Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define and give examples of physical and chemical properties.
• Compare the properties of common materials.
• Describe how a material's properties relate to its use.
• Distinguish between mixtures and pure substances.
• Explain the process of dissolving.

Lesson summary

This week, we delve deeper into the fascinating world of materials and mixtures, focusing on their specific properties and how these properties influence their uses. Understanding the properties of materials is crucial because it helps us make informed decisions about which materials to use for different purposes, from building houses and bridges to packaging food and developing new technologies. In a South African context, this knowledge is vital for selecting appropriate and sustainable building materials, understanding how to purify water safely, and even appreciating the unique properties of indigenous plants used in traditional medicine.

Lesson notes

2.1 Physical Properties vs. Chemical Properties Materials have two main types of properties: physical and chemical.

Physical Properties: These are characteristics that can be observed or measured without changing the substance into a new substance.

Examples include: Colour: The visual appearance of the material (e.g., red, blue, silver).

State of Matter: Whether the material is a solid, liquid, or gas at room temperature.

Hardness: The resistance of a material to being scratched or dented. Diamond is one of the hardest known materials.

Texture: How the material feels to the touch (e.g., smooth, rough, bumpy).

Lustre: How shiny a material is. Metals generally have high lustre.

Magnetism: Whether a material is attracted to a magnet (e.g., iron, nickel, cobalt).

Malleability: The ability of a material to be hammered into thin sheets (e.g., gold, aluminium).

Ductility: The ability of a material to be drawn into wires (e.g., copper, silver).

Conductivity: The ability of a material to conduct heat or electricity (e.g., copper is a good conductor of electricity).

Solubility: The ability of a material to dissolve in a solvent (e.g., salt is soluble in water).

Density: The mass of a substance per unit volume (Density = Mass / Volume).

Melting Point: The temperature at which a solid changes to a liquid.

Boiling Point: The temperature at which a liquid changes to a gas.

Chemical Properties: These are characteristics that describe how a substance reacts with other substances or changes into a new substance.

Examples include: Flammability: The ability of a substance to burn (e.g., wood, petrol).

Reactivity with Water: How a substance reacts when it comes into contact with water (e.g., some metals react violently with water).

Reactivity with Acids: How a substance reacts when it comes into contact with acids (e.g., some metals dissolve in acids).

Rusting: The process of iron reacting with oxygen and water to form rust (iron oxide). 2.2 Examples of Materials and their Properties | Material | Physical Properties | Chemical Properties | Common Uses | |--------------|----------------------------------------------------|---------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | Copper | Good conductor of electricity and heat, ductile, malleable, reddish-brown colour, lustrous | Relatively unreactive with water | Electrical wiring, plumbing, cooking pots | | Wood | Relatively strong, lightweight, can be carved, burns | Flammable | Furniture, building materials, fuel | | Glass | Transparent, hard, brittle, doesn't conduct electricity | Relatively unreactive with most chemicals | Windows, containers, lenses | | Plastic | Lightweight, can be moulded into different shapes, waterproof | Some plastics can be flammable or react with certain chemicals | Packaging, containers, toys, pipes | | Iron | Strong, magnetic, dense, malleable | Rusts in the presence of oxygen and water | Construction, machinery, tools | 2.3 Mixtures and Pure Substances Pure Substance: A substance made up of only one type of particle (atom or molecule). It has a fixed composition and distinct properties. Examples include pure water (H2O), pure gold (Au), and pure table salt (NaCl).

Mixture: A combination of two or more substances that are physically combined but not chemically bonded. Each substance in the mixture retains its own properties. Mixtures can be separated by physical means.

Homogeneous Mixture: A mixture where the components are evenly distributed throughout and the mixture appears uniform. Examples include saltwater (salt dissolved in water), air (a mixture of gases), and sugar dissolved in water. These are also called solutions.

Heterogeneous Mixture: A mixture where the components are not evenly distributed and the mixture appears non-uniform. You can often see the different components. Examples include sand and water, oil and water, and mielie pap with lumps. Suspensions and colloids are types of heterogeneous mixtures.

Suspension: A heterogeneous mixture where the particles are large enough to be seen and will settle out over time (e.g., muddy water).

Colloid: A heterogeneous mixture where the particles are larger than those in a solution but smaller than those in a suspension. They do not settle out, and they scatter light (Tyndall effect) (e.g., milk, fog). 2.4 Dissolving Dissolving is the process of a solute (the substance being dissolved, e.g., salt) becoming dispersed evenly throughout a solvent (the substance doing the dissolving, e.g., water) to form a solution.

Factors Affecting the Rate of Dissolving: Temperature: Increasing the temperature generally increases the rate of dissolving (for solids in liquids). Hot water dissolves sugar faster than cold water.