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Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.
The three categories of pure and impure matter. How to tell them apart, separate them, and write their formulae correctly.
Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).
Element: 1 atom type. Compound: chemically bonded mix. Mixture: physically mixed.
Element. A pure substance made of ONE type of atom. Cannot be broken down into simpler substances by chemical means.
Compound. A pure substance made of TWO or more elements chemically combined in FIXED RATIOS (with chemical bonds).
Mixture. Two or more substances NOT chemically combined; can be physically separated.
Worked qualitative.
Cambridge tip. "Pure" in chemistry has a specific meaning: an element OR a single compound. So distilled water is "pure", but tap water (which contains dissolved minerals) is a MIXTURE.
Use a physical method that exploits a difference between the components.
Choosing the method depends on what's different.
| Difference | Method | Example |
|---|---|---|
| Solid in liquid (insoluble) | Filtration | Sand from sand-water |
| Soluble solid in liquid | Evaporation / crystallisation | Salt from sea water |
| Liquids with different b.p. | Simple distillation | Pure water from sea water |
| Liquid mix with close b.p. | Fractional distillation | Crude oil components |
| Coloured solutes in solution | Chromatography | Inks, leaf pigments |
| Magnetic from non-magnetic | Magnetic separation | Iron from sand-iron mix |
| Sublimable from solid | Sublimation | Iodine from sand-iodine |
Filtration. Pour the mixture through filter paper in a funnel. Insoluble solid stays as the residue; the liquid passes through as the filtrate.
Crystallisation. Heat the solution gently to evaporate some water until saturated. Cool slowly. Pure crystals form.
Distillation. Boil the mixture; the more volatile component evaporates first. Cool the vapour in a condenser → pure liquid drips out.
Chromatography. Spot the mixture on filter paper, dip the bottom in solvent. Solvent rises by capillary action, carrying components at different rates. Components separate into spots.
Rf value (Extended). Rf=distance moved by solvent frontdistance moved by spot. Always between 0 and 1; characteristic of a substance in a given solvent.
Worked. A spot moves 4.5cm. The solvent front moves 9cm.
Verbatim phrases and definitions Cambridge mark schemes credit.
Element/compound/mixture distinction appears every Paper 2 (3-4 marks: classify a list, describe a separation method) and Paper 4 (5-7 marks: a multi-step separation, Rf calculation). Examiner reports flag confusing 'compound' with 'mixture' for everyday substances like water.
Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (1.3); 0620/22 May/Jun 2024 — Q3 (separation, $R_{f}$); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.
Step-by-step solutions to past-paper-style questions on elements, compounds and mixtures, written exactly the way a tutor would explain them at the board.
Question
Classify each of the following as an element, a compound or a mixture: copper, water, air, sodium chloride, sea water.
Step-by-step solution
Step 1
Element = made of only ONE type of atom. Copper is a single type of atom, so it is an element.
Step 2
Compound = two or more elements CHEMICALLY combined in fixed proportions. Water (H2O) and sodium chloride (NaCl) are compounds.
Step 3
Mixture = two or more substances NOT chemically combined, in any proportion. Air (a mixture of gases) and sea water (water + dissolved salts) are mixtures.
Answer
Copper – element. Water – compound. Air – mixture. Sodium chloride – compound. Sea water – mixture.
Examiner tip
A formula like H2O or NaCl that joins different element symbols signals a compound; a list of separate substances signals a mixture.
Question
Give a precise definition of an element, a compound and a mixture.
Step-by-step solution
Step 1
Element: a substance made of only one type of atom that cannot be broken down into anything simpler by chemical means.
Step 2
Compound: two or more different elements chemically combined (bonded) together in a FIXED proportion.
Step 3
Mixture: two or more substances (elements and/or compounds) that are physically mixed but NOT chemically combined, so they can be in any proportion and separated by physical means.
Answer
Element – one type of atom, cannot be chemically broken down. Compound – elements chemically combined in fixed proportions. Mixture – substances not chemically combined, variable proportions, separable physically.
Examiner tip
The two key contrasts examiners reward: 'chemically combined' vs 'not chemically combined', and 'fixed' vs 'variable' proportions.
Question
A mixture of iron filings and sulfur powder is made WITHOUT heating. State two ways you could separate the iron from the sulfur, and explain why this is possible.
Step-by-step solution
Step 1
In an unheated mixture the iron and sulfur are NOT chemically combined, so each element keeps its own properties.
Step 2
Method 1 – a magnet: iron is magnetic, sulfur is not, so a magnet pulls out the iron filings.
