Lesson Notes By Weeks and Term v4 - SHS 3
Classification of Materials
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Classification of Materials
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Subject: Manufacturing Engineering
Class: SHS 3
Term: 1st Term
Week: 13
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
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Welcome, future engineers! Today, we are exploring the secret lives of the materials all around us. Why does an iron roofing sheet rust but an aluminium cooking pot does not? Why do we store kerosene away from the kitchen fire? Why must a farmer be careful when spraying chemicals on their cocoa farm? The answers lie in their chemical properties. Understanding these properties—specifically reactivity, flammability, and toxicity—is not just for passing exams. It is crucial for our safety at home, in the workshop, and for choosing the right material for any manufacturing job, from building a simple chair to constructing a bridge over the River Pra.
This lesson focuses on three key chemical properties that determine how a material behaves in different situations. A. Reactivity Definition: Reactivity is the tendency of a substance to undergo a chemical reaction, either by itself or with other materials, and release energy. Think of it as a material's "eagerness" to change into something new. A highly reactive material changes easily, while a material with low reactivity is very stable. Detailed Explanation: Reactions can happen with substances in the environment like oxygen (oxidation), water (hydrolysis), or acids. Oxidation (Rusting): The most common example we see in Ghana is the rusting of iron. When iron is exposed to oxygen and moisture (especially in humid coastal areas like Takoradi or Cape Coast), it reacts to form iron oxide, which we call rust. This rust is weak and flakes off, destroying the iron material over time. `Iron + Oxygen + Water → Hydrated Iron (III) Oxide (Rust)` Reaction with Acids: Some metals react vigorously with acids, producing hydrogen gas. For example, dropping a zinc-coated nail into a strong acid will cause fizzing. This is why certain chemicals cannot be stored in metal containers. Materials with low reactivity, like gold, plastic, and glass, are valued for their stability. This is why gold (sika) doesn't rust and can be buried for years without changing. Ghanaian Examples: High Reactivity: Calcium carbide used in a carbide lamp for lighting or ripening fruits reacts instantly and violently with water to produce flammable acetylene gas. Medium Reactivity: Iron nails or roofing sheets rusting over time. An Effervescent tablet (e.g., Vitamin C) fizzing in water. Low Reactivity: An aluminium cooking pot ("dadesen") which forms a protective, non-reactive layer of aluminium oxide on its surface, preventing further reaction. A plastic "Kuffour" gallon used for storing water. B. Flammability Definition: Flammability is a measure of how easily a material will ignite, burn, and support combustion. If something is highly flammable, it can catch fire at a very low temperature. Detailed Explanation: For a fire to occur, three things are needed: Fuel (the flammable material), Oxygen (from the air), and Heat (an ignition source). This is called the Fire Triangle. Flammable vs. Combustible: These terms are related but different. Flammable materials ignite very easily at normal temperatures (e.g., petrol, LPG, turpentine/thinner). A small spark is enough. Combustible materials need higher temperatures to ignite (e.g., wood, charcoal, paper). You need to hold a flame to them for a while before they catch fire. The state of the material matters. Sawdust (fine particles) is more flammable than a solid block of wood because more of its surface area is exposed to oxygen. Ghanaian Examples: Highly Flammable: Liquefied Petroleum Gas (LPG) for cooking, petrol for cars and generators, kerosene for lanterns, turpentine used by painters, "akpeteshie" (local gin) due to its high alcohol content. Combustible (Medium Flammability): Wood used for furniture, charcoal for cooking, dried thatch for roofing, paper, cotton fabric. Non-Flammable (Low/No Flammability): Metals, glass, stone, sand, water, clay for making earthenware pots. C. Toxicity Definition: Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. It is, simply put, how poisonous a material is to living things (humans, animals, and plants). Detailed Explanation: Toxicity can affect the body in different ways depending on the dose (how much), the duration (how long), and the route of exposure (how it gets into thebody). Routes of Exposure: Inhalation: Breathing in fumes, dust, or vapours (e.g., insecticide spray, smoke from burning plastic). Ingestion: Swallowing the substance (e.g., accidentally drinking a chemical stored in a water bottle). Absorption: The substance passing through the skin (e.g., some strong pesticides). Acute vs. Chronic Toxicity: Acute toxicity causes harm from a single, short-term exposure (e.g., feeling dizzy after inhaling strong paint fumes). Chronic toxicity causes harm over a long period of repeated, low-level exposure (e.g., health problems from long-term exposure to mercury used in 'galamsey'). Ghanaian Examples: High Toxicity: Pesticides and weedicides (e.g., "Condemn"), battery acid, mercury and cyanide used in illegal mining, carbon monoxide from a running generator in a closed room. Medium Toxicity: Lead-based paints (can be harmful if ingested, especially by children), bleach, solvents like turpentine (harmful if inhaled over time). Low/Non-Toxic: Wood (in its natural state), water, clay, common metals like iron and aluminium (in solid form).
Guided Practice (With Solutions)
Question 1: Classification Table
A small-scale manufacturer in Suame Magazine has the following materials in his workshop: a steel bar, a can of petrol, a PVC pipe, and a container of wood varnish. Copy and complete the table below by ranking each material's property as High, Medium, or Low.