Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Distinguish between mixtures and solutions.
• Describe methods like sieving, filtration, and evaporation.
• Select the correct technique for a given mixture.
• Explain the science behind each method.
• Apply knowledge to solve practical problems.

Lesson summary

Welcome to Week 9, where we'll be diving into the fascinating world of separating mixtures and solutions! This topic is crucial because in our everyday lives, we often encounter substances that are combinations of different things. Knowing how to separate these mixtures allows us to isolate useful components, purify substances, and even solve environmental problems. Think about cleaning water, mining for gold, or even just making a good cup of tea – all involve separating mixtures. In South Africa, with our rich mineral resources and diverse environments, understanding separation techniques is especially important.

Lesson notes

What are Mixtures and Solutions?

Mixture: A mixture is a combination of two or more substances that are physically combined, but not chemically bonded. The substances in a mixture retain their individual properties and can be separated by physical means.

Examples include: Soil: A mixture of sand, clay, organic matter, and minerals.

Granite: A mixture of different minerals like quartz, feldspar, and mica.

Air: A mixture of nitrogen, oxygen, carbon dioxide, and other gases.

Biltong Spice Mix: A mixture of coriander, pepper, salt and other flavourants.

Solution: A solution is a special type of mixture where one substance (the solute) dissolves completely into another substance (the solvent). The solute is uniformly distributed throughout the solvent, forming a homogeneous mixture.

Key characteristic: solutions are transparent.

Examples include: Saltwater: Salt (solute) dissolved in water (solvent).

Sugar water: Sugar (solute) dissolved in water (solvent).

Cool drink: Various solutes (sugar, flavourings, colourings) dissolved in water (solvent).

Vinegar: Acetic acid (solute) dissolved in water (solvent).

Important distinction: Mixtures can be heterogeneous (different parts visible, like soil) or homogeneous (parts evenly distributed, like air). Solutions are always homogeneous.

Separation Techniques: We can separate mixtures based on the different physical properties of their components.

Here are some common techniques: Sieving: Principle: Separates components based on particle size. A sieve has holes of a specific size, allowing smaller particles to pass through while larger particles are retained.

How it works: The mixture is passed through the sieve. Particles smaller than the sieve openings fall through, while larger particles are trapped.

Example: Separating sand and pebbles in a riverbed. Think of artisanal miners (often women) sieving for small diamonds in riverbeds. This also applies to grading mielie meal where finer mielie meal passes through a sieve and larger parts are retained.

Filtration: Principle: Separates insoluble solids from a liquid.

How it works: The mixture is poured through a filter paper (or cloth). The solid particles are too large to pass through the pores of the filter paper and are retained as a residue, while the liquid passes through as the filtrate.

Example: Filtering muddy water to remove dirt and leaves. Also, think of using a cloth to strain the amasi (sour milk) separating the curds from the whey.

Evaporation: Principle: Separates a soluble solid from a liquid solution.

How it works: The solution is heated, causing the liquid (solvent) to evaporate, leaving the solid (solute) behind.

Example: Obtaining salt from seawater. The sun evaporates the water, leaving salt crystals behind. This is how "sea salt" is produced along the coasts of South Africa.

Distillation: Principle: Separates liquids with different boiling points.

How it works: The mixture is heated. The liquid with the lower boiling point evaporates first. The vapor is then cooled and condensed back into a liquid, which is collected separately.

Example: Distillation of crude oil to separate it into different fractions like petrol, diesel, and kerosene. This is a major industry in South Africa. Also, think of distilling mampoer, a traditional South African spirit, though this should only be done legally and safely by adults.

Magnetism: Principle: Separates magnetic substances from non-magnetic substances.

How it works: A magnet is used to attract and remove the magnetic component from the mixture.

Example: Separating iron filings from sand. This might be used in recycling processes to separate metals from other waste.

Worked example

Problem: You have a mixture of sand, salt, and iron filings. Describe how you would separate them.

Solution:

Step 1: Use a magnet to remove the iron filings. The iron filings will be attracted to the magnet, leaving behind the sand and salt mixture.

Step 2: Add water to the remaining sand and salt mixture. The salt will dissolve in the water, forming a saltwater solution, while the sand will remain undissolved.

Step 3: Filter the saltwater and sand mixture. The sand will be trapped on the filter paper, while the saltwater will pass through.

Step 4: Evaporate the water from the saltwater solution. The water will evaporate, leaving the salt behind.

Problem: A farmer needs to separate a mixture of wheat and chaff (the dry husks of grain). Which separation technique would be most suitable?

Solution: Sieving. The wheat kernels are larger than the chaff particles. A sieve with appropriately sized holes would allow the smaller chaff particles to pass through, leaving the wheat kernels behind.

Problem: You have a muddy water sample from a local dam. How can you obtain clean water from this sample?

Solution: Filtration. The mud consists of insoluble solid particles suspended in the water. By filtering the muddy water through a filter paper or a clean cloth, the mud particles will be trapped, allowing clean water to pass through. You may also allow the mud to settle by gravity, then carefully decant the clear water from the top.

Guided Practice (With Solutions)

Question: You have a mixture of rice and small stones. What separation technique would you use, and why?

Solution: I would use sieving. Rice and stones have different particle sizes. A sieve with holes large enough for the stones to pass through but small enough to retain the rice would effectively separate the mixture.

Question: Describe how you would separate a mixture of sand and water. What is the solid residue called, and what is the liquid that passes through called?

Solution: I would use filtration. First, I would pour the sand and water mixture through filter paper held in a funnel. The sand particles would be too large to pass through the filter paper and would be trapped on it as the residue. The water would pass through the filter paper as the filtrate.

Question: You are given a clear liquid which you suspect is saltwater. How could you prove that it is saltwater and not pure water?

Solution: I would use evaporation. I would heat the clear liquid in a container. If it is saltwater, the water will evaporate, leaving behind salt crystals. If it is pure water, no residue will be left behind after evaporation.

Question: You accidentally spill iron filings into a container of maize meal. What is the easiest way to separate them, and why is this method suitable?

Solution: The easiest method is using a magnet. Iron filings are magnetic, and maize meal is not. By passing a magnet over the mixture, the iron filings will be attracted to the magnet and can be easily removed, leaving the maize meal behind. This method is suitable because it exploits the difference in magnetic properties.

Independent Practice (Questions Only)

Classify the following as either a mixture or a solution: orange juice (with pulp), steel, air, and cooking oil.

Explain why distillation is more effective than simple evaporation for separating two liquids.

A scientist is trying to extract a valuable metal from a complex ore. The ore contains the metal, silica (sand), and other non-magnetic impurities. Describe a process they could use to isolate the metal.

Thandi wants to make a cup of tea but accidentally mixed the tea leaves with sugar. Explain how she can separate the tea leaves from the sugar.

Explain why you cannot use filtration to separate a solution of sugar and water.

Describe a scenario where evaporation might not be the best method to separate a soluble solid from a liquid.

Design an experiment to determine which of two unknown solids is magnetic.

You are given a mixture of different sized gravel. How would you separate them by size? Explain your method.

How does the rate of evaporation affect the purity of the solid that remains after evaporation?

Consider a mining company that needs to separate gold from sand and other materials. Considering the density differences as well, research and describe how "panning" for gold works and relate it to separation techniques.