0620 · 9.3Alloys and their properties

Alloys, pure metals and examples

0:00A knife needs to keep its shape while cutting food. A fork must withstand repeated use and washing. Pure metals do not always provide the best combination of properties for these jobs. In this video, we will connect the structure of alloys to their usefulness, including the stainless steel in cutlery. The important idea is that changing a material’s composition can change its behaviour. Adding another element to a metal can make it harder or stronger.

0:23Some additions also improve resistance to rusting. To understand these changes, we need to look beyond the finished object and consider its particles. In a solid metal, the particles are closely packed in a regular arrangement. We can represent this structure using circles arranged in layers. These diagrams are simplified models rather than pictures of individual atoms. For this topic, their purpose is to show particle sizes and the arrangement of the layers.

0:48A pure metal contains only one metallic element. In our simplified diagram, its atoms have the same size, so the layers are regular. Notice that the circles are touching rather than floating far apart. This represents a solid structure, not the widely separated particles of a gas. An alloy is a mixture of a metal with other elements. The word mixture matters because an alloy is not a single compound with a fixed chemical formula.

1:11Its composition can be adjusted to produce useful properties. At least one component is a metal, but the other elements need not all be metallic. Brass is an alloy made from copper and zinc. Both components are metals, so brass is an example of mixing metallic elements. Stainless steel contains iron and other elements such as chromium, nickel and carbon. Carbon is a non-metal, showing why the definition must include other elements rather than only metals.

1:37Keep these two examples distinct when naming the components. Brass contains copper and zinc, not iron. Stainless steel is based on iron, with other elements added to improve its properties. You do not need a particular percentage composition to identify either example at this level.

Properties and uses of alloys

1:53Alloys can be harder and stronger than the pure metals from which they are made. Hardness describes resistance to scratching or denting, while strength concerns how well a material withstands forces without breaking. These properties are related, but they are not interchangeable descriptions of exactly the same behaviour. The word can is important in that statement. We should not claim that every alloy is always harder and stronger than every pure metal.

2:15Different compositions give different combinations of properties. The aim is to choose a material whose behaviour suits the object being made. Brass can be used for door handles because its hardness helps it withstand everyday wear. Think about the repeated contact a handle experiences during normal use. A useful answer connects the property to that demand. Simply saying that brass is better does not explain why it suits the job.

2:39Stainless steel is used in cutlery because it is hard and resists rusting. Hardness helps the surface withstand scratching during use. Resistance to rusting is useful when knives and forks repeatedly contact water. These are two separate advantages, and both help explain the choice of material. Do not treat resistance to rusting as another word for strength. Rusting concerns chemical changes, whereas hardness and strength describe physical behaviour. The altered composition of stainless steel provides a useful combination of properties.

3:08The details of rusting belong in the video on corrosion of metals.

Why alloys are harder

3:11An alloy diagram contains atoms of different sizes within the same solid structure. Larger or smaller atoms interrupt the regular arrangement found in the pure metal model. Look for those different sizes when identifying an alloy. A drawing does not need to show the name of each element for you to recognise this feature. Different shading may help distinguish the elements, but shading itself is only a drawing convention. The structural feature to focus on is the presence of different-sized atoms.

3:37Do not assume that every particle must have a different size. An alloy can contain many atoms of the main metal and fewer of the added element. This next part is for the Extended paper. In a pure metal, the regular layers can slide over each other when a force is applied. This allows the material to change shape relatively easily. The metal does not need to melt for the layers to move.

3:59Imagine rows of identical round beads arranged neatly beside one another. Their matching sizes produce regular rows that can shift past neighbouring layers. This is a useful picture of why a pure metal can change shape. The analogy explains the arrangement, but real atoms are held together within the metallic structure. Adding atoms of different sizes disrupts those regular layers. The layers can no longer slide over each other easily, so changing the shape becomes more difficult.

4:23This explains why alloys can be harder and stronger than pure metals. The important change is structural, not simply that another substance has been added. For an exam explanation, connect the cause to the result in a clear sequence. The alloy contains atoms of different sizes. These disrupt the regular arrangement of the layers. The layers can no longer slide over each other easily, making the alloy harder and stronger.

Exam questions and recap

4:47Consider this worked example using two particle diagrams. The question asks you to identify the alloy and explain why it can be harder than the pure metal. First, compare the sizes of the circles rather than the overall size of each box. Diagram P shows identical atoms, while diagram Q contains two different sizes. Diagram Q therefore represents the alloy. Its different-sized atoms interrupt the regular rows visible in diagram P.

5:12That disruption makes it harder for neighbouring layers to slide past one another. We have now linked the evidence in the drawing to the property, rather than merely naming the correct box. A separate exam question could ask you to state the components of brass and describe why stainless steel is used in cutlery. Match each answer to the command word. A state question needs only the facts, whereas describe asks you to give the relevant properties and link them to the use.

5:35A concise answer would give each creditworthy point clearly. Brass is a mixture of copper and zinc. Stainless steel is hard, so it resists scratching during use. It also resists rusting, making it suitable for cutlery that repeatedly comes into contact with water. A common mistake is to call an alloy a compound. That misses the definition because an alloy is a mixture of a metal with other elements. Another mistake is saying that its atoms become harder.

6:00The explanation concerns how different particle sizes affect the movement of layers, not a change in the hardness of individual atoms. Avoid saying that the layers slide more easily in an alloy. That would predict easier changes of shape, which is the opposite of the explanation required here. Also, do not use disrupted layers to explain resistance to rusting. The structural model explains hardness and strength, while rust resistance is a separate property.

6:24The main connection is between composition, structure and usefulness. An alloy mixes a metal with other elements, producing a structure containing different-sized atoms. Those atoms disrupt regular layers and make sliding more difficult. This is why alloys can be harder and stronger than pure metals. Remember brass as copper mixed with zinc, and stainless steel as iron mixed with other elements such as chromium, nickel and carbon. Recognise alloys in diagrams by their different-sized atoms.

6:51Explain uses by linking a named property to the demands of the object. For stainless steel cutlery, the key properties are hardness and resistance to rusting.