Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define physical and chemical properties of materials.
• Identify properties of common materials and relate them to their uses.
• Explain the concept of solubility.
• Describe methods for separating mixtures based on their properties.

Lesson summary

This week, we delve deeper into the fascinating world of materials and mixtures, focusing on their unique properties. Understanding these properties is crucial for everyday life in South Africa, from choosing the right building materials for homes to ensuring the safety of the food we eat. For example, knowing about the flammability of materials helps prevent fires in informal settlements, and understanding solubility allows us to effectively use cleaning products. The ability to identify and separate mixtures is essential in industries like mining and agriculture, which are vital to our country's economy.

Lesson notes

2.1 Physical vs.

Chemical Properties Physical Properties: These are characteristics of a substance that can be observed or measured without changing the substance's chemical composition.

Examples include: Colour: The visual appearance of the material (e.g., gold is yellow, copper is reddish-brown).

State of Matter: Whether the material is a solid, liquid, or gas at room temperature (e.g., water is liquid, iron is solid, oxygen is gas).

Density: The mass per unit volume of a substance (e.g., lead is more dense than aluminum). Density dictates whether something will float or sink in a liquid. A heavier thing won't always sink, density plays a crucial role!

Melting Point: The temperature at which a solid changes into a liquid (e.g., ice melts at 0°C).

Boiling Point: The temperature at which a liquid changes into a gas (e.g., water boils at 100°C).

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

Malleability: The ability of a material to be hammered or rolled into thin sheets (e.g., gold is very malleable, used in jewellery).

Ductility: The ability of a material to be drawn into wires (e.g., copper is very ductile, used in electrical wiring).

Solubility: The ability of a substance (solute) to dissolve in another substance (solvent) (e.g., sugar dissolves in water).

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

Thermal Conductivity: The ability of a material to conduct heat (e.g., metal pots conduct heat well).

Chemical Properties: These describe how a substance reacts with other substances or changes its chemical composition.

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

Reactivity: How readily a substance reacts with other substances (e.g., sodium reacts violently with water).

Corrosivity: The ability of a substance to corrode or damage other materials (e.g., acids are corrosive).

Toxicity: The extent to which a chemical substance or a particular type of radiation can damage a living organism.

Example 1: Consider a piece of iron. Its physical properties include its grey colour, solid state at room temperature, high density, and ability to conduct electricity. Its chemical property is that it can rust (react with oxygen in the presence of water).

Example 2: Think about wood. Its physical properties include its brown colour, solid state, and lower density than iron (it floats). A chemical property is that it is flammable; it can burn. The specific type of wood affects the flammability due to different chemical compositions. 2.2 Properties of Common Materials and Their Uses | Material | Properties | Common Uses in South Africa | | -------- | ---------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------ | | Wood | Strong (depending on the type), relatively lightweight, flammable, renewable | Building materials for houses (especially in rural areas), furniture, firewood, crafts. | | Plastic | Lightweight, durable, waterproof, can be molded into different shapes | Packaging (bottles, bags), containers, toys, pipes, furniture, car parts. Problems with plastic pollution are serious. | | Metal | Strong, durable, good conductors of heat and electricity, malleable | Building materials (steel frames), electrical wiring (copper), cooking utensils (stainless steel), cars, mining equipment. | | Glass | Transparent, brittle, strong, can be recycled | Windows, bottles, containers, lenses. | | Rubber | Elastic, waterproof, flexible | Tires, hoses, seals, gloves, erasers. | 2.3 Solubility Solubility is the ability of a substance (the solute) to dissolve in another substance (the solvent) to form a solution. "Like dissolves like" is a useful rule of thumb. Polar substances (like water) tend to dissolve polar solutes (like salt), while non-polar substances (like oil) tend to dissolve non-polar solutes (like grease).

Solution: A homogeneous mixture where the solute is evenly distributed throughout the solvent (e.g., salt water, sugar dissolved in tea).

Solute: The substance that dissolves (e.g., salt, sugar).

Solvent: The substance that does the dissolving (e.g., water, ethanol).

Factors affecting solubility: Temperature: Generally, the solubility of solids and liquids increases with increasing temperature. The solubility of gases decreases with increasing temperature. Think about how you add sugar to hot tea.

Pressure: Pressure has a significant effect on the solubility of gases. Increasing pressure increases the solubility of gases.

Nature of solute and solvent: As mentioned earlier, "like dissolves like." Example 3: Consider sugar dissolving in water. Sugar is the solute, and water is the solvent. As you increase the temperature of the water, more sugar will dissolve.