Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define and differentiate between mixtures and solutions.
• Identify and explain four methods of separating mixtures.
• Select the best separation technique for a given mixture.
• Describe real-world uses of these techniques in South Africa.

Lesson summary

This week, we delve into the fascinating world of mixtures and solutions and, more importantly, how to separate them. Understanding separation techniques is crucial because we encounter mixtures every day – from the air we breathe (a mixture of gases) to the food we eat (often mixtures of various ingredients). In South Africa, these techniques are vital in areas like water purification, mining, and even in our kitchens. Think about cleaning muddy water after heavy rains or extracting gold from ore! Knowing how to separate mixtures allows us to isolate valuable resources, purify substances, and generally make our lives healthier and more efficient.

Lesson notes

What are Mixtures? 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 classified as homogeneous or heterogeneous.

Homogeneous Mixtures: These mixtures have a uniform composition throughout. You can't see the different components. A good example is salt dissolved in water (saltwater). Another example is air, which is a mixture of nitrogen, oxygen, and other gases. These are also known as SOLUTION

S. Heterogeneous Mixtures: These mixtures do not have a uniform composition. You can easily see the different components. Examples include sand and gravel, or a salad containing lettuce, tomatoes, and cucumbers. What are Solutions? A solution is a special type of homogeneous mixture where one substance (the solute) dissolves evenly into another substance (the solvent). For example, in saltwater, salt is the solute and water is the solvent. Sugar dissolving in tea is another common solution. Why do we need to separate mixtures? Sometimes we want to isolate one or more components of a mixture for various reasons: To obtain a pure substance. To remove unwanted substances. To recycle materials.

Methods of Separating Mixtures: Here are some common separation techniques: Filtration: This method is used to separate an insoluble solid from a liquid. It works by passing the mixture through a filter paper, which allows the liquid (the filtrate) to pass through but traps the solid (the residue). Think of using a coffee filter to separate coffee grounds from brewed coffee. In South Africa, filtration is crucial in water treatment plants to remove impurities from drinking water.

Example: Separating sand from muddy water.

Evaporation: This method is used to separate a soluble solid from a liquid. The liquid is heated until it evaporates, leaving the solid behind. This works because the liquid has a lower boiling point than the solid. Saltpans in coastal areas of South Africa use evaporation to extract salt from seawater.

Example: Separating salt from saltwater. If you leave a bowl of saltwater in the sun, the water will evaporate, leaving the salt crystals behind.

Sieving: This method is used to separate solids of different sizes. A sieve is a mesh screen that allows smaller particles to pass through while retaining larger particles. This is commonly used in construction sites in South Africa to separate gravel from sand. In the kitchen, we use a sieve to separate flour lumps.

Example: Separating gravel from sand using a sieve with a mesh size that allows sand to pass through but retains gravel.

Magnetism: This method is used to separate magnetic materials from non-magnetic materials. A magnet is used to attract the magnetic material, separating it from the rest of the mixture. This is important in recycling plants to separate steel and iron from other waste materials in South Africa.

Example: Separating iron filings from sand using a magnet. The magnet will attract the iron filings, leaving the sand behind.

Decantation: This is a simple process of allowing a mixture to settle and then carefully pouring off the liquid, leaving the solid sediment behind. Think of carefully pouring water off of a pot of cooked mielie pap to remove excess water.

Example: Removing excess water from mielie pap after cooking.

Distillation: This method is used to separate 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. This is used in the production of alcoholic beverages in South Africa (e.g., brandy, gin).

Example: Separating water from alcohol. Alcohol has a lower boiling point than water, so it will evaporate first.

Worked example

Example 1: Separating Sand and Iron Filings:

Problem: You have a mixture of sand and iron filings. How would you separate them?

Solution: Use magnetism. A magnet will attract the iron filings, separating them from the sand.

Why: Iron is magnetic, while sand is not.

Example 2: Separating Salt and Sand:

Problem: You have a mixture of salt and sand. How would you separate them?

Solution:

Add water to the mixture. The salt will dissolve in the water, forming a saltwater solution, while the sand will remain undissolved.

Use filtration to separate the sand from the saltwater. The sand will be trapped on the filter paper, while the saltwater will pass through.

Use evaporation to separate the salt from the water. Heat the saltwater until the water evaporates, leaving the salt behind.

Why: Salt is soluble in water, while sand is not. Filtration separates insoluble solids from liquids, and evaporation separates soluble solids from liquids.

Example 3: Separating Maize Meal and Pebbles:

Problem: You have a mixture of maize meal and small pebbles. How would you separate them?

Solution: Use sieving. A sieve with a mesh size that allows maize meal to pass through but retains pebbles will effectively separate the two.

Why: Sieving separates particles based on size.

Guided Practice (With Solutions)

Question 1:

You have a mixture of tea leaves and water after brewing tea. Which separation technique would you use to separate the tea leaves from the tea? Explain why.

Solution: