Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define mixtures and solutions.
• Identify methods like filtration, evaporation, and sieving.
• Explain the principle behind each separation method.
• Apply the correct technique to separate a mixture.
• Design a simple separation experiment.

Lesson summary

Overview: In this lesson, we'll be diving into the fascinating world of mixtures and solutions and how we can separate them. A mixture is simply a combination of two or more substances that are physically combined, but not chemically bonded. A solution is a special type of mixture where one substance (the solute) dissolves completely into another (the solvent). Understanding how to separate these mixtures and solutions is crucial because it has countless applications in everyday life and various industries, particularly in South Africa. From purifying drinking water to extracting valuable minerals, separation techniques are fundamental to many processes that impact our lives.

Lesson notes

2.1 Mixtures and Solutions: Mixture: A mixture is a combination of two or more substances that are physically combined but not chemically bonded. This means the individual substances retain their properties and can be separated by physical means. Mixtures can be homogeneous or heterogeneous.

Heterogeneous Mixture: A mixture where the different components are visible.

Think of a salad: you can clearly see the lettuce, tomatoes, and cucumbers. Another South African example is biltong, especially when it contains visible spices and fat.

Homogeneous Mixture: A mixture where the different components are not visible, and the mixture appears uniform throughout. A good example is air, which is a mixture of nitrogen, oxygen, and other gases.

Solution: A solution is a homogeneous mixture where one substance (the solute) dissolves completely into another substance (the solvent). The solute is the substance that dissolves, and the solvent is the substance that does the dissolving. For example, sugar dissolving in water. The water is the solvent, and the sugar is the solute. Another example could be salt (solute) dissolving in water (solvent) to make a saline solution often used for cleaning wounds. 2.2 Separation Techniques: Here are some common separation techniques and the principles behind them: Filtration: This technique separates insoluble solids from liquids. The mixture is poured through a filter paper, which allows the liquid (the filtrate) to pass through while trapping the solid (the residue).

Example: Separating sand from water. Imagine collecting water from a muddy river. You can filter the water through a cloth (acting as a filter) to remove the mud (the solid residue).

Why it works: Filtration works because the solid particles are larger than the pores in the filter paper, preventing them from passing through.

Evaporation: This technique separates a soluble solid from a liquid solution. The solution is heated, causing the liquid to evaporate and leaving the solid behind.

Example: Obtaining salt from saltwater. In coastal areas, shallow ponds are filled with seawater. The sun's heat evaporates the water, leaving behind salt crystals. This is how much of South Africa's salt supply is produced.

Why it works: Evaporation works because the liquid has a lower boiling point than the solid.

Decantation: This technique separates an insoluble solid from a liquid by carefully pouring the liquid off the top after the solid has settled to the bottom.

Example: Separating sand from water after the sand has settled. After stirring the water with sand, allow the sand to settle at the bottom of the container, and then carefully pour the water out without disturbing the sand.

Why it works: Decantation works because the solid is denser than the liquid and settles to the bottom.

Sieving: This technique separates solids of different sizes using a mesh with specific pore sizes.

Example: Separating gravel from sand. At a construction site, a sieve is used to separate different sizes of aggregate for use in concrete mixtures. Similarly, maize meal might be sieved to remove any larger bran particles.

Why it works: Sieving works based on the size difference between the particles.

Magnetism: This technique separates magnetic materials from non-magnetic materials using a magnet.

Example: Separating iron filings from sand. Imagine you are at a construction site and accidentally mixed iron filings with sand. You can use a magnet to attract and remove the iron filings.

Why it works: Magnetism works because magnetic materials are attracted to a magnetic field, while non-magnetic materials are not. 2.3 Worked

Examples: Example 1: Separating a mixture of sand, salt, and iron filings.

Use a magnet: First, use a magnet to remove the iron filings.

Dissolve the salt: Add water to the remaining mixture of sand and salt. The salt will dissolve.

Filter the sand: Filter the mixture to separate the sand from the salt solution. The sand will remain on the filter paper.

Evaporate the water: Evaporate the water from the salt solution to obtain the salt.

Example 2: Separating maize seeds of different sizes: Use a sieve with a mesh size that will allow the smaller seeds to pass through but retain the larger seeds.

Example 3: Separating cream from milk: This relies on a technique called centrifugation (although Grade 7s are not expected to know this). Cream is less dense than milk. In large dairies in South Africa, specialized centrifuges spin the milk at high speeds, causing the cream to separate from the milk due to the density difference. This is how milk is standardized to different fat contents (full cream, low fat, etc.). Guided Practice (With Solutions)

Question 1: You have a mixture of sugar and small pebbles. Which separation technique would be most appropriate to separate them? Explain your reasoning.

Solution: Technique: Filtration.

Reasoning: Sugar is soluble in water, while pebbles are not.