Step 3
Method 2 – add water / carbon disulfide: sulfur is insoluble in water (or dissolves in carbon disulfide) while iron does not, so they can be separated; alternatively the iron filings can simply be picked out because they have not reacted.
Answer
Use a magnet to attract the magnetic iron away from the non-magnetic sulfur, or use a solvent (sulfur dissolves in carbon disulfide; iron does not). The components keep their properties because they are not chemically combined.
Examiner tip
The mark hinges on linking the separation method to a retained PHYSICAL property (iron is magnetic) — proof that no chemical reaction has occurred.
Question
The iron + sulfur mixture is now heated strongly until it glows. Write the equation for the reaction and explain why the iron can no longer be removed with a magnet.
Step-by-step solution
Step 1
Heating supplies energy for a chemical reaction. Iron reacts with sulfur to form the compound iron(II) sulfide.
Fe+S→FeS
Step 2
In iron(II) sulfide the iron and sulfur atoms are now CHEMICALLY COMBINED in fixed proportions (1:1).
Step 3
The new compound has different properties from the elements: it is a grey-black solid and is NOT magnetic, so a magnet cannot pull the iron out.
Answer
Fe+S→FeS. Once heated, the iron is chemically combined in iron(II) sulfide, a new compound with new properties — it is no longer magnetic, so a magnet cannot separate the iron.
Examiner tip
Examiners want the contrast: BEFORE heating = a mixture (magnetic, separable); AFTER heating = a compound (non-magnetic, new properties, fixed ratio).
Question
Two elements X and Y are tested. X is shiny, conducts electricity, is malleable, and its oxide is basic. Y is a dull solid, does not conduct, is brittle, and its oxide is acidic. Identify which is the metal and justify each piece of evidence.
Step-by-step solution
Step 1
Physical properties: metals are shiny, good conductors of heat and electricity, malleable and ductile. X matches all of these.
Step 2
Physical properties of Y: non-metals are typically dull, brittle (when solid) and do not conduct electricity. Y matches these.
Step 3
Chemical property — the oxide: metal oxides are basic (many neutralise acids), whereas non-metal oxides are acidic. X's basic oxide confirms a metal; Y's acidic oxide confirms a non-metal.
Answer
X is the metal (shiny, conducts, malleable, basic oxide); Y is the non-metal (dull, brittle, non-conducting, acidic oxide).
Examiner tip
The oxide test is the strongest discriminator: basic oxide → metal, acidic oxide → non-metal. State the property AND the conclusion for each mark.
Question
Four boxes of particles are shown. Box A: identical grey spheres. Box B: grey and white spheres always joined in pairs. Box C: separate grey spheres and separate white spheres, not joined. Box D: joined grey–white pairs mixed with single grey spheres. Classify each box.
Step-by-step solution
Step 1
Box A – only one type of atom, all the same → an ELEMENT.
Step 2
Box B – two different atoms chemically joined in identical units → a COMPOUND.
Step 3
Box C – two different elements present but NOT joined → a MIXTURE of two elements.
Step 4
Box D – a compound (the joined pairs) together with an unbonded element (single atoms) → a MIXTURE of a compound and an element.
Answer
A – element; B – compound; C – mixture of two elements; D – mixture of a compound and an element.
Examiner tip
Look for whether different atoms are JOINED (compound) or merely sitting together unbonded (mixture). 'One type of atom only' = element.
High-scoring sample answers for elements, compounds and mixtures on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Define the term element. (1 mark)
Model answer
An element is a substance made of only one type of atom, which cannot be broken down into anything simpler by chemical means.
Why this scores
One mark for 'one type of atom' (or 'cannot be chemically broken down'). Stating both is safest.
Classify air and water, stating whether each is an element, a compound or a mixture. (2 marks)
Model answer
Air is a mixture — it contains several different gases (mainly nitrogen and oxygen) that are not chemically combined and are present in variable proportions. Water is a compound — it is hydrogen and oxygen chemically combined in a fixed ratio (H2O).
Why this scores
One mark each. Air = mixture and water = compound; a brief reason (combined/not combined) secures the marks if asked to justify.
State three differences between a compound and a mixture. (3 marks)
Model answer
First, in a compound the elements are chemically combined (bonded), whereas in a mixture the substances are not chemically combined. Second, a compound has a fixed composition (the elements are in fixed proportions), while a mixture can have any (variable) proportions. Third, a compound has new properties that differ from its component elements, whereas the substances in a mixture keep their own properties and can be separated by physical means.
