Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define mixtures, solutions, solutes, and solvents.
• Explain four methods of separating mixtures.
• Describe chromatography and its uses.
• Design a simple separation experiment.
• Explain how water purification plants work in South Africa.

Lesson summary

This week, we will delve into the fascinating world of mixtures and solutions and, most importantly, how we can separate them.

Think about it: from the water you drink to the air you breathe, most things around you are actually mixtures! Understanding how to separate these mixtures is crucial, not just in science class, but also in many everyday situations. For example, think about cleaning water from a muddy river or extracting salt from seawater – these are all examples of separation techniques at work. In South Africa, with our diverse environment and resources, understanding separation techniques is vital for industries like mining, agriculture, and water purification.

Lesson notes

2.1 Mixtures and Solutions: The Basics Mixture: A mixture is a combination of two or more substances that are physically combined but not chemically bonded. This means each substance retains its individual properties. Mixtures can be either homogeneous or heterogeneous.

Homogeneous mixture: A mixture where the components are evenly distributed throughout, and you can't easily see the different parts. These are often called solutions. For example, air (a mixture of nitrogen, oxygen, and other gases) or sugar dissolved in water. Think of a cup of rooibos tea – once the sugar is dissolved, it's a homogeneous mixture.

Heterogeneous mixture: A mixture where the components are not evenly distributed, and you can easily see the different parts. For example, sand and water, or a salad. Think of biltong spices mixed together – you can easily see the different spices, making it a heterogeneous mixture.

Solution: A special type of homogeneous mixture where one substance (the solute) dissolves completely into another substance (the solvent).

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

Solvent: The substance that does the dissolving (e.g., water in sugar water or seawater). Water is often called the "universal solvent" because it dissolves so many different substances. 2.2 Separation Techniques The choice of separation technique depends on the properties of the substances in the mixture, such as their particle size, boiling points, and magnetic properties.

Filtration: This method separates insoluble solids from liquids. It relies on the difference in particle size. The mixture is poured through a filter paper, which has tiny holes. The liquid (the filtrate) passes through, while the solid (the residue) is trapped on the filter paper.

Example:* Separating sand from water. Think about cleaning muddy water from a dam. The mud (insoluble solid) is trapped by the filter, while the clean water passes through.

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

Example:* Obtaining salt from seawater. Seawater is placed in shallow pans and left in the sun. The water evaporates, leaving the salt crystals behind. In South Africa, this is done on a large scale in coastal areas.

Distillation: This method separates two or more liquids with different boiling points. The mixture is heated, and the liquid with the lower boiling point evaporates first. The vapor is then cooled and condensed back into a liquid (the distillate).

Example:* Separating alcohol and water. Alcohol has a lower boiling point than water. When the mixture is heated, the alcohol evaporates first, and its vapor can be collected and condensed separately.

Magnetism: This method separates magnetic materials from non-magnetic materials. A magnet is used to attract the magnetic material, leaving the non-magnetic material behind.

Example:* Separating iron filings from sand. The magnet will attract the iron filings, leaving the sand behind. This could be relevant in mining operations where iron ore needs to be separated from other materials.

Chromatography: This is a more advanced technique used to separate substances that are dissolved in a liquid or gas. It works by separating the substances based on their different affinities for a stationary phase (e.g., paper or a solid material) and a mobile phase (e.g., a liquid solvent). Substances that are more attracted to the stationary phase will move more slowly than substances that are more attracted to the mobile phase.

Example:* Separating different food colourings in a sweet or drink. A drop of the food colouring is placed on a piece of chromatography paper, and the paper is placed in a solvent. As the solvent moves up the paper, it carries the different colourings with it at different rates, separating them into bands. This is commonly used in forensic science and food safety. 2.3 Worked Examples Example 1 (Filtration): You have a mixture of soil and water. Describe how you would separate the soil from the water and identify the filtrate and residue.

Solution:* Set up a funnel with filter paper inside. Pour the soil and water mixture into the funnel. The water will pass through the filter paper and collect in a container below. This is the filtrate. The soil will be trapped on the filter paper. This is the residue.

Example 2 (Evaporation): You have a solution of salt and water. Explain how you would obtain the salt from this solution.

Solution:* Pour the salt water into a container (e.g., a beaker or dish). Heat the container gently (e.g., using a hot plate or placing it in the sun). As the water heats up, it will evaporate into the air. Continue heating until all the water has evaporated. The salt will be left behind as solid crystals in the container.

Example 3 (Magnetism): A farmer accidentally mixes iron filings with his maize seeds.