Classifying materials and choosing metals
Key idea: Composition decides whether a material is an element, compound or mixture. Alloys are mixtures; different-sized atoms disrupt metal layers and make them harder to slide.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Compare an element, a compound and a mixture.
- Classify each sample from its particles.
- Explain the feature that decides each classification.
- Explain why the alloy’s layers are harder to slide.
- Choose the property each job needs.
- Choose from the bending and heat-transfer results.
- Explain which result decides your material choice.
- Use the melting and conductivity results to classify the material.
- Explain why brass has no single compound formula.
- Explain hardness, composition and what the diagram cannot tell you.
- Review composition and properties as different questions.
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ContinueA material can be an element, a compound or a mixture. That classification tells you about its composition; choosing it for a job requires evidence about its properties. This lesson connects those decisions, then explains why an alloy is a mixture rather than a compound.
You need the particle ideas from ionic bonding and covalent bonding: atoms can form molecules or extended lattices. If you need melting or conductivity explanations for ionic and molecular substances, use Structure and physical properties.
Identify the composition first
An element contains atoms with the same proton number. A compound contains different elements chemically combined in a fixed ratio. A mixture contains different substances whose proportions can vary; they have not reacted to form one new compound.
Swipe or scroll sideways to inspect the complete overview.
A molecule can contain only one element: O₂ is an element despite its bonded pairs. Identical H₂O molecules make a pure compound; a sample of both O₂ and H₂O molecules is a mixture. Mixture components can be separated by physical methods. An ionic compound is also a compound, but its formula describes a lattice ratio rather than a molecule.
Check your understanding 1 Classify from the particles
A and B are different elements. U contains only A₂ molecules, V only AB₂ molecules, and W both A₂ and AB₂ molecules without a reaction. Classify each sample, then explain each choice in the next task.
Show classifications
Check your understanding 2 Classify three particle samples
A and B stand for different elements. Sample U contains only A₂ molecules. Sample V contains only AB₂ molecules. Sample W contains both A₂ and AB₂ molecules, with no reaction between them. Classify each sample and explain the feature that decides each answer.
Explain on paper or use optional notes, then compare your complete reasoning with the answer.
Show answer
U is an element: every atom is A, even though its atoms are joined in pairs. V is a compound: different elements are chemically combined in a fixed 1:2 ratio in each AB₂ molecule. W is a mixture of an element and a compound; its composition can vary. A bond between atoms of the same element does not make a compound.
Metals and alloys
Metals are generally solids with high melting and boiling points, are malleable (can be shaped by hammering), and conduct heat and electricity well. These are general properties, not properties of every metal: mercury is liquid at room temperature. Choose a metal from the properties required for the job and any given measurements.
An alloy is a mixture of a metal with one or more other elements. Brass contains copper and zinc; stainless steel contains iron with other elements, including chromium. The proportions can vary, so an alloy is not represented by one fixed compound formula.
In the simple particle model of a pure metal, similar-sized atoms form regular layers that can slide past one another when a force is applied. Different-sized atoms in an alloy disrupt that regular arrangement and make sliding harder. This explains why an alloy can be harder than the pure metal. It cannot tell you every property of an alloy: use measurements for conductivity, corrosion resistance and melting point. This layer model does not require the metallic-bonding or delocalised-electron model.
Choose using the required property
A saucepan base must transfer heat to its contents; its handle must limit heat transfer to the hand. Good thermal conduction justifies a metal base; the handle needs poor thermal conduction, so “metals are strong” is not a reason to make it metal. For a shaped metal component, malleability may help manufacture; for a wear-resistant part, hardness may matter more. State the property and why it serves that purpose.
An illustrative test compares two possible materials:
| Material | Relative thermal conductivity | Resists permanent bending under the test load? |
|---|---|---|
| P | 100 | no |
| Q | 35 | yes |
Check your understanding 3 Choose for two different jobs
Use the illustrative test results above. Choose the support that must resist permanent bending, and the better heat conductor. Use the relevant measurement for each job.
Show choices
Guided practice 1
Choose a support from evidence
Problem
Using the illustrative results above, choose P or Q for a support that must resist permanent bending. Cite the decisive result. Would the same evidence identify the better heat conductor or prove that the chosen material is an alloy?
Explain on paper or use optional notes; compare choice, evidence and limits
Hints
Hint 1
Required property
The support’s requirement concerns bending, rather than heat transfer.
Show solution step by step
Choose for the stated load
Method
Choose Q.Reason
Q resists permanent bending under the test load; P does not.
Working
Answer
Required: resist bending. Evidence: Q passes that test.
Limit the conclusion
Method
P is the better heat conductor in this comparison. Neither test proves that Q is an alloy.
Reason
Thermal performance and composition are different questions from resistance to bending.
Working
Answer
Thermal conductivity: P = 100, Q = 35. Alloy identification needs composition or a suitable particle representation.
Quick check
Explain why the property fits the job. “Strong” alone does not describe heat conduction, and a bending test does not identify which elements are present.
Check your understanding 4 What the data do and do not show
A pure sample C melts at 1080 °C and conducts electricity as both a solid and a liquid. Which general class of material fits these observations, and what can you conclude about its identity and corrosion resistance?
Explain on paper or use optional notes, then compare your complete reasoning with the answer.
Show answer
C is consistent with a metal: a high melting point and conduction in both states match general metallic properties. These results do not identify a particular element or tell you whether it resists corrosion; those need further evidence.
Correct an alloy formula
A learner writes: “Brass contains copper and zinc, so it is the compound CuZn.” The presence of two elements does not make it a compound. Brass is an alloy whose proportions can vary; there is no single fixed compound formula for it. In a schematic alloy diagram, look for atoms of another element among the metal atoms.
A particle diagram can identify a mixture; it cannot by itself tell you whether the alloy resists corrosion. For a part exposed to water, compare corrosion-resistance measurements under the conditions in which the part will be used.
Check your understanding 5 Explain the alloy model’s limits
A schematic shows one pure metal with regular layers of similar-sized atoms and an alloy with different-sized atoms disrupting those layers. Explain why this alloy can be harder. A learner then claims: “That proves brass is the compound CuZn and that it resists corrosion.” Explain why neither conclusion follows from the diagram.
Explain on paper or use optional notes, then compare your complete reasoning with the answer.
Show answer
Different-sized atoms disrupt the regular layers and can make sliding harder, so the alloy can be harder than the pure metal. Brass is a mixture of copper and zinc whose proportions can vary, rather than a compound with one fixed CuZn formula. The layer model explains resistance to sliding; it does not tell you whether it resists corrosion. That needs suitable measurements under the conditions of use. Compare all three parts of your reasoning, rather than judging the drawing by appearance alone.
Common mistakes
- Calling an alloy a compound and giving it a formula such as CuZn. Its proportions can vary, so it is a mixture.
- Calling O₂ a compound because its atoms are bonded. It contains only one element.
- Choosing a material because “metals are strong” instead of naming the property the job needs.
- Assuming every property of an alloy, such as corrosion resistance, follows from the layer model. Use measured data.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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