Elements, compounds and mixtures

Water, air and brass all contain more than one element. Why is water a compound, while air and brass are mixtures? Look at how their components are combined, not just how many elements are present.

Define the three categories

Key ideas

• Element vs compound: an element’s atoms have the same proton number; a compound contains different elements chemically combined.
• Compound vs mixture: compound has a fixed ratio; mixture has a variable ratio.
• Separation: mixtures → physical methods; compounds → chemical methods.
• Purity evidence: a sharp melting point can support purity under controlled conditions; it does not identify an element or compound or prove purity by itself.
• Alloys are mixtures, not compounds.

Look at the particles and composition

An element can contain atoms or molecules

An element’s atoms all have the same proton number. Its isotopes can have different neutron numbers and still belong to the same element.

Some elements exist as molecules (separate, uncharged groups of chemically joined atoms), e.g. O₂, N₂, Cl₂.

Compounds can contain molecules or giant structures

A compound is chemically combined. This means new bonds form and you get a new substance with new properties.

• Some covalent compounds contain separate molecules, such as H₂O. Others, such as SiO₂, form giant covalent networks, rather than separate molecules.
• Ionic compounds exist as a giant ionic lattice. There are no “molecules of NaCl” — the simplest ratio is called a formula unit.

Compare compounds and mixtures

Why brass is a mixture

Brass is an alloy containing copper and zinc. Its composition can vary: “brass” does not name one compound with a fixed chemical formula.

Alloys are often harder than pure metals because different-sized atoms disrupt the regular layers and prevent them from sliding easily. (See Metallic Bonding.)

Example: the composition of an alloy can vary (still “brass”, but different ratios):

These illustrative compositions are percentages by mass:

Each row totals 100%. Both are brass even though their proportions differ. A manufacturer could also repeatedly make sample A’s chosen 70:30 mixture; that would not turn it into a compound.

Misconceptions to check

• Defining an element only by saying physical separation does not work. An element cannot be broken down into simpler substances by chemical methods.
• Calling ionic compounds “molecules” (e.g., “a molecule of NaCl”). Use formula unit.
• Calling alloys “compounds” because they contain two elements. Brass is a mixture.
• Using “mixture = two or more atoms mixed together”. Mixtures can contain elements and compounds (and in solutions, ions).
• Using examples without stating what they are made of (e.g., “air” with no mention that it’s a mixture of gases).

Use evidence carefully

Worked examples

Modelled example 1

Classify a material and explain why

Core

Problem

Classify each substance and justify the classification: (a) iron, (b) distilled water, (c) air, (d) brass.

Study the worked solution

1. Classify iron

   Method

   Identify the number of atom types.

   Reason

   All iron atoms have the same proton number.

   Working

   Iron is an element because it contains only iron atoms.

2. Classify distilled water

   Method

   Use its fixed chemical formula.

   Reason

   Hydrogen and oxygen are chemically combined in a fixed ratio in H₂O.

   Working

   Distilled water is a compound.

3. Classify air and brass

   Method

   Decide whether their components are chemically combined in fixed ratios.

   Reason

   Air contains physically mixed gases, while brass is an alloy with variable copper and zinc proportions.

   Working

   Air is a mixture; brass is a mixture (alloy).

Guided practice 2

Separating a Mixture (Method Selection)

About 6 min

Problem

Describe how to separate a mixture of sand and sodium chloride solution to obtain dry sand and dry salt.

Choose the methods in order

First method

FiltrationDistillationChromatography

Treat the filtrate by

Evaporation / crystallisationUsing a magnetFiltering again

Check my attempt

Hints

Hint 1: separate the insoluble solid

Sand does not dissolve, but sodium chloride is present in the solution.

Hint 2: recover the dissolved solid

After removing sand, the salt is in the liquid that passed through the filter.

View solution step by step

1. Recover the sand

   Method

   Filter the mixture, wash the residue with a little distilled water, and dry it.

   Reason

   Insoluble sand remains as residue while sodium chloride solution passes through as filtrate. Washing removes salt solution left on the sand.

   Working

   Residue → wash and dry → dry sand.

2. Recover the salt

   Method

   Evaporate some water to obtain a hot saturated solution, cool to crystallise, then filter and dry the crystals.

   Reason

   Sodium chloride is dissolved in the filtrate and is recovered by removing the solvent.

   Working

   Filtrate → concentrate and cool → filter and dry the crystals.

Common misconception 3

Does containing two elements make brass a compound?

Find and correct the mistake

Learner response

A student says: “Brass is a compound because it contains copper and zinc.” Explain what is missing from that reasoning and give the correct classification.

Test for fixed chemical combination

Correct classification

CompoundMixture (alloy)Element

Decisive reason

It contains two elementsIt is physically mixed in variable proportionsBoth components are metallic

Check my attempt

View solution step by step

1. Locate the classification error

   Method

   Reject the inference that any material containing two elements is a compound.

