Lesson Notes By Weeks and Term v4 - SHS 3
Classification of Materials
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Classification of Materials
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Manufacturing Engineering
Class: SHS 3
Term: 1st Term
Week: 14
Grade code: 2.1.1.LI.2
Strand code: 1
Sub-strand code: 1
Content standard code: 2.1.1.CS.1
Indicator code: 2.1.1.LI.2
Theme: Manufacturing Materials and Technologies
Subtheme: Classification of Materials
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Welcome, future engineers! Today, we are exploring the secret lives of the materials all around us. Why does an iron roofing sheet rust in Takoradi but not in Tamale as quickly? Why do we keep kerosene away from fire? Why are some plastics not safe for storing hot kenkey? The answers lie in their chemical properties. Understanding these properties—specifically reactivity, flammability, and toxicity—is crucial for any engineer. It helps us choose the right material for the right job, ensure safety in our workshops and homes, and protect our environment. From the artisan welding a gate in Suame Magazine to the engineer designing a food processing plant in Nsawam, this knowledge is fundamental.
Chemical properties describe how a material behaves during a chemical reaction. Unlike physical properties (like colour or density) which you can observe without changing the material's substance, chemical properties can only be observed when the material undergoes a chemical change. We will focus on three critical ones for manufacturing. A. Reactivity
Definition: Reactivity is the tendency of a substance to undergo a chemical reaction, either by itself or with other materials, and to release energy. A highly reactive material will readily change into a new substance.
Explanation: Think of reactivity as how "willing" a material is to change. In Ghana, the most common example of reactivity we see is corrosion, specifically the rusting of iron. High Reactivity: Materials that react quickly with their environment (e.g., with air, water, or acids). Example 1: Rusting of Iron. Iron reacts with oxygen and water (moisture in the air) to form iron oxide, which we call rust. This is a huge problem for iron gates, roofing sheets, and car bodies, especially in humid coastal areas like Axim and Cape Coast. The salt in the sea air speeds up this reaction. Example 2: Sodium. A pure piece of sodium metal is so reactive it will burst into flames if it touches water. This is why it is not used as a common structural material. Low Reactivity (or Inert): Materials that resist chemical change. They are stable and durable. Example 1: Gold. Gold is highly valued for jewellery because it has very low reactivity. It does not rust or tarnish, even after many years. Example 2: Aluminium. Aluminium is actually a reactive metal. So why do we use it for cooking pots ('adwia') and roofing sheets? Because when it reacts with oxygen in the air, it instantly forms a very thin, tough, and transparent layer of aluminium oxide. This layer acts like a protective "skin," preventing any further reaction. This is called passivation. Example 3: Plastics (like PVC). Polyvinyl chloride pipes are used for plumbing because they do not react with water, unlike iron pipes which can rust over time.
In Manufacturing: We choose materials with low reactivity for products that need to last a long time or will be exposed to the elements (e.g., Aluzinc for roofing, stainless steel for kitchen sinks, PVC for water pipes). B. Flammability