Elements, Compounds & Mixtures
Elements vs compounds vs mixtures: mark-scheme definitions, fixed ratio vs physical mixing, and common examples like alloys and separation methods.
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The core idea
On this page
Learning objectives
- describe the differences between elements, compounds and mixtures
- describe an alloy as a mixture of a metal with another element, e.g. brass; stainless steel
For classification questions, make the distinction explicit: substances in a compound are chemically combined, while substances in a mixture are physically mixed.
1. Definition
| Term | Mark-scheme definition |
|---|---|
| Element | A pure substance made of one type of atom only, which cannot be broken down into simpler substances by chemical methods. |
| Compound | A pure substance made of two or more different elements chemically combined in a fixed ratio, which can be broken down by chemical methods (e.g., electrolysis). |
| Mixture | Two or more substances (elements and/or compounds) physically mixed together, not chemically combined, with no fixed composition; components can be separated by physical methods. |
| Alloy | A mixture containing a metal and one or more other elements (e.g., brass is copper + zinc). |
Physical methods do not change a substance’s chemical identity (e.g., filtration, distillation, chromatography). See Separation Techniques.
2. Key Ideas
- Element vs compound: element = one type of atom; compound = different elements chemically combined.
- Compound vs mixture: compound has a fixed ratio; mixture has a variable ratio.
- Separation: mixtures → physical methods; compounds → chemical methods.
- Melting/boiling point: pure substance → sharp; mixture → usually a range.
- Alloys are mixtures, not compounds.
3. Detailed Explanations
- Compound: elements chemically combined in a fixed ratio.
- Mixture: substances physically mixed, no fixed composition.
- Mixtures → physical separation. Compounds → chemical decomposition.
A. Elements (Atoms and Molecules)
An atom is the smallest particle of an element that still has the chemical properties of that element.
Some elements exist as molecules (two or more atoms bonded together), e.g. O₂, N₂, Cl₂.
B. Compounds (Molecules vs Formula Units)
A compound is chemically combined. This means new bonds form and you get a new substance with new properties.
- Covalent compounds exist as molecules (e.g., water molecules are H₂O).
- Ionic compounds exist as a giant ionic lattice. There are no “molecules of NaCl” — the simplest ratio is called a formula unit.
C. Compounds vs Mixtures (Exam Comparison Table)
| Compounds | Mixtures |
|---|---|
| Pure substances, with two or more elements chemically joined together. | Not pure substances; two or more substances mixed together with no new chemical bonds. |
| Fixed composition (fixed ratio). | Variable composition (ratio can change). |
| Separated into elements only by chemical methods (e.g., electrolysis, thermal decomposition). | Separated by physical methods (e.g., filtration, distillation, chromatography, magnet). |
| Usually have sharp melting/boiling points. | Usually melt/boil over a range. |
| Properties are different from the elements that form them. | Components largely keep their own properties. |
| Example: water (H₂O), carbon dioxide (CO₂). | Example: air, brass, salt solution. |
D. Alloys (Metallic Mixtures)
An alloy is a mixture because the components are not chemically combined in a fixed ratio.
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):
Data table
| Component | Brass sample A | Brass sample B |
|---|---|---|
| Copper | 70 | 60 |
| Zinc | 30 | 40 |
4. Common Mistakes
- Saying “element cannot be broken down by physical methods”. Physical methods are irrelevant for elements; the mark is cannot be broken down 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).
5. Exam Tips
Compound: “chemically combined” + “fixed ratio”.
Mixture: “physically mixed” + “can be separated by physical methods”.
Sharp melting point/boiling point → likely a pure substance (element or compound).
Range → likely a mixture.
If the question mentions “brass”, “steel”, or “stainless steel”, write mixture (alloy) unless a fixed ratio is explicitly given (it usually is not).
6. Worked Examples
Modelled example 1
Quick Classification (Must Give a Reason)
Problem
Study the worked solution
Classify iron
Method
Identify the number of atom types.Reason
An element contains one type of atom only.Working
Iron is an element because it contains only iron atoms.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.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)
Problem
Choose the methods in order
Hints
Hint 1: separate the insoluble solid
Hint 2: recover the dissolved solid
View solution step by step
Recover the sand
Method
Filter the mixture, then wash and dry the residue.Reason
Insoluble sand remains as residue while sodium chloride solution passes through as filtrate.Working
Residue → wash and dry → dry sand.Recover the salt
Method
Evaporate water from the filtrate or crystallise, filter and dry the crystals.Reason
Sodium chloride is dissolved in the filtrate and is recovered by removing the solvent.Working
Filtrate → evaporation/crystallisation → dry salt.
Common misconception 3
Error Analysis (Fix the Bad Statement)
Learner response
Test for fixed chemical combination
View solution step by step
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.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.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)
Examination question
Answer both stages using observations
View solution step by step
Classify the unheated material
1 markMethod
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.Select a physical method
1 markMethod
Use a magnet to remove the iron filings.Reason
Iron is magnetic while sulfur is not.Working
Magnet → iron removed; sulfur remains.Identify the reaction product
1 markMethod
Recognise formation of iron(II) sulfide.Reason
The glow and new black solid indicate a chemical reaction.Working
Fe + S → FeSClassify and justify the black solid
2 marksMethod
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.
Challenge 5
Using Melting Point Data
Evidence transfer
Interpret the shape of the melting interval
Hints
Hint 1: compare interval shapes
Hint 2: limit the identity claim
View solution step by step
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.Make the supported inference
Working
A is more likely to be pure; B is more likely to be a mixture.Limit the claim
Method
Avoid calling A specifically an element or compound from melting data alone.Reason
Both elements and compounds can be pure substances; another test is needed to distinguish them.Working
Supported claim: “A is likely pure,” not “A must be an element.”
7. Mind Stretchers
Mind stretcher 1: Identify X From EvidenceExtension
Question: Substance X has a sharp melting point and cannot be separated by filtration, distillation, or a magnet. When X is electrolysed, two different gases are produced. Is X an element, compound, or mixture? Explain.
Show Answer
Electrolysis producing different substances shows X can be broken down by a chemical method. That means X is a compound, not an element. It is also not a mixture because it has a sharp melting point and cannot be separated by physical methods.
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.
To make the conclusion safe, you need evidence of a fixed composition (or a chemical test showing it can be decomposed into elements by a chemical method).
8. Quiz
Ready to check your understanding? Try the interactive quiz, then review any questions you missed.