   Reason

   A compound requires the elements to be chemically combined in a fixed ratio.

   Working

   Two named elements alone do not establish a compound.

2. Apply the alloy evidence

   Method

   Recognise that copper and zinc proportions in brass can vary.

   Reason

   Variable composition is evidence of physical mixing rather than a fixed chemical formula.

   Working

   Brass samples can contain different copper-to-zinc ratios.

3. Write the correction

   Working

   Brass is a mixture (alloy) because copper and zinc are physically mixed and not chemically combined in a fixed ratio.

Examiner practice 4

Iron + Sulfur (Mixture vs Compound)

5 marks

Examination question

A teacher mixes iron filings and sulfur powder. (a) Classify the unheated material and state a physical method to separate its components. The teacher then heats suitable proportions until the reaction is complete. A new black solid forms, and a magnet cannot remove iron filings from it. (b) Name the product, classify it and use the observations to support your explanation. [5 marks]

Answer both stages using observations

Classification, method and evidence

View solution step by step

1. Classify the unheated material

   1 mark

   Method

   Identify physical mixing before any reaction.

   Reason

   The components retain their identities and no new substance has formed.

   Working

   Iron and sulfur initially form a mixture.

2. Select a physical method

   1 mark

   Method

   Use a magnet to remove the iron filings.

   Reason

   Iron is magnetic while sulfur is not.

   Working

   Magnet → iron removed; sulfur remains.

3. Identify the reaction product

   1 mark

   Method

   Recognise formation of iron(II) sulfide.

   Reason

   The glow and new black solid indicate a chemical reaction.

   Working

   Fe + S → FeS

4. Classify and justify the black solid

   2 marks

   Method

   Classify it as a compound and connect that claim to evidence.

   Reason

   A new substance with different properties has formed; the iron can no longer be removed with a magnet.

   Working

   The black solid is the compound iron(II) sulfide.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the initial classification, separation method, product and evidence separately.

Mark points

1. 1 mark: Classifies unheated iron and sulfur as a mixture.
2. 1 mark: Selects a magnet to remove iron.
3. 1 mark: Identifies iron(II) sulfide or FeS as the product.
4. 1 mark: Classifies the black solid as a compound.
5. 1 mark: Uses new properties or failure of magnetic separation as evidence.

Calculate my mark

Challenge 5

Using Melting Point Data

Minimal support

Evidence transfer

Two samples of the same crystalline substance are tested under the same controlled conditions. Sample A melts sharply at 801°C, its known melting point. Sample B melts from 750°C to 790°C. Neither decomposes during the test. Which result better supports purity, and what does it leave uncertain?

Interpret the shape of the melting interval

More likely pure

Substance ASample BEqually likely

Best inference for B

A compound is provenA mixture is likelyAn element is proven

Check my attempt

Hints

Hint 1: compare interval shapes

Decide which sample has one melting temperature and which has a melting range.

Hint 2: limit the identity claim

A sharp melting point supports purity under these conditions but does not prove purity or distinguish an element from a compound.

View solution step by step

1. Compare melting behaviour

   Method

   Distinguish a sharp point from a temperature range.

   Reason

   Pure substances normally melt sharply, while mixtures usually melt over a range.

   Working

   A: one temperature, 801°C; B: a 40°C range.

2. Make the supported inference

   Working

   A is more likely to be pure; B is more likely to be a mixture.

3. Limit the claim

   Method

   Avoid claiming that the result proves purity or identifies the substance as an element rather than a compound.

   Reason

   Both elements and compounds can be pure, and some mixtures can show a narrow melting interval. Use additional evidence about composition.

   Working

   Supported claim: “A is likely pure,” not “A must be an element.”

Next step

Try Mind Stretcher 1 independently.

Try these independently

Mind stretcher 1: Identify X From EvidenceExtension

Question: A sample of X has been established as a pure substance. Electrolysis decomposes X into two different elemental gases. Classify X as an element, compound or mixture, and explain which evidence decides this.

Show Answer

The sample is already known to be pure, excluding a mixture. Its decomposition into different elements by a chemical method establishes that X is a compound, rather than an element. A sharp melting point or failure of a few physical methods alone would not establish all of this.

Final: X is a compound.

Mind stretcher 2: Fixed Ratio TrapExtension

Question: A sample contains copper and oxygen. A student claims it must be a compound. Explain why this conclusion is unsafe, and what extra information would make it safe.

Show Answer

Copper and oxygen could be:

• a compound (e.g., copper(II) oxide) if they are chemically combined in a fixed ratio, or
• a mixture (e.g., copper powder mixed with copper(II) oxide) if physically mixed.

A fixed percentage in one sample is not enough: a mixture can be prepared to a chosen ratio. Establish whether there is one substance in which copper and oxygen are chemically combined, with a fixed formula, or more than one substance. Evidence could include separating the original substances physically or characterising a pure oxide and its chemical decomposition.

Practise and check