Why this scores
Three clear contrasts: chemically combined vs not; fixed vs variable proportions; new properties vs retained properties (separable physically).
A student mixes iron filings with sulfur powder without heating. Describe how the iron could be separated from this mixture and explain why this method works. (4 marks)
Model answer
The mixture should be spread out and a magnet moved over it (or through it). The iron filings are magnetic and are attracted to the magnet, so they are pulled out, while the sulfur is not magnetic and is left behind. This separation works because the iron and sulfur are only physically mixed, not chemically combined, so each substance still has its own properties — in particular the iron keeps its magnetism. No chemical reaction has taken place, so the elements can be separated by this physical method.
Why this scores
Marks for: use a magnet; iron is magnetic / attracted; sulfur not magnetic / left behind; reason — substances keep own properties because not chemically combined.
The iron and sulfur mixture is heated strongly. Write a word and symbol equation for the reaction, and explain why the product can no longer be separated with a magnet. (5 marks)
Model answer
On heating, a chemical reaction occurs: iron + sulfur → iron(II) sulfide, which in symbols is Fe+S→FeS. The product, iron(II) sulfide, is a new compound in which the iron and sulfur atoms are now chemically combined in a fixed 1:1 ratio. Because the iron is now bonded inside a compound, it no longer has its own properties — iron(II) sulfide is a grey-black solid that is not magnetic. A magnet therefore cannot attract it, so the iron cannot be separated out. This contrasts with the unheated mixture, where the iron was still magnetic and separable.
Why this scores
Five marks: correct word equation; correct balanced symbol equation Fe+S→FeS; a new compound has formed; iron now chemically combined / fixed ratio; product is not magnetic so a magnet won't work.
Describe how the physical and chemical properties of metals and non-metals can be used to distinguish them from one another. (6 marks)
Model answer
Physical properties: Metals are usually shiny (lustrous), whereas non-metals are often dull. Metals are good conductors of heat and electricity, while most non-metals are poor conductors (insulators) — graphite being a notable exception. Metals are malleable and ductile (they can be hammered into shape and drawn into wires), whereas non-metals that are solid tend to be brittle. Metals also generally have high melting and boiling points and high density, while many non-metals have low melting points or are gases. Chemical properties: the most reliable test is the oxide — metal oxides are basic (many react with acids to neutralise them), whereas non-metal oxides are acidic. In addition, metals tend to lose electrons to form positive ions, while non-metals tend to gain electrons to form negative ions. By combining these physical observations with the acid–base nature of the oxide, an element can be confidently classified as a metal or a non-metal.
Why this scores
Up to 6 marks across distinct points: lustrous vs dull; conductor vs insulator; malleable/ductile vs brittle; high vs low melting point/density; basic vs acidic oxide; forms positive vs negative ions. The oxide test is the key chemical discriminator.
Definitions to memorise and the exact keywords mark schemes credit for elements, compounds and mixtures answers — sharpened from recent examiner reports for the 2026 0620 sitting.
A substance made of only ONE type of atom; it cannot be broken down into anything simpler by chemical means.
Two or more different elements CHEMICALLY combined (bonded) in FIXED proportions, e.g. iron(II) sulfide FeS.
Two or more substances NOT chemically combined, in any (variable) proportion; the components keep their own properties and can be separated by physical means.
An element that is typically shiny, a good conductor, malleable and ductile, and forms a BASIC oxide and positive ions.
An element that is typically dull, a poor conductor and brittle (if solid), and forms an ACIDIC oxide and negative ions.
The traps other students keep falling into on elements, compounds and mixtures questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.
0620/42 — recurring
Why it happens
Both contain more than one element.
How to avoid it
Ask: are the elements chemically COMBINED? Compound = chemically combined + fixed ratio + new properties. Mixture = not combined + variable ratio + retained properties.
0620 Examiner Reports — iron/sulfur question
Why it happens
Not realising heating causes a chemical reaction.
How to avoid it
After heating, Fe+S→FeS — a new compound forms that is NOT magnetic, so a magnet no longer works.
Why it happens
Everyday meaning of 'pure'.
How to avoid it
In chemistry, pure = ONE substance only. Air contains several gases not chemically combined, so it is a mixture.
Why it happens
Shininess is the most obvious property.
How to avoid it
Use the oxide as the clincher: basic oxide → metal, acidic oxide → non-metal. Combine physical and chemical evidence.
The things students keep getting wrong in this sub-topic, answered.