Ch 3 · Alloys and their Properties · IGCSE Chemistry 0620
Topic 3 of 6 in Metals — IGCSE Chemistry 0620 — 38 lessons.
Introduction
Very few metals are used pure. Copper coins, steel girders, aircraft skins and cutlery are all alloys, because mixing a metal with small amounts of other elements makes it harder, stronger or more corrosion resistant. This lesson gives the Cambridge definition of an alloy, the two named examples you must know, the particle-diagram explanation of why alloys are harder, and the use of stainless steel that the syllabus specifies.
Key ideas — What an alloy is
An alloy is a mixture of a metal with other elements. The other elements are usually metals but may be non-metals such as carbon. Because an alloy is a mixture and not a compound, the proportions can be varied to tune the properties.
🧑🏫 Teacher's explanation
Imagine a box of identical marbles all the same size. If you shake the box, the marbles slide over each other very easily because they are uniform. This is like a pure metal; the atoms are arranged in neat, regular layers that can slide past one another when a force is applied, making the metal soft and easy to shape.
📖 Definitions
🔍 Worked example — Easy
Identify the components of brass and state its primary advantage over pure copper.
🔍 Worked example — Standard
A sample of stainless steel contains iron, chromium, nickel, and carbon. Why is it used for cutlery instead of pure iron?
🔍 Worked example — Exam-style
Explain, in terms of atomic structure, why an alloy is harder than a pure metal.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one difference in the properties of these two materials. [1] (b) Explain why stainless steel is harder than pure iron, referring to its structure. [3] [4 marks]
Key ideas — Alloys are harder and stronger, and how to spot one in a diagram
Alloys can be harder and stronger than the pure metals from which they are made, and are therefore more useful. Pure iron is too soft for construction; adding carbon gives steel that is strong enough for girders and rails. Pure gold is too soft for jewellery that lasts, so it is alloyed with copper or silver.
🧑🏫 Teacher's explanation
Imagine a perfectly organized stack of identical tennis balls in a crate. Because they are all the same size, if you push the top layer, they roll and slide over each other very easily. This is like a pure metal; the atoms are identical, forming neat layers that slide when force is applied, making the metal soft.
📖 Definitions
🔍 Worked example — Easy
Identify the type of structure shown in a diagram where identical circles are arranged in perfectly straight, repeating rows.
🔍 Worked example — Standard
A metal is used for making cutlery because it is hard and resists rusting. Identify a common alloy used for this purpose and name its main components.
🔍 Worked example — Exam-style
Explain why an alloy is harder than a pure metal using the concept of atomic structure.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Uses of alloys: stainless steel
The syllabus example is stainless steel in cutlery, chosen because of its hardness and its resistance to rusting. A knife blade must stay sharp under repeated use, and cutlery is washed in hot water many times, so an alloy that neither dents nor corrodes is essential. Stainless steel is also used for surgical instruments, sinks and chemical plant for the same reasons. The chromium in stainless steel forms a thin oxide layer that protects the iron beneath from oxygen and water, so the alloy does not rust like ordinary steel.
🧑🏫 Teacher's explanation
Imagine a neatly stacked crate of identical oranges. If you push the crate, the oranges slide over each other very easily because they are all the same size and shape. This is like a pure metal; the atoms are arranged in regular layers that can slide easily when force is applied, making the metal soft and malleable.
📖 Definitions
🔍 Worked example — Easy
A student states that stainless steel is used for cutlery because it is 'strong'. Using the required Cambridge format, explain why stainless steel is used for cutlery.
🔍 Worked example — Standard
Explain why an alloy is harder than a pure metal in terms of its structure.
🔍 Worked example — Exam-style
A piece of pure iron is used to make a tool. A piece of stainless steel is used to make the same tool. Which is more suitable for a heavy-duty hammer and why?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name two elements added to iron to make stainless steel. [2] (b) Explain why stainless steel is more useful for surgical instruments than pure iron. [3] [5 marks]
Worked example
Question. Pure copper is soft. Brass, an alloy of copper and zinc, is much harder. (a) State what is meant by an alloy. [1] (b) Explain, with reference to the arrangement of the atoms, why brass is harder than pure copper. [3]
🔍 Worked example — Easy
Define an alloy and identify the components of brass.
🔍 Worked example — Standard
Explain why pure iron is more ductile (easier to shape) than steel (an alloy of iron and carbon).
🔍 Worked example — Exam-style
Stainless steel is used for cutlery. Identify the main metal in stainless steel and explain why it is preferred over pure iron for this use.
⚠️ Common errors
Common mistakes
Practice
Answers Copper and zinc [B1]. Any three of chromium, nickel, carbon (and accept manganese, molybdenum) [B2, one mark for two correct]. Stainless steel is hard, so the cutlery does not bend or blunt [B1]; it resists rusting, whereas iron would rust when washed [B1]. In pure aluminium the atoms are all the same size and arranged in regular layers [B1]; these layers slide over each other when a force is applied, so the metal bends [B1]; in the alloy, atoms of a different size disrupt the layers so they cannot slide easily, making the alloy harder and stronger [B1]. Pure metal: atoms all the same size arranged in regular rows or layers [B1]; alloy: atoms of two (or more) different sizes in the lattice, with the regular arrangement distorted around the larger or smaller atoms [B1].