What is the difference between elements, compounds, and mixtures in Chemistry?

In Chemistry, an element is a pure substance consisting of only one type of atom, such as oxygen or gold. A compound is a substance formed when two or more elements are chemically bonded together, like water (H2O) or carbon dioxide (CO2). A mixture, on the other hand, is a combination of two or more substances that are not chemically bonded, such as air or a salad. Understanding these distinctions is crucial for Cambridge IGCSE Chemistry students.

How can you distinguish between a compound and a mixture?

Compounds have a fixed composition and their components are chemically bonded, meaning they can only be separated by chemical reactions. For example, water is always composed of two hydrogen atoms and one oxygen atom. Mixtures, however, have variable compositions and their components can be separated by physical means, such as filtration or distillation. This distinction is important for students studying the Cambridge IGCSE Chemistry curriculum.

Why are elements considered the simplest form of matter?

Elements are considered the simplest form of matter because they consist of only one type of atom and cannot be broken down into simpler substances by chemical means. Each element is unique and is represented by a specific symbol on the periodic table. This concept is fundamental in the study of Chemistry, especially for Cambridge IGCSE students learning about the building blocks of matter.

What are some common methods to separate mixtures?

Common methods to separate mixtures include filtration, distillation, evaporation, and chromatography. Filtration is used to separate solids from liquids, distillation separates substances based on boiling points, evaporation removes a liquid to leave a solid behind, and chromatography separates components based on their movement through a medium. These techniques are essential for Cambridge IGCSE Chemistry students to understand how mixtures can be analyzed and separated.

Can a compound be broken down into its elements? If so, how?

Yes, a compound can be broken down into its constituent elements through chemical reactions. For example, electrolysis can be used to decompose water into hydrogen and oxygen gases. This process involves passing an electric current through the compound to break the chemical bonds. Understanding how compounds can be decomposed is a key part of the Cambridge IGCSE Chemistry syllabus, highlighting the difference between chemical and physical changes.

Why is "Calling a mixture a compound (or vice versa)" a common mistake in Elements, Compounds and Mixtures?

Why it happens: Both contain more than one element. How to avoid it: Compound: fixed ratio + new properties. Mixture: variable ratio + retained properties. Source: 0620/42 — every series

Why is "Calling brass a compound of copper and zinc" a common mistake in Elements, Compounds and Mixtures?

Why it happens: The two metals are 'combined'. How to avoid it: Alloys are MIXTURES — variable composition; metals can be 'unmixed' physically.

Why is "Calling air pure because it's clean" a common mistake in Elements, Compounds and Mixtures?

Why it happens: Everyday meaning of 'pure'. How to avoid it: In chemistry, pure = ONE substance only. Air is a mixture.

Why is "Saying 'a colour change always means chemical reaction'" a common mistake in Elements, Compounds and Mixtures?

Why it happens: Common heuristic that fails. How to avoid it: Look for: new substances formed, energy change, hard to reverse → chemical. Some physical changes (e.g. dissolving CuSO_4) also change colour.

Atoms, Elements and CompoundsCambridge IGCSE Chemistry (0620)

Elements, Compounds and Mixtures

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Elements, Compounds and Mixtures — Cambridge IGCSE 0620 Chemistry Extended (2026)

The three categories of pure and impure matter. How to tell them apart, separate them, and write their formulae correctly.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

1.3 — Define element, compound and mixture.
1.3 — Distinguish between an element, a compound and a mixture from a description or formula.
1.3 — Describe physical methods for separating mixtures.

Element vs compound vs mixture

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.

Examples: oxygen ( $\text{O}_{2}$ ), iron ( $\text{Fe}$ ), gold ( $\text{Au}$ ), helium ( $\text{He}$ ).
All $\sim 118$ are listed on the Periodic Table.

Compound. A pure substance made of TWO or more elements chemically combined in FIXED RATIOS (with chemical bonds).

Examples: water ( $\text{H}_{2}\text{O}$ ), sodium chloride ( $\text{NaCl}$ ), carbon dioxide ( $\text{CO}_{2}$ ), glucose ( $\text{C}_{6}\text{H}_{12}\text{O}_{6}$ ).
Properties differ from those of the constituent elements.
Composition is fixed (e.g. water is always 2:1 H to O).

Mixture. Two or more substances NOT chemically combined; can be physically separated.

Examples: air (gas mixture), salt water (NaCl + H₂O), milk, brass.
Composition can vary.
Each component keeps its own properties.

Worked qualitative.

Iron + sulfur shaken together = MIXTURE. Iron filings can still be picked out with a magnet.
Iron + sulfur HEATED to react = COMPOUND (iron sulfide, FeS). New properties; magnet no longer pulls out iron.

