Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define mixtures and pure substances.
• Describe physical properties of materials and their uses.
• Classify mixtures as homogeneous or heterogeneous.
• Apply techniques for separating mixtures.
• Explain how properties of materials affect a mixture.

Lesson summary

This week, we delve into the fascinating world of materials and mixtures, focusing on their properties. Understanding these properties is crucial because it helps us make informed decisions about which materials to use for different purposes in our daily lives. From choosing the right pot for cooking (considering its heat conductivity) to selecting materials for building houses that can withstand harsh weather conditions, knowledge of material properties is essential. Moreover, understanding how different substances mix (or don't mix!) is fundamental to many processes, from cooking our favorite dishes to understanding how pollutants spread in our environment.

Lesson notes

2. 1. Pure Substances vs.

Mixtures Pure Substance: A pure substance is made up of only one type of particle. It has a fixed composition and distinct properties. Pure substances can be either elements (like gold (Au), iron (Fe), oxygen (O)) or compounds (like water (H2O), salt (NaCl), sugar (C12H22O11)). In South Africa, we use pure substances in many ways, such as pure gold used in Kruger Rands, or pure water used in some bottled water companies.

Mixture: A mixture is 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 easily separated by physical means. 2.

2. Types of Mixtures Homogeneous Mixture: A homogeneous mixture has a uniform composition throughout. This means you cannot see the different components with the naked eye.

Examples include: Sugar dissolved in water: The sugar seems to disappear, and the mixture looks the same throughout. Think of making a cold drink like Oros or Coke – the ingredients are uniformly mixed.

Air: Although air contains many gases (nitrogen, oxygen, etc.), it appears as one uniform substance.

Saltwater: When salt dissolves in water completely, it forms a homogeneous mixture. This is similar to the ocean water around South Africa's coastline.

Heterogeneous Mixture: A heterogeneous mixture does not have a uniform composition. You can see the different components.

Examples include: Sand and water: The sand will settle at the bottom, and you can clearly see both components. This is like the sand and water mixture you find on beaches.

Oil and water: The oil will float on top of the water, as they do not mix.

A mixed salad: You can easily identify the lettuce, tomatoes, and other ingredients. Think of a "chakalaka" (a spicy vegetable relish), where you can distinguish different vegetables.

Concrete: Used extensively in building in South Africa. You can see the aggregate, sand and cement that make it up. 2.

3. Properties of Materials Materials possess various properties that determine their suitability for specific purposes.

Some key properties include: Hardness: Resistance to scratching or indentation. Diamond is one of the hardest materials. For example, the hardness of materials used to make tools like spades is very important in South African agriculture.

Flexibility: Ability to bend without breaking. Rubber is flexible, allowing it to be used in tires.

Strength: Ability to withstand a force without breaking or deforming. Steel is strong and used in building bridges and skyscrapers. The strength of materials used in building RDP houses is very important.

Conductivity: Ability to conduct heat or electricity. Copper is a good conductor of electricity and is used in electrical wiring. Pots used for cooking should be good conductors of heat, while handles should be poor conductors of heat.

Magnetism: Ability to be attracted to a magnet or to act as a magnet. Iron is magnetic.

Melting Point: The temperature at which a solid changes into a liquid. Ice melts at 0°

C. The melting point of metals is crucial in manufacturing and recycling processes.

Boiling Point: The temperature at which a liquid changes into a gas. Water boils at 100°

C. Solubility: The ability of a substance (solute) to dissolve in a solvent (usually a liquid). Sugar is soluble in water. Understanding the solubility of different substances in water is critical in addressing water pollution issues in South Africa. 2.

4. Separation Techniques We can separate mixtures using different techniques based on the properties of the substances involved.

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

Example: Removing sand from water. This is used extensively in water purification processes.

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 is commonly used in coastal areas to produce salt.

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

Example: Separating iron filings from sand. This can be used in recycling plants to separate magnetic metals.

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

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

Distillation: Separating liquids with different boiling points by heating the mixture and collecting the vapors of the liquid with the lower boiling point.

Example: Obtaining pure water from saltwater. This process is used in some water purification plants in drought-stricken areas.