two marks — one for the building-block type, one for combined-or-not.
Purity from melting and boiling behaviour
Pure substance: a sharp, fixed melting point and boiling point — pure water melts at 0 degrees C and boils at 100 degrees C at atmospheric pressure. Mixture: melts and boils over a range of temperatures, with no single fixed point. This is how melting point is used as a test for purity: an impurity lowers the melting point and spreads it over a range. So salt water boils above 100 degrees C and over a range, not sharply at 100 degrees C.
Separation techniques and the Rf value
Match the method to the mixture: filtration = insoluble solid from a liquid; crystallisation = a soluble solid from its solution (evaporate, then cool); simple distillation = the solvent (e.g. pure water from salt water); fractional distillation = miscible liquids with different boiling points; paper chromatography = soluble coloured substances. Rf = spot distance divided by solvent-front distance, both from the baseline; Rf has no units, always below 1.
Drawn from real examiner reports.
Compound = different elements bonded
To score, a compound must be two or more different elements chemically combined (bonded) together — you must name elements or atoms. Answers such as "things joined together" or "two substances mixed" score zero, because "mixed" describes a mixture. Even saying a substance is "made of silicon and oxygen" is not enough without "different elements" and "chemically bonded".
Compound classification flagged Nov 2024 P1C Q2 (HCl wrongly called a mixture) and Jun 2024 P1CR Q3b (a substance of Si and O needs "different elements/atoms" AND "chemically bonded").
A diatomic molecule is an element
An element is a substance made of only one type of atom, so a diatomic molecule such as O2 or Cl2 is still an element — both atoms are the same type. A compound needs different elements bonded together. Defining an element as "made up of carbon" is not enough either; the mark scheme wants "only one type of atom".
Element definition flagged Jun 2024 P1CR Q11a — "made up of carbon" scored nothing; the answer needed "only one type of atom".
Rf written upside down
Rf must be the distance moved by the spot divided by the distance moved by the solvent front — spot on top. Writing it upside down (solvent front over spot) gives a value greater than 1, which is impossible. Both distances are measured from the baseline to the centre of the spot; measuring from the bottom of the paper, or the solvent distance from the ink spot, is wrong.
Rf errors flagged Nov 2024 P1C Q4c and Jun 2024 P1CR Q5bii — fraction written upside down, or distances not measured from the baseline to the centre of the spot.
Solvent front and pencil baseline
The solvent front (the highest line the solvent reaches) is the least-known chromatography term — do not confuse it with the baseline, an "end line", or the solute. Label the chromatogram: baseline, spots, solvent front. The baseline is drawn in pencil because pencil is insoluble and does not travel up the paper; ink would dissolve and run, spoiling the result.
Chromatography terms flagged Nov 2024 P1C Q4a and Jun 2024 P1CR Q5ai (solvent front least known); Q5aii — baseline in pencil because pencil is insoluble and does not travel up the paper.
Wrong separation method for the target
Match the technique to what you want and to solubility. To get a solid salt back from its solution use crystallisation, not simple distillation — distillation collects the water, not the salt. To recover pure water use simple distillation. Fractional distillation is only for miscible liquids with different boiling points, and filtration works only when the solid is insoluble.
Separation choice flagged Jun 2023 P1C Q1aiii (candidates wrongly picked fractional distillation or evaporation) and Q1b (must read whether the target is the solvent, the solute, or an insoluble solid).
Purity means a sharp fixed point
A pure substance melts and boils at one fixed, sharp temperature; a mixture does so over a range. A liquid boiling over a range (e.g. 101 to 104 degrees C) is not pure, even though it is near 100 degrees C. An impurity lowers the melting point and raises the boiling point and spreads the change over a range, so "boils near 100 degrees C" does not prove pure water.
Work Rf with a fixed routine
For every Rf question, measure baseline-to-centre-of-spot and baseline-to-solvent-front, then divide spot distance by solvent-front distance. Keep both in the same unit (they cancel, so Rf has no unit) and show working. A value of 1 or more means you divided upside down.
Read the target before you separate
Before naming a separation technique, decide the target and its solubility: solid or liquid? soluble or insoluble? Insoluble solid → filtration; a dissolved solid you want back → crystallisation; the pure liquid → simple distillation; miscible liquids → fractional distillation.
Give two-element definitions
For element, compound and mixture, give both required parts: the building-block count or type (one type of atom / different elements / two or more substances) AND the bonding status (chemically combined or not). A definition with only one half scores one mark at most.
Learn these in the exact two-element mark-scheme form:
| Term | Mark-scheme definition |
|---|---|
| Element | A substance made of only one type of atom |
| Compound | Two or more different elements chemically combined (bonded) in fixed proportions |
| Mixture | Two or more substances not chemically combined |
| Pure substance | A single element or a single compound, with nothing else mixed in |
Key points:
Define an element.
Classify each of the following as an element, a compound or a mixture, and give a reason for each choice.
(a) oxygen gas,
(b) carbon dioxide,
(c) clean air