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.

Element: 1 atom type.
Compound: 2+ elements bonded in fixed ratio.
Mixture: 2+ substances not bonded; variable composition.
Pure = element or single compound.

Separating mixtures

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.

$R_{f}$ value (Extended). $R_{f} = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent front}}.$ Always between 0 and 1; characteristic of a substance in a given solvent.

Worked. A spot moves $4.5\,\text{cm}$ . The solvent front moves $9\,\text{cm}$ .

$R_{f} = 4.5/9 = 0.5$ .

Pick the method to match the difference.
Filtration: insoluble solid + liquid.
Distillation: liquids with different b.p.
Chromatography: coloured/dissolved components.
$R_{f}$ = spot distance / solvent distance.

How it’s examined

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, $R_{f}$ 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.

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Step-by-step worked examples — Elements, Compounds and Mixtures

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.

1Classify substances
Core• classify
Question
Classify each as element, compound or mixture: copper, water, salt water, carbon dioxide, air.
Step-by-step solution
1. Step 1
  Element: one type of atom only.
2. Step 2
  Compound: two or more elements chemically combined in fixed ratios.
3. Step 3
  Mixture: two or more substances physically mixed in any ratio.
Answer
Copper – element. Water – compound. Salt water – mixture. Carbon dioxide – compound. Air – mixture.
2Compound vs mixture properties
Extended• Adapted from 0620/42 May/Jun 2024 Q1• compound vs mixture
Question
Compare the properties of a compound and a mixture in three ways.
Step-by-step solution
1. Step 1
  Composition: compound = fixed ratios; mixture = any ratio.
2. Step 2
  Properties: compound has properties different from its elements; mixture's components keep their original properties.
3. Step 3
  Separation: compound separates only by chemical reaction; mixture by physical means.
Answer
Compounds: fixed ratio, new properties, chemical separation. Mixtures: variable ratio, retained properties, physical separation.
3Physical vs chemical change
Core• change types
Question
Classify each as physical or chemical: melting ice, burning wood, dissolving salt, rusting iron.
Step-by-step solution
1. Step 1
  Physical: no new substance, reversible.
2. Step 2
  Chemical: new substance(s), usually irreversible.
Answer
Physical: melting ice, dissolving salt. Chemical: burning wood, rusting iron.
4Why an alloy is a mixture
Extended• alloys
Question
Explain why brass (copper + zinc) is classed as a mixture, not a compound.
Step-by-step solution
1. Step 1
  The ratio of copper to zinc is variable depending on the brass grade.
2. Step 2
  No new chemical substance forms; both metals keep their atomic identity.
Answer
Variable composition + no new substance → mixture (an alloy).

Key Definitions and Keywords — Elements, Compounds and Mixtures

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.

Element
Examiner keyword
A pure substance made of only one type of atom. Cannot be broken down chemically.
Compound
Examiner keyword
Two or more elements chemically combined in FIXED proportions.
Mixture
Examiner keyword
Two or more substances physically combined in any proportion; not chemically bonded.
Pure substance
Examiner keyword
A single element or single compound — has a sharp, fixed melting and boiling point.
Alloy
Examiner keyword
A mixture of metals (or a metal with carbon, e.g. steel). Properties differ from component metals.

Common Mistakes and Misconceptions — Elements, Compounds and Mixtures

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.

✕Calling a mixture a compound (or vice versa)
0620/42 — every series
Why it happens
Both contain more than one element.
How to avoid it
Compound: fixed ratio + new properties. Mixture: variable ratio + retained properties.
✕Calling brass a compound of copper and zinc
Why it happens
The two metals are 'combined'.
How to avoid it
Alloys are MIXTURES — variable composition; metals can be 'unmixed' physically.
✕Calling air pure because it's clean
Why it happens
Everyday meaning of 'pure'.
How to avoid it
In chemistry, pure = ONE substance only. Air is a mixture.
✕Saying 'a colour change always means chemical reaction'
Why it happens
Common heuristic that fails.
How to avoid it
Look for: new substances formed, energy change, hard to reverse → chemical. Some physical changes (e.g. dissolving CuSO $_{4}$ ) also change colour.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

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Elements, Compounds and Mixtures — frequently asked questions

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Elements, Compounds and Mixtures

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Classify substances

2Compound vs mixture properties

3Physical vs chemical change

4Why an alloy is a mixture

Element

Compound

Mixture

Pure substance

Alloy

✕Calling a mixture a compound (or vice versa)

✕Calling brass a compound of copper and zinc

✕Calling air pure because it's clean

✕Saying 'a colour change always means chemical reaction'

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Elements, Compounds and Mixtures — frequently asked questions