Elements, Compounds and Mixtures

Classifying boxes of dots as element, compound or mixture is among the most-asked multiple-choice formats on Papers 1 and 2, and Cambridge sets the options so that a fuzzy definition picks the wrong box. The definitions take two minutes to learn precisely. The skill of reading particle diagrams takes one practice set. Together they protect 2-4 marks per paper, and the ideas anchor the whole of atoms, elements and compounds.

The three definitions (Core)

An element is a substance that cannot be broken down into anything simpler by chemical means, because it contains only one type of atom. The Periodic Table lists every element. A compound contains two or more elements chemically combined (bonded) in fixed proportions: water is always H2O, never roughly H2O. A mixture contains two or more substances (elements or compounds) that are not chemically combined, in no fixed proportion.

“Chemically combined” is the load-bearing phrase. It is what separates a compound from a mixture, and writing “joined” or “mixed together” in its place drops the mark.

Compound versus mixture: the comparison table (Core)

	Compound	Mixture
Bonding	Elements chemically combined	Substances not chemically combined
Composition	Fixed proportions	Any proportions
Properties	Different from the elements in it	Each substance keeps its own properties
Separation	Only by chemical reaction	By physical means (filtration, distillation, chromatography)
Energy change on forming	Usually heat taken in or given out	Little or no energy change

Iron and sulfur is the set-piece example. Mix the powders: a magnet still pulls out the iron, the mixture is grey-yellow, and nothing new exists. That is a mixture. Heat the mixture: it glows, energy is released, and the product, iron(II) sulfide (FeS), is non-magnetic with properties unlike either element. A compound has formed.

Reading and drawing particle diagrams (Core)

The diagram code: one type of single circle = element (monatomic, like helium). Pairs of identical circles = element as molecules (like O2). Two different circles joined = compound (like CO or H2O). Different particles loose in the same box, not joined = mixture. A box holding O2 molecules and CO2 molecules together shows a mixture of an element and a compound, the exact discrimination Paper 2 loves.

Two details trip students. Joined circles of the same type are still an element: Cl2 is an element, not a compound. And a compound’s particles must each contain both atom types; a box of separate black circles and separate white circles is a mixture of two elements, however evenly scattered.

Mixtures matter beyond this topic: because their components keep their own properties and are not bonded, they can be separated physically, which is the entire logic of purification and separation in topic 12.

Worked exam question

Iron filings and sulfur powder are mixed. The mixture is then heated strongly and a black solid forms. (a) State one piece of evidence that the unheated mixture is not a compound. (1) (b) Name the compound formed on heating and write its formula. (2) (c) Explain why the black solid cannot be separated into iron and sulfur using a magnet. (1)

Model answer: (a) The iron can be removed with a magnet / the proportions can be anything / each substance keeps its own properties (any one, 1). (b) Iron(II) sulfide (1), FeS (1). (c) The iron and sulfur are chemically combined/bonded in the compound, so the iron atoms are no longer present as magnetic iron (1).

Mark-by-mark: (a) accepts any property-based evidence; “it looks mixed” does not score. (b) the name needs the (II) for full credit at Extended level, and the formula mark is separate. (c) must say chemically combined; “it’s a compound now” merely restates the question.

The mistakes that cost marks

1. Calling air or sea water a compound. Both are mixtures: no chemical bonds between the components, no fixed composition.
2. Treating O2 or Cl2 as compounds because “there are two atoms”. Both atoms are the same element; a compound needs different elements bonded together.
3. Writing “mixed” or “joined” instead of “chemically combined” in definitions. The mark scheme tests the phrase.
4. Saying a compound can be separated by filtering or distilling. Physical separation works only on mixtures; compounds need a chemical reaction (or electrolysis) to break them apart.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”A compound is stuff joined together"	"Two or more elements chemically combined in fixed proportions"
"A mixture is things mixed up"	"Two or more substances not chemically combined"
"You can unmix a mixture"	"The components of a mixture can be separated by physical methods"
"The compound is totally different"	"The compound has different properties from the elements it contains”

The Malaysia note

Students at Malaysian international schools meet this in the first term of the IGCSE course and treat it as too easy to revise, then meet it again in Paper 6, where identifying a substance as a mixture justifies the separation method chosen, and the definition suddenly carries 2 marks. Checkpoint-background students usually have this cold; students who joined from other curricula at Year 10 sometimes missed it entirely. Our free 1-hour trial lesson starts with a diagnostic that catches exactly these silent gaps.

Test yourself

Try all three without looking back up the page; click each answer to check.

Q1 (3 marks). Classify each of the following as an element, a compound or a mixture, giving a reason in each case: air, oxygen, carbon dioxide.

Show answer

• Air: mixture, because the gases in it are not chemically combined / it has no fixed composition [1] • Oxygen: element, because it contains only one type of atom [1] • Carbon dioxide: compound, because carbon and oxygen are chemically combined in fixed proportions [1]

Q2 (2 marks). A particle diagram shows a box containing molecules each made of one black atom joined to one white atom, together with separate molecules each made of two white atoms. State what the box represents and explain your answer.

Show answer

• A mixture [1] • Of a compound (molecules containing two different types of atom bonded together) and an element (molecules of identical atoms) [1]

Q3 (2 marks). Salt can be obtained from sea water by evaporation, but water cannot be split into hydrogen and oxygen by any physical method. Explain the difference.

Show answer

• Sea water is a mixture: its components are not chemically combined, so they can be separated by physical means [1] • Water is a compound: hydrogen and oxygen are chemically combined, so a chemical reaction (e.g. electrolysis) is needed to separate them [1]