Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define matter, material, property, and mixture.
• Classify materials based on their properties.
• Distinguish between natural and man-made materials.
• Identify homogeneous and heterogeneous mixtures.
• Describe how material properties contribute to a mixture's properties.

Lesson summary

This week, we begin our exploration of the fascinating world of materials and mixtures. Understanding the properties of materials and how they interact is crucial in our daily lives. From the clothes we wear to the food we eat, and the houses we live in, everything is made of different materials with unique properties. In South Africa, knowing about materials is especially important for industries like mining (understanding the properties of different ores), agriculture (understanding soil composition), and construction (choosing the right materials for building safe and durable structures).

Lesson notes

2.1 What is Matter? Matter is anything that has mass and takes up space (volume). Everything around us – the air we breathe, the water we drink, the desks we sit at, and even ourselves – is made of matter.

Matter exists in different states: solid, liquid, and gas. 2.2 Materials: A material is a specific kind of matter that is used for a particular purpose. Think of the different materials used to build a house: bricks, cement, wood, glass, and metal. Each material is chosen because of its specific properties. 2.3 Properties: Properties are the characteristics that describe a material. These properties can be observed (seen, felt, smelled, tasted, or heard) or measured.

Some common properties include: Colour: The appearance of a material due to the way it reflects or emits light (e.g., gold is yellow, coal is black).

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

Hardness: A material's resistance to being scratched or dented (e.g., a diamond is very hard, talc is very soft).

Flexibility: A material's ability to bend without breaking (e.g., a rubber band is flexible, a ceramic plate is not).

Magnetism: A material's ability to be attracted to a magnet (e.g., iron is magnetic, wood is not).

Transparency: The extent to which light can pass through a material (e.g., glass is transparent, wood is opaque).

Electrical Conductivity: A material's ability to conduct electricity (e.g., copper is a good conductor, plastic is a poor conductor - an insulator).

Thermal Conductivity: A material's ability to conduct heat (e.g., metal is a good conductor, wood is a poor conductor).

Example: Consider a typical pap (maize porridge) pot in a South African household. The pot material (often aluminum or stainless steel) is chosen for its high thermal conductivity (heats up quickly) and durability. The handles might be made of plastic (poor thermal conductivity, so they don't get too hot to hold). 2.4 Naturally Occurring vs.

Man-made (Synthetic)

Materials: Naturally Occurring Materials: These materials are found in nature and have not been significantly altered by humans. Examples include wood from trees, cotton from cotton plants, sand, rocks, and minerals like gold and diamonds.

Man-made (Synthetic)

Materials: These materials are created by humans through chemical processes. Examples include plastic, nylon, polyester, and concrete.

Example: Wool is a naturally occurring material (from sheep), while nylon is a synthetic material. 2.5 Mixtures: A mixture is a combination of two or more substances that are physically combined but not chemically bonded. The substances in a mixture retain their individual properties.

There are two main types of mixtures: Homogeneous Mixtures: These mixtures have a uniform composition throughout. You cannot see the different components with the naked eye. Examples include sugar dissolved in water (sugar solution), air (a mixture of gases), and brass (a mixture of copper and zinc).

Heterogeneous Mixtures: These mixtures do not have a uniform composition. You can easily see the different components. Examples include sand and water, oil and water, and a salad.

Example: Mahewu, a popular fermented maize drink in South Africa, is a heterogeneous mixture. You might see some sediment settling at the bottom. 2.6 Properties of Materials in Mixtures: The overall properties of a mixture depend on the properties of the individual materials that make it up. For example, adding sugar to water makes the water sweeter (sugar's property of sweetness), and adding salt to water makes it saltier (salt's property of saltiness).

Example: Think about concrete. It's a mixture of cement (a fine powder that hardens when mixed with water), sand (small grains of rock), and gravel (larger pieces of rock). The cement binds the sand and gravel together, giving the concrete its strength and durability. Guided Practice (With Solutions)

Question 1: Classify the following materials as naturally occurring or man-made: cotton, plastic, wood, glass, gold.

Solution: Cotton: Naturally occurring Plastic: Man-made Wood: Naturally occurring Glass: Man-made Gold: Naturally occurring Explanation: Cotton and wood are obtained directly from plants. Gold is a mineral found in the earth. Plastic and glass are produced through industrial processes.

Question 2: Identify whether the following mixtures are homogeneous or heterogeneous: salt water, orange juice (with pulp), air, soil.

Solution: Salt water: Homogeneous (salt is dissolved evenly in the water)

Orange juice (with pulp): Heterogeneous (you can see the pulp)

Air: Homogeneous (gases are evenly mixed)

Soil: Heterogeneous (you can see different components like sand, leaves, and small rocks)

Explanation: Homogeneous mixtures have a uniform appearance. Heterogeneous mixtures have visibly different components.

Question 3: A builder needs to choose a material for roofing. List three properties that the material should have and explain why they are important.