Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify different methods for separating mixtures.
• Explain the science behind each separation method.
• Predict the best method to separate a given mixture.
• Design a simple experiment to separate a mixture.
• Evaluate the effectiveness of different separation methods.

Lesson summary

Welcome to Week 2 of our Natural Sciences journey! This week, we're diving deep into the fascinating world of materials and mixtures, specifically focusing on how we can separate different kinds of mixtures based on the unique properties of their components. Understanding how to separate mixtures isn't just a science lesson; it's a skill that's used every day in South Africa, from filtering water to make it safe to drink to extracting valuable minerals from the earth. Think about the gold mines in Gauteng or the fruit juice factories in the Western Cape – separation techniques are at the heart of these industries!

Lesson notes

A mixture is a combination of two or more substances (elements or compounds) that are physically combined but not chemically bonded. This means the individual substances retain their own properties. Separating a mixture means physically isolating these different components. The choice of separation method depends entirely on the different physical properties of the substances in the mixture.

Let's explore some common methods: Filtration: Principle: Separates insoluble solids from a liquid. Insoluble means that the solid doesn't dissolve in the liquid. This method relies on differences in particle size.

How it works: A filter (like filter paper, cloth, or a sieve) has small holes that allow the liquid to pass through but trap the solid particles.

Example: Think about making mageu at home. After fermenting the mealie-meal, you often need to strain it through a cloth to remove any large, undissolved bits of mealie-meal. The liquid mageu passes through, while the solid bits remain on the cloth.

Why it works: The solid particles are larger than the pore size of the filter paper.

Evaporation: Principle: Separates a soluble solid from a liquid solution by heating the solution, causing the liquid to change into a gas (evaporate), leaving the solid behind. This method relies on differences in boiling point. The liquid must have a much lower boiling point than the solid.

How it works: Heat is applied to the mixture, causing the liquid to evaporate. The solid remains as a residue.

Example: Imagine you accidentally mixed salt into a glass of water. You can't filter it out because the salt has dissolved.

However, if you gently heat the salt water, the water will evaporate, leaving the salt crystals behind in the container. Salt pans in coastal areas of South Africa use solar evaporation to harvest salt from seawater.

Why it works: The liquid's boiling point is reached before the solid melts or decomposes.

Decantation: Principle: Separates an insoluble solid from a liquid by carefully pouring the liquid off, leaving the solid sediment behind. This works best when the solid settles quickly to the bottom of the container. This relies on density and particle size.

How it works: Allow the mixture to stand so the solid settles. Gently pour the liquid layer off the top without disturbing the solid sediment.

Example: You might use decantation when panning for gold. After swirling water and sediment in the pan, the heavier gold particles settle to the bottom. You carefully pour off the lighter water and sediment, leaving the gold behind. Or think about settling mud from a murky bucket of water.

Why it works: The solid is denser than the liquid and settles due to gravity.

Magnetism: Principle: Separates magnetic materials from non-magnetic materials. This relies on magnetic properties.

How it works: A magnet is used to attract the magnetic material, separating it from the non-magnetic components.

Example: Suppose you accidentally spilled iron filings into a container of sand. You could use a magnet wrapped in paper (to avoid direct contact with the filings) to attract and remove the iron filings from the sand. This technique is used in recycling plants to separate ferrous metals (containing iron) from other waste materials.

Why it works: Only magnetic materials are attracted to the magnet.

Sieving: Principle: Separates solid particles of different sizes using a sieve (a mesh with specific-sized holes). This relies on particle size.

How it works: The mixture is placed on a sieve, and the smaller particles pass through the holes, while the larger particles are retained on the sieve.

Example: Think about sifting flour when baking vetkoek. This removes lumps and ensures a fine, consistent texture. Gravel and stones can be separated by sieving when preparing concrete.

Why it works: The sieve holes are larger than the smaller particles but smaller than the larger particles.

Distillation: Principle: Separates two or more liquids with different boiling points.

How it Works: The mixture is heated. The liquid with the lowest boiling point boils first. The vapour is then cooled and condensed back into a liquid, which is collected separately.

Example: Distillation is used to purify water or to separate alcohol from water in the production of spirits. Although this is a more advanced concept, you can illustrate it with a simple diagram of a water distillation setup.

Why it Works: The different boiling points of the liquids allow for selective vaporization and condensation. Guided Practice (With Solutions)

Question 1: You have a mixture of sand and water. Which separation method would be most suitable, and why? Describe the steps you would take.

Solution: Method: Filtration Why: Sand is insoluble in water. Filtration exploits the difference in particle size.

Steps: Fold a piece of filter paper into a cone shape and place it in a funnel. Place the funnel over a clean beaker. Carefully pour the sand and water mixture into the filter paper.