Alloys and their properties- CIE iGCSE Chemistry Notes - New Syllabus
Alloys and their properties for iGCSE Chemistry Notes
Core Syllabus
Describe an alloy as a mixture of a metal with other elements, including:
(a) brass as a mixture of copper and zinc
(b) stainless steel as a mixture of iron and other elements such as chromium, nickel and carbonState that alloys can be harder and stronger than the pure metals and are more useful
Describe the uses of alloys in terms of their physical properties, including stainless steel in cutlery because of its hardness and resistance to rusting
Identify representations of alloys from diagrams of structure
Supplement Syllabus
- Explain in terms of structure how alloys can be harder and stronger than the pure metals because the different sized atoms in alloys mean the layers can no longer slide over each other
Alloys and their properties and uses
Alloys and their properties
- An alloy is a mixture of a metal with one or more other elements, which may be metals or non-metals.
- Alloys are made to improve the properties of pure metals, since pure metals are often too soft, too reactive, or corrode easily. By mixing metals with other elements, their strength, hardness, durability, and resistance to rusting can be enhanced.
Uses of alloys
The usefulness of alloys comes from their physical properties, which are usually superior to those of pure metals. The choice of alloy depends on the desired strength, resistance to corrosion, appearance, or other special qualities.
Examples of alloys and their uses:
Stainless Steel
- Contains iron mixed with chromium, nickel, and sometimes carbon.
- The chromium gives it strong resistance to rusting by forming a thin, protective oxide layer on the surface.
- Nickel adds strength and toughness.
- Properties – hard, strong (harder and stronger than pure iron), and highly resistant to rusting or corrosion.
- Uses – cutlery, cooking utensils, surgical instruments, construction materials, and machinery. Its resistance to rusting makes it especially useful where hygiene or contact with water is involved.
Brass
- Mixture of copper and zinc.
- Properties – harder and more durable than copper, corrosion-resistant, and attractive in appearance.
- Brass also has a bright gold-like appearance and good resistance to corrosion, making it useful in musical instruments, decorative items, plumbing fittings, and coins.
Bronze
- Mixture of copper and tin.
- Properties – strong, hard, and corrosion-resistant.
- Uses – statues, medals, ship propellers, and bearings.
Duralumin
- Mixture of aluminium, copper, and magnesium.
- Properties – lightweight but very strong.
- Uses – aircraft, vehicles, and other transport industries where both low weight and high strength are important.
Example
Brass is used to make musical instruments and decorative items. Explain why brass is preferred over pure copper.
▶️Answer/Explanation
- Brass is harder and stronger than pure copper.
- It is more durable and resistant to wear.
- The addition of zinc improves its mechanical properties while maintaining good appearance.
- This makes brass suitable for items that require both strength and aesthetics.
Structure of Pure Metals and Alloys
Why alloys are harder and stronger than pure metals
The strength and hardness of alloys compared to pure metals can be explained by the structure of the atoms in the metallic lattice.
Identifying representations of alloys from diagrams of structure
To identify alloys in structural diagrams, it is important to understand how atoms are arranged in pure metals compared to alloys. These diagrams usually show circles to represent atoms and their arrangement in layers.
Structure of Pure Metals
- Pure metals consist of only one type of atom, so all atoms are the same size.
- These atoms are arranged in neat, regular, closely packed layers forming a highly ordered lattice structure.
- Because the atoms are identical in size, the layers of atoms can slide over each other easily when a force is applied.
- This explains why pure metals are usually soft, malleable (can be hammered into sheets), and ductile (can be drawn into wires).
- In diagrams, pure metals are shown as rows of identical circles, equally spaced, forming a perfect repeating pattern, meaning they can be bent or shaped easily but are not very strong for heavy-duty applications.
Structure of Alloys
- Alloys contain a mixture of atoms of different elements. These atoms are often different in size (some larger, some smaller).
- When these differently sized atoms are mixed into the lattice, they distort the regular arrangement of layers.
- Because of this distortion, the layers of atoms can no longer slide over each other easily.
- This makes alloys much harder and stronger and less malleable compared to pure metals.
- In diagrams, alloys are represented by circles of different sizes placed irregularly within the structure, breaking the perfect pattern.
Key structural explanation:
- Pure metals → identical atoms → regular layers → layers slide easily → softer and weaker.
- Alloys → different sized atoms → distorted layers → layers cannot slide easily → harder and stronger.
How to identify from diagrams:
- If the diagram shows a perfectly regular arrangement with identical atoms → it represents a pure metal.
- If the diagram shows atoms of different sizes mixed together, breaking the uniform pattern → it represents an alloy.
Why alloys are more useful
- Greater hardness and strength – makes them suitable for construction, tools, machinery, and transport.
- Improved resistance to corrosion – some alloys (like stainless steel) resist rusting, unlike pure iron.
- Better durability – alloys last longer under stress, heat, or wear.
- Can be designed for specific purposes – for example, brass for decorative uses and instruments, or stainless steel for medical instruments and cutlery.
Examples explained by structure:
- Brass (copper + zinc) – zinc atoms are different in size from copper atoms, disrupting the regular lattice and making brass stronger than pure copper.
- Stainless steel (iron + chromium + nickel + carbon) – the mixture of differently sized atoms prevents rusting and makes it both strong and corrosion-resistant compared to pure iron.
Example
A diagram shows a metal lattice with atoms of different sizes mixed into it. Explain what this diagram represents and why the metal is harder than the pure metal.
▶️Answer/Explanation
- The diagram represents an alloy, a mixture of a metal with other elements.
- Different sized atoms distort the regular metal lattice.
- This prevents the layers of atoms from sliding over each other easily.
- The metal becomes harder and stronger than the pure metal.
Example
Given a diagram of a lattice showing iron atoms with smaller carbon atoms in between, identify the type of alloy and explain why it is stronger than pure iron.
▶️Answer/Explanation
- This is a steel alloy (iron with carbon).
- Carbon atoms fit into spaces in the iron lattice, distorting it.
- The distortion prevents layers of iron atoms from sliding, increasing hardness and strength.
- This makes steel stronger than pure iron and suitable for construction and tools.
Example
Explain why adding different sized atoms to a metal makes the resulting alloy harder and stronger than the pure metal.
▶️Answer/Explanation
- Pure metals have layers of atoms that can slide over each other easily, making them soft.
- Adding atoms of different sizes distorts the regular lattice structure.
- This distortion blocks the sliding of layers, preventing deformation.
- As a result, the alloy becomes harder and stronger than the pure metal.
Example
Why is steel stronger than pure iron in terms of atomic structure?
▶️Answer/Explanation
- Steel is an alloy of iron with carbon atoms.
- Carbon atoms occupy spaces between iron atoms, distorting the lattice.
- This prevents layers of iron atoms from sliding over each other easily.
- Hence, steel is harder and stronger than pure iron and suitable for construction, machinery, and tools.