The shape of the lesson
By the end of the lesson, learners can
- What an alloy is: Mix in atoms of a different size in the simulator and try to slide the layers.
- Why alloys are harder: Extended.
- Alloy: a mixture of a metal with other elements.
- Brass: copper + zinc. Stainless steel: iron + Cr, Ni, C.
- Harder and stronger than the pure metal.
- Extended: different-sized atoms stop layers sliding.
One picture every idea returns to
An alloy is a mixture of a metal with other elements. Examples: brass is copper and zinc; stainless steel is iron with other elements such as chromium, nickel and carbon. Alloys can be harder and stronger than the pure metals and are used for that reason: stainless steel for cutlery because it is hard and resists rusting. Extended: in an alloy, atoms of different sizes disrupt the regular layers, so the layers cannot slide over each other as easily.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Better metals by mixing.. Pure iron is soft and rusts; add chromium, nickel and carbon and it becomes stainless steel for cutlery and surgical tools. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: What an alloy is | Mix in atoms of a different size in the simulator and try to slide the layers. Project tab 6 of the lab (States of matter lab, metals and alloys) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why alloys are harder | Extended. | Jot Thoughts |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why alloys are harder, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jot Thoughts
Teams write ideas on slips, one idea per slip, as many as they can in two minutes, saying each aloud as they place it in the middle. Then they sort the slips, which is where the structure of the topic appears.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| An alloy is a mixture, not a compound: the elements are not chemically combined and there is no fixed formula. | And the added element need not be a metal: steel contains carbon, a non-metal. |
Alloys and their properties: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Define the term alloy.
Name the elements in (a) brass and (b) stainless steel.
Give one use of stainless steel and explain why it is suitable.
(Extended) Explain, using ideas about particles, why brass is harder than pure copper.
Explain why an alloy is classified as a mixture and not a compound.
Pure iron is soft and rusts easily. Suggest two reasons why steel, not pure iron, is used for building bridges.
Answer key and marking notes
A mixture of a metal with other elements (metals or non-metals).
- (a) copper and zinc.
- (b) iron, with chromium, nickel and carbon.
- Cutlery (or sinks or surgical instruments).
- It is hard and resistant to rusting (corrosion).
- In pure copper all the atoms are the same size and are arranged in regular layers that can slide over each other.
- In brass, zinc atoms of a different size distort the layers.
- The layers cannot slide as easily, so brass is harder.
- The elements in an alloy are not chemically combined.
- The composition can vary; there is no fixed ratio or formula.
- Steel is harder and stronger than pure iron.
- Some steels resist corrosion better (and steel can be protected more effectively).
7 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement about brass is correct?
- Aits composition can vary, because it is a mixture of copper and zinc
- Bit is a compound with the formula CuZn
- Cit is a compound of copper and tin
- Dit is softer than pure copper
Car body panels are pressed into shape from sheets of mild steel, an alloy of iron with a little carbon. Which pair of properties makes mild steel suitable?
- Astrong and malleable
- Bbrittle and hard
- Clow density and a good electrical conductor
- Dresistant to rusting without any coating, and ductile
A student measures the relative hardness of three samples.
| Sample | Relative hardness |
|---|---|
| pure copper | 50 |
| pure zinc | 30 |
| brass (copper and zinc) | 110 |
- Athe alloy is harder than either of the pure metals it is made from
- Bbrass is hard because zinc is harder than copper
- Cmixing two metals always gives a softer material
- Dbrass is hard because it is a compound of copper and zinc
Surgical instruments are cleaned many times in hot steam. Why are they made from stainless steel rather than mild steel?
- Astainless steel is hard and resists rusting
- Bstainless steel is a pure element
- Cmild steel is harder than stainless steel
- Dstainless steel is a better conductor of electricity
Pure gold is easily hammered into very thin sheets. Jewellery gold contains copper and silver mixed with the gold and is much harder to bend. Which explanation is correct?
- Acopper and silver atoms have a different size and distort the layers of gold atoms, so the layers cannot slide as easily
- Bcopper and silver form covalent bonds with the gold atoms
- Cthe alloy has no delocalised electrons, so its bonding is stronger
- Dgold atoms become smaller when copper and silver are added
Steel is made by adding a small amount of carbon to iron. What happens to the structure of the iron?
- Acarbon atoms of a different size disrupt the regular layers of iron atoms
- Bcarbon reacts with iron to form a compound with a fixed formula
- Cthe iron atoms all become larger
- Dthe metallic bonding is replaced by ionic bonding
A student explains: "Alloys are harder than pure metals because adding a second element makes the bonds between atoms much stronger." Which statement best corrects this?
- Athe hardness comes mainly from the irregular arrangement of atoms, which stops the layers sliding, not from a new, stronger type of bond
- Balloys are harder because they contain ionic bonds
- Calloys are harder because all their atoms are exactly the same size
- Dalloys are not harder; they are always softer than the pure metal
Quiz key
Brass is an alloy of copper and zinc. Like all alloys it is a mixture: the metals are not chemically combined, so the proportions can vary from one sample to another. A compound would have a fixed formula, so describing brass as CuZn is wrong. Copper with tin is bronze, not brass. Brass is harder than pure copper, which is one reason it is used.
Pressing sheets into shape needs a malleable material, and the finished body must be strong. Mild steel has both properties. A brittle material would crack when pressed. Mild steel is dense (about 7.9 g/cm3) and its conductivity is not relevant to a car body. Mild steel does rust, which is why car bodies are painted; it is stainless steel that resists rusting.
Brass (110) is harder than both copper (50) and zinc (30), so the alloy is harder than either metal on its own. Zinc is actually softer than copper in the table, so it cannot be the reason. The data show the opposite of a softer material. Brass is a mixture, not a compound, so that explanation is wrong even though brass is hard.
Steam provides water and air is present, so mild steel would rust. Stainless steel contains chromium and nickel, which make it resistant to rusting, and it is hard enough to keep a sharp edge. Stainless steel is an alloy, a mixture, not a pure element. Hardness is not the reason mild steel is rejected, and electrical conductivity does not matter for a surgical instrument.
In pure gold all the atoms are the same size and form regular layers that slide over each other, so gold is very malleable. Copper and silver atoms are a different size, so they disrupt the layers and stop them sliding easily, making the alloy harder. The bonding in an alloy is still metallic, not covalent, and the delocalised electrons are still present. The size of a gold atom does not change when other metals are added.
Carbon atoms are much smaller than iron atoms. They sit among the iron atoms and distort the regular layers, so the layers cannot slide as easily and steel is harder and stronger than pure iron. Steel is a mixture, not a compound, so there is no fixed formula. The iron atoms do not change size, and the bonding remains metallic.
When a metal is bent, layers of atoms slide over each other. In an alloy, atoms of different sizes make the layers irregular, so sliding is much harder and the alloy is harder. The bonding is still metallic, so there are no ionic bonds. If all the atoms were the same size the layers would be regular and slide easily, as in a pure metal. Alloys are usually harder, not softer, than the pure metal.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.