Lesson Notes By Weeks and Term v5 - Grade 7
Separation of mixtures and solutions – Week 8 focus
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Separation of mixtures and solutions – Week 8 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Natural Sciences
Class: Grade 7
Term: 1st Term
Week: 8
Theme: General lesson support
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In our daily lives, we often encounter substances that are mixtures – combinations of different materials not chemically bonded together. Understanding how to separate these mixtures is crucial for various reasons, from purifying water in drought-stricken areas to extracting valuable resources from mined materials. In South Africa, with our rich mining history and ongoing challenges related to water scarcity and pollution, knowledge of separation techniques is especially valuable. Separating mixtures allows us to isolate desired substances, remove unwanted contaminants, and recycle valuable materials, contributing to a more sustainable and efficient use of resources.
2. 1. Mixtures and Solutions A mixture is a combination of two or more substances that are physically combined but not chemically bonded. The components of a mixture retain their individual properties.
Mixtures can be classified as: Homogeneous Mixtures (Solutions): These mixtures have a uniform composition throughout. You cannot see the individual components with the naked eye. Examples include sugar dissolved in water, salt water, and air. The dissolved substance is called the solute, and the substance it dissolves in is called the solvent.
Heterogeneous Mixtures: These mixtures do not have a uniform composition. You can see the individual components with the naked eye or with a simple magnifying glass. Examples include sand and water, oil and water, and a mixed salad. 2.
2. Separation Techniques The choice of separation technique depends on the properties of the substances in the mixture, such as particle size, solubility, density, boiling point, and magnetic properties.
Let's look at some common techniques: Sieving: This method separates mixtures based on particle size. A sieve with a mesh of specific size allows smaller particles to pass through while retaining larger particles.
How it works:* The sieve acts as a barrier. Particles smaller than the mesh size fall through, while larger particles are trapped.
Example:* Separating different sizes of gravel at a construction site or sifting flour in baking. In South Africa, sieving is used in mining to separate ore from larger rocks.
Filtration: This method separates insoluble solids from liquids. The mixture is poured through a filter paper, which allows the liquid (the filtrate) to pass through but traps the solid (the residue).
How it works:* Filter paper has tiny pores that allow liquid molecules to pass, but are too small for the solid particles.
Example:* Separating sand from water or removing solid impurities from fruit juice. Filtration is crucial in water purification plants in South Africa to remove suspended particles and make water safe to drink.
Evaporation: This method separates a soluble solid from a liquid solution. The solution is heated, causing the liquid to evaporate, leaving the solid behind.
How it works:* The liquid changes from a liquid state to a gaseous state (vapor), leaving the solid behind. The solid has a higher boiling point than the liquid.
Example:* Obtaining salt from saltwater. Solar evaporation ponds are used extensively in coastal areas of South Africa to produce salt.
Decantation: This method separates an insoluble solid from a liquid by carefully pouring the liquid off the top, leaving the solid at the bottom of the container. This works best when the solid has settled and formed a distinct layer.
How it works:* Gravity allows the solid to settle. Care is taken not to disturb the solid while pouring off the liquid.
Example:* Separating sand from water after the sand has settled to the bottom. In some rural communities in South Africa, decantation is used as a preliminary step in purifying water from rivers and streams.
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.
How it works:* The magnet exerts a force that attracts magnetic materials, allowing them to be separated.
Example:* Separating iron filings from sand. Magnetism is widely used in the mining industry in South Africa to separate iron ore from other minerals.
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, which is collected separately.
How it works:* Liquids with lower boiling points evaporate more readily when heated. The vapor is then cooled and condensed back to liquid, collecting each liquid separately.
Example:* Separating alcohol from water. Distillation is used in the production of alcoholic beverages and in the purification of chemicals. 2.3 Worked
Examples: Problem: A mixture contains iron filings and sand. Which method is most suitable to separate them?
Solution: Magnetism. Iron filings are magnetic, while sand is not. A magnet will attract the iron filings, leaving the sand behind.
Problem: You have a muddy water sample. Describe the steps you would take to obtain clean water using appropriate separation techniques.
Solution: Step 1: Decantation: Allow the muddy water to stand undisturbed for some time. The heavier mud particles will settle at the bottom. Carefully pour the clear water from the top into another container.
Step 2: Filtration: Pass the water obtained from decantation through a filter paper placed in a funnel. This will remove any remaining fine particles of mud.
Problem: How can you separate salt from a saltwater solution?
Solution: Evaporation. Heat the saltwater solution.