Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define 'material' and 'mixture' with examples.
• Classify materials based on their properties.
• Differentiate between homogeneous and heterogeneous mixtures.
• Describe methods for separating mixtures.

Lesson summary

Welcome to Natural Sciences, Grade 7! This week, we're diving into the fascinating world of materials and mixtures. Understanding the properties of different materials and how they behave when mixed is crucial for everyday life, from cooking in the kitchen to building roads and bridges. In South Africa, where we rely on a variety of natural resources and manufactured goods, knowing how different materials behave helps us make informed decisions about their use and disposal. For example, understanding the properties of building materials like cement and sand is essential for safe and durable construction, and knowing how different chemicals react is important for managing waste effectively.

Lesson notes

2.1 What are Materials? A material is any substance that is used to make something. Materials can be natural (found in nature, like wood, rock, or water) or synthetic (man-made, like plastic or nylon). The properties of a material determine how it can be used.

Examples of Natural Materials: Wood: Used for building houses, furniture, and tools. It is strong, relatively lightweight, and renewable (if harvested sustainably). Rock (e.g., Granite): Used for construction, countertops, and monuments. It is very hard and durable.

Water: Essential for drinking, cleaning, agriculture, and industrial processes. It is a good solvent.

Sand: A key component of concrete and glass.

Examples of Synthetic Materials: Plastic (e.g., Polyethylene): Used for packaging, containers, toys, and many other products. It is lightweight, durable, and can be molded into different shapes.

Nylon: Used for clothing, ropes, and fishing nets. It is strong, elastic, and resistant to abrasion.

Concrete: A mixture of cement, sand, gravel, and water, used extensively in construction. 2.2 Properties of Materials The properties of a material are the characteristics that describe it.

Some important properties include: Hardness: How resistant a material is to scratching or indentation. A diamond is a very hard material.

Flexibility: How easily a material can be bent without breaking. Rubber is very flexible.

Strength: How much force a material can withstand before breaking. Steel is very strong.

Conductivity: How well a material conducts electricity or heat. Copper is a good conductor of electricity.

Magnetism: Whether a material is attracted to a magnet. Iron is a magnetic material.

Transparency: How much light a material allows to pass through. Glass is transparent.

Solubility: How well a material dissolves in a liquid. Sugar is soluble in water. 2.3 What are Mixtures? A mixture is a combination of two or more substances that are physically combined but not chemically bonded. Each substance in a mixture retains its own properties.

Examples of Mixtures: Soil: A mixture of sand, silt, clay, organic matter, and air.

Air: A mixture of nitrogen, oxygen, carbon dioxide, and other gases.

Seawater: A mixture of water and various salts.

Concrete: A mixture of cement, sand, gravel, and water. 2.4 Types of Mixtures Mixtures can be classified as either homogeneous or heterogeneous.

Homogeneous Mixture: A mixture that has a uniform composition throughout. You cannot see the individual components.

Examples: Saltwater (salt dissolved evenly in water), air, sugar dissolved in tea.

Heterogeneous Mixture: A mixture that does not have a uniform composition throughout. You can see the individual components.

Examples: Sand and gravel, oil and water, salad, soil. 2.5 Separating Mixtures Because the substances in a mixture are not chemically bonded, they can be separated using physical methods that exploit the differences in their properties.

Sieving: Separating solids of different sizes using a sieve.

Example: Separating stones from sand. This is important in construction to get properly sized aggregate for concrete.

Filtration: Separating a solid from a liquid using a filter.

Example: Filtering muddy water to remove dirt. This is used in water purification plants to provide clean drinking water.

Evaporation: Separating a dissolved solid from a liquid by heating the mixture until the liquid evaporates, leaving the solid behind.

Example: Obtaining salt from seawater. In South Africa, this is often done in salt pans.

Magnetism: Using a magnet to separate magnetic materials from non-magnetic materials.

Example: Separating iron filings from sand.

Decantation: Carefully pouring off a liquid from a solid that has settled at the bottom of a container.

Example: Separating water from sand after it has settled. Guided Practice (With Solutions)

Question 1: Identify whether the following materials are natural or synthetic: (a) Cotton (b)

Glass (c)

Wool (d)

Plastic Solution: (a)

Cotton: Natural (b)

Glass: Synthetic (c)

Wool: Natural (d)

Plastic: Synthetic

Commentary: Natural materials come directly from the environment, while synthetic materials are man-made. Cotton and wool come from plants and animals, respectively, whereas glass and plastic are manufactured.

Question 2: Classify the following mixtures as homogeneous or heterogeneous: (a) Chicken noodle soup (b) Sugar dissolved in water (c) Sand and water (d)

Orange juice with pulp Solution: (a)

Chicken noodle soup: Heterogeneous (You can see the different ingredients) (b)

Sugar dissolved in water: Homogeneous (The sugar is completely dissolved and evenly distributed) (c)

Sand and water: Heterogeneous (You can see the sand separate from the water) (d)

Orange juice with pulp: Heterogeneous (You can see the pulp separate from the juice)

Commentary: In homogeneous mixtures, the components are uniformly distributed, making it impossible to distinguish them with the naked eye.