Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify properties of materials (colour, texture, hardness).
• Distinguish between homogeneous and heterogeneous mixtures.
• Explain and apply different methods of separating mixtures.
• Choose the right separation method for a given mixture.
• Understand solubility and the factors that affect it.

Lesson summary

This week, we delve into the fascinating world of materials and mixtures, focusing specifically on their properties and how we can use those properties to separate mixtures. Understanding the properties of materials is crucial because everything around us is made of something! From the food we eat to the clothes we wear, understanding these properties helps us make informed decisions about the materials we use and the processes involved in creating the things we need. In South Africa, this knowledge is especially important for industries like mining, agriculture, and manufacturing, where the properties of materials play a vital role in production and innovation.

Lesson notes

2.1 Properties of Materials: Materials have different characteristics that we can observe or measure. These characteristics are called properties.

Some important properties include: Colour: The visual appearance of the material (e.g., red, blue, yellow, transparent, opaque).

Texture: How the material feels to the touch (e.g., smooth, rough, bumpy, silky). Think about the difference between a smooth river stone and a rough piece of bark.

Hardness: The ability of a material to resist being scratched. A diamond is very hard, while chalk is relatively soft. We often use the Mohs hardness scale to compare the relative hardness of minerals.

Flexibility: The ability of a material to bend without breaking. Rubber is flexible, while glass is brittle.

Conductivity: The ability of a material to conduct heat or electricity. Metals like copper and aluminium are good conductors, while wood and plastic are insulators. The electrical wiring in our homes relies on the high conductivity of copper.

Magnetism: The ability of a material to be attracted to a magnet. Iron, nickel, and cobalt are magnetic materials. Magnets are used in many everyday objects, from fridge magnets to electric motors.

Density: How much mass is contained in a given volume. This determines whether an object will float or sink in water (or another liquid). Density is calculated as mass/volume. 2.2 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: Saltwater (salt dissolved in water) Air (a mixture of gases like nitrogen, oxygen, and carbon dioxide) Sugar dissolved in tea Heterogeneous Mixtures: These mixtures have a non-uniform composition. You can easily see the different components.

Examples include: Sand and gravel Oil and water Salad Breyani (a rice dish with various ingredients) 2.3 Separating Mixtures: Since the components of a mixture are not chemically bonded, we can use physical methods to separate them based on their different properties.

Some common separation techniques include: Sieving: Separating solids of different sizes using a sieve.

Example: Separating maize meal from larger pieces of maize bran. This is used extensively in food processing.

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

Example: Separating sand from water. Water purification plants use filtration to remove impurities from 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 saltwater. This process is used on a large scale in coastal areas of South Africa to produce salt for various uses.

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

Example: Separating sand from water after the sand has settled.

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

Example: Separating iron filings from sand. This technique is used in recycling plants to separate metals from other waste materials.

Distillation: Separating liquids with different boiling points by heating the mixture and collecting the vapors as they condense.

Example: Separating alcohol from water. This process is used to produce alcoholic beverages and to purify water. 2.4 Solubility: Solubility is the ability of a substance (solute) to dissolve in another substance (solvent) to form a homogeneous mixture (solution). For example, salt is soluble in water, while sand is not.

Factors Affecting Solubility: Temperature: Generally, the solubility of solids in liquids increases with temperature. For example, more sugar can dissolve in hot tea than in cold tea.

Particle Size: Smaller particles dissolve faster than larger particles. For example, powdered sugar dissolves faster than granulated sugar.

Stirring/Agitation: Stirring increases the rate of dissolving by bringing fresh solvent into contact with the solute.