Element, compound, mixture — definitions
Each needs a two-element mark-scheme definition. Element: only one type of atom, not broken down chemically (e.g. , ). Compound: two or more elements chemically combined in fixed proportions, with properties unlike its elements (, ). Mixture: two or more substances not chemically combined, variable composition, each keeping its own properties and separable by physical methods.
Compound vs mixture — four contrasts
A compound differs from a mixture in four ways. Composition: fixed ratio vs variable. Properties: a compound has new properties unlike its elements; a mixture's components keep their own. Bonding: chemically bonded vs only physically mixed. Separation: a compound needs a chemical reaction (e.g. electrolysis); a mixture only physical methods. Example: heating iron with sulfur gives the compound , so a magnet no longer picks out the iron.
Classify and find Mr from a formula
Read the formula: if every atom is the same element it is an element (, , ); two or more different symbols means a compound (, ). Mixtures have no single formula. Relative formula mass () sums the of every atom in one formula unit: , . Cambridge : H 1, C 12, O 16, Na 23, Cl 35.5.
Drawn from real examiner reports.
Elements confused with compounds
Candidates pick compounds when asked for elements, and vice versa. " is an element" is wrong — two atom types (C and O) make it a compound; "sodium is a compound" is wrong — is one atom type, an element. Rule: count the different element symbols — one only = element (even ); two or more = compound.
W22 P32 Q1; S22 P31 Q1a
Vague definitions miss the key phrase
Definitions need the key phrase, not a paraphrase. Element: "a single/pure substance" fails — say "only one type of atom". Compound: "things joined together" fails — say "elements chemically combined". Mixture: "different things mixed" fails — say "not chemically combined". Omitting either required part typically halves the marks.
S22 P31 Q1a; W22 P32 Q1
Subscript is not number of elements
Subscripts show how many atoms of one element are present, not how many different elements. , and each have a subscript above 1 but only one type of atom, so they are elements. What decides element vs compound is the number of different symbols — has C and O, so it is a compound.
Compounds have new properties
A compound's properties differ from those of its elements — don't assume they carry over. Sodium is a reactive metal and chlorine a poisonous gas, yet sodium chloride is a safe, edible white solid. In a mixture, each component keeps its own properties (iron filings stay magnetic in an iron/sulfur mixture). "New properties" is the compound marker.
(Extended) Compounds need chemical separation
Mixtures separate by physical methods because components are not bonded; a compound needs a chemical reaction (e.g. electrolysis of water into and ). "Water can be filtered into hydrogen and oxygen" is wrong — filtration is physical. A magnet cannot pull iron from , because there the iron is chemically bonded.
"Pure" means one substance, not clean
In chemistry "pure" means a single substance (one element or one compound) with no others present, shown by a sharp, fixed melting point. It does not mean "clean" or "natural" as in everyday speech ("pure orange juice"). A mixture is impure and melts over a range of temperatures, not at one sharp value.
Four-point contrast scaffold
For "compound vs mixture" differences give four contrasts: composition (fixed vs variable), properties (new vs retained), bonding (chemical vs physical) and separation (chemical vs physical method). Use exact Cambridge wording like "chemically combined" and "fixed proportions".
Classify by counting element symbols
To classify a substance fast, count the different element symbols in its formula: one symbol = element (even with a subscript, e.g. ); two or more different = compound; no fixed formula / variable composition = mixture.
Match the command word
Match the command word: "State the difference" wants 1–2 contrasts; "Describe"/"Explain" wants 2–4 full sentences. Quick check: if each component keeps its own properties it is a mixture; if the ratio is always fixed it is a compound.
Element: A substance containing only one type of atom; it cannot be broken down into simpler substances by chemical means.
Compound: A substance in which two or more elements are chemically combined in fixed proportions.
Mixture: A substance containing two or more substances not chemically combined.
Define an element.
Calculate the relative formula mass () of water, .
(: H = 1, O = 16) [2]
