Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define solute, solvent, solution, suspension, and colloid with examples.
• Describe the properties of different mixture types.
• Explain how properties influence a material's use.
• Separate mixtures using methods like filtration and evaporation.
• Compare homogeneous and heterogeneous mixtures.

Lesson summary

This week, we'll be diving deep into the fascinating world of materials and mixtures, focusing on the properties that define them. Understanding these properties is crucial because it allows us to predict how materials will behave under different conditions and to choose the right materials for specific purposes. In South Africa, this knowledge is essential in industries like construction (choosing the right cement mixture for a building), agriculture (selecting appropriate fertilizers for different soil types), and even in our kitchens (understanding how ingredients interact to create the perfect meal!).

Lesson notes

2.1 Mixtures: A Recap A mixture is a combination of two or more substances that are physically combined but not chemically bonded. This means each substance retains its original properties. Mixtures are all around us – think of the air we breathe (a mixture of nitrogen, oxygen, and other gases), the soil under our feet (a mixture of minerals, organic matter, and water), and even the food we eat (many foods are complex mixtures of ingredients). 2.2 Types of Mixtures: Solutions, Suspensions, and Colloids Mixtures can be classified based on how well the different substances are mixed: Solutions: A solution is a homogeneous mixture where one substance (the solute) is completely dissolved in another substance (the solvent). Homogeneous means the mixture has a uniform composition throughout – you can't see the individual components with the naked eye.

Solute:* The substance that dissolves.

Solvent:* The substance that does the dissolving.

Example: Sugar dissolved in water (sugar is the solute, water is the solvent). Another example is Coke – a solution of sugar, flavourings, and carbon dioxide in water. The carbon dioxide gives the bubbles.

Properties: Homogeneous: Components are evenly distributed.

Transparent: Light passes through without scattering (generally).

Stable: The solute does not settle out over time.

Suspensions: A suspension is a heterogeneous mixture where the particles of one substance are dispersed throughout another but are large enough to be seen and will eventually settle out. Heterogeneous means the mixture does not have a uniform composition.

Example: Muddy water (soil particles suspended in water). Think of the water in a river after heavy rains – it’s brown and cloudy because of the soil suspended in it. Another example is a mixture of flour and water.

Properties: Heterogeneous: Components are not evenly distributed.

Cloudy or opaque: Light does not pass through easily.

Unstable: Particles settle out over time.

Colloids: A colloid is a mixture with properties intermediate between those of a solution and a suspension. The particles are larger than in a solution but smaller than in a suspension. They appear homogeneous but are actually heterogeneous at a microscopic level.

Example: Milk (fat globules dispersed in water). Another example is mayonnaise (an emulsion – a type of colloid where liquids are dispersed in other liquids).

Properties: Appear homogeneous but are heterogeneous at a microscopic level.

Cloudy: Light is scattered (Tyndall effect - see below).

Relatively stable: Particles do not settle out easily. 2.3 The Tyndall Effect The Tyndall effect is the scattering of light by the particles in a colloid or suspension. This effect is not observed in solutions because the particles are too small to scatter light. Shining a beam of light through a colloid (like milk) will make the beam visible as it passes through the mixture. This is how you can distinguish a colloid from a solution. Imagine shining a torch through a glass of milk and comparing it to shining a torch through a glass of water with dissolved sugar. In the milk, you will see the beam of light. 2.4 Separating Mixtures Because the components of a mixture are physically combined and not chemically bonded, they can be separated using physical methods.

Common separation techniques include: Filtration: Separating a solid from a liquid using a filter paper (e.g., separating sand from water). This works because the solid particles are too large to pass through the pores of the filter paper.

Example in SA:* Using a cloth to filter out impurities from river water (although this is not sufficient to make the water safe to drink).

Evaporation: Separating a dissolved solid from a liquid by heating the mixture until the liquid evaporates, leaving the solid behind (e.g., obtaining salt from saltwater).

Example in SA:* Salt pans where seawater is evaporated to produce salt.

Decantation: Carefully pouring off a liquid from a solid that has settled at the bottom of the container (e.g., separating sand from water after the sand has settled).

Magnetic Separation: Using a magnet to separate magnetic materials from non-magnetic materials (e.g., separating iron filings from sand).

Example in SA:* Separating iron ore from other rocks during mining.

Distillation: Separating liquids with different boiling points by heating the mixture and collecting the vapors as they condense (e.g., separating alcohol from water). 2.5 Influence of Properties on Use The properties of materials determine their suitability for different uses.

For example: Transparency: Glass is used for windows because it is transparent and allows light to pass through.

Strength: Steel is used in construction because it is strong and can withstand heavy loads.

Flexibility: Rubber is used for tires because it is flexible and can absorb shocks.

Conductivity: Copper is used for electrical wiring because it is a good conductor of electricity.