Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define solution, solute, and solvent.
• Distinguish between solutions, suspensions, and colloids.
• Explain the concept of solubility and what affects it.
• Describe methods for separating mixtures like filtration and evaporation.

Lesson summary

This week, we will be diving deeper into the fascinating world of materials and mixtures. Understanding the properties of different materials and how they behave when mixed is crucial for many things we do every day. From cooking a simple meal to building a strong house, we rely on our knowledge of materials. In South Africa, this knowledge is particularly important for industries like mining, agriculture, and construction. For example, understanding the properties of different soils is vital for successful farming, and knowing the properties of various metals is essential for the mining industry. Understanding mixtures helps us create solutions for cleaning, cooking, and even medicines.

Lesson notes

2. 1. Solutions, Solutes, and Solvents A solution is a special type of mixture where one substance (the solute) is completely dissolved into another substance (the solvent). The result is a homogeneous mixture, meaning it looks uniform throughout. You can't see the individual particles of the solute.

Solute: The substance that dissolves.

Examples: sugar in tea, salt in water, oxygen in the air.

Solvent: The substance that does the dissolving.

Examples: water in sugar water, air in oxygen/nitrogen mixture, petrol in oil mixtures.

Example: When you add sugar to water and stir until it disappears, you've created a sugar solution. The sugar is the solute, and the water is the solvent. The sugar particles are so small and evenly distributed that you can't see them anymore. 2.

2. Types of Mixtures: Solutions, Suspensions, and Colloids Not all mixtures are solutions. Mixtures can be classified based on the size of the particles and how well they mix.

Solutions: As described above, the solute is completely dissolved, and the mixture is homogeneous. Particles are very small.

Suspensions: A suspension is a heterogeneous mixture where the particles are large enough to be seen and will settle out over time.

Example: muddy water, sand in water. If you leave muddy water to stand, the mud will eventually sink to the bottom.

Colloids: A colloid is somewhere between a solution and a suspension. The particles are larger than in a solution but smaller than in a suspension. They don't settle out, but they do scatter light (this is called the Tyndall effect).

Example: milk, fog, paint.

Practical Test: A simple way to tell the difference is the "shine-through" test. Shine a torch through the mixture.

Solution: The light passes straight through with little scattering.

Suspension: The light is blocked or scattered, and you can't see clearly through it.

Colloid: The light is scattered, making the beam visible within the mixture (Tyndall effect). 2.

3. Solubility Solubility is the ability of a solute to dissolve in a solvent. Some substances are very soluble (dissolve easily), while others are nearly insoluble (don't dissolve).

Factors Affecting Solubility: Temperature: Generally, increasing the temperature increases the solubility of solids in liquids. Think about dissolving sugar in hot tea versus iced tea – it dissolves much faster in hot tea. For gases dissolving in liquids, the opposite is true: solubility decreases as temperature increases.

Nature of Solute and Solvent: "Like dissolves like." This means that polar solvents (like water) tend to dissolve polar solutes (like salt and sugar), while nonpolar solvents (like oil) tend to dissolve nonpolar solutes (like grease and wax).

Stirring/Agitation: Stirring helps to speed up the dissolving process by bringing fresh solvent into contact with the solute.

Particle Size: Smaller particles dissolve faster than larger particles because they have a larger surface area exposed to the solvent.

Example: Imagine trying to dissolve a sugar cube versus the same amount of sugar in a granular form. The granular sugar will dissolve much faster because the water can access more surface area of each sugar particle. 2.

4. Methods of Separating Mixtures Since mixtures are combinations of substances that are not chemically bonded, we can separate them using physical methods.

Filtration: Used to separate insoluble solids from liquids. The mixture is poured through a filter paper, which allows the liquid to pass through but traps the solid particles.

Example: separating sand from water, filtering coffee grounds from coffee.

Evaporation: Used to separate a soluble solid from a liquid. The liquid is heated until it evaporates, leaving the solid behind.

Example: obtaining salt from seawater, obtaining sugar from sugar cane juice.

Decantation: Used to separate a liquid from a solid that has settled to the bottom of the container. The liquid is carefully poured off, leaving the solid behind.

Example: separating water from settled sand, separating oil from settled water (if they are immiscible).

Example: Think about making mageu at home. You might use filtration to remove any large undissolved bits of maize meal after you've allowed it to ferment. Guided Practice (With Solutions)

Question 1: Classify the following mixtures as solutions, suspensions, or colloids: a) Orange juice with pulp b)

A clear glass of water c)

Paint Solution: a)

Orange juice with pulp: This is a suspension. The pulp particles are large enough to see, and they will settle out if left standing for a while. b)

A clear glass of water: This is a solution. Assuming the water is pure, it's a single substance. If you added something that dissolved completely, like salt, then it's still a solution. c)

Paint: This is a colloid. The pigment particles are small enough that they don't settle out, but they scatter light, giving paint its color and opacity.

Question 2: You want to separate salt from a saltwater mixture.