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: 15
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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This lesson introduces three critical chemical properties of materials: reactivity, flammability, and toxicity. Understanding these properties is essential for any manufacturing engineer. When we choose a material to make an item—whether it is a cooking pot, a car part, or a water storage tank—we must consider how it will interact with its environment. Will it rust or corrode? Could it catch fire easily? Is it safe for people to use, or is it poisonous? By the end of this lesson, you will be able to analyse materials found right here in our communities and make informed decisions about their use in manufacturing.
This section breaks down the three key chemical properties. A chemical property describes how a material behaves during a chemical reaction. 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. In simple terms, it's how easily a material changes into a new substance when it comes into contact with its surroundings, like air, water, or acids.
Explanation: For metals, a key aspect of reactivity is corrosion, which is the gradual destruction of a material by chemical reaction with its environment. The most common form of corrosion is rusting, which happens to iron and steel. Reaction for Rusting: Iron + Oxygen + Water → Hydrated Iron(III) Oxide (Rust) `4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃` The Reactivity Series: Metals can be arranged in a list called the reactivity series. Metals at the top are very reactive (e.g., Potassium, Sodium), while those at the bottom are very unreactive (e.g., Gold, Platinum). High Reactivity: Potassium, Sodium, Calcium, Magnesium, Aluminium Medium Reactivity: Zinc, Iron, Lead Low Reactivity: Copper, Silver, Gold, Platinum
Ghanaian Context & Manufacturing Importance: Example 1: Roofing Sheets. In coastal areas like Takoradi or Cape Coast, the air is salty and humid. Iron roofing sheets will rust very quickly here. This is why people prefer to use aluminium or aluzinc roofing sheets, as aluminium is less reactive to salt and water (it forms a protective oxide layer). In drier northern regions like Bolgatanga, iron sheets last longer. Example 2: Cooking Pots ('Dadesen'). Most local cooking pots are made from aluminium. While aluminium is high on the reactivity series, it quickly forms a very thin, tough, and invisible layer of aluminium oxide on its surface. This layer is very unreactive and protects the aluminium underneath from reacting with food. Pure iron pots would rust and affect the taste of the food. Manufacturing Decision: A manufacturer must choose a material with the right level of reactivity for the product's intended environment. For a car exhaust pipe, you need a material that can resist heat and corrosive gases, like stainless steel. For a disposable battery casing, a more reactive metal like zinc is used intentionally. B. Flammability