Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define mixture, solution, solute, and solvent.
• Identify different methods for separating mixtures.
• Explain the science behind each separation method.
• Apply these methods to practical problems.
• Describe a real-world use of separation in South Africa.

Lesson summary

This week, we dive into the fascinating world of mixtures and solutions, focusing on how we can separate them. Understanding how to separate mixtures is extremely important.

Think about it: cleaning muddy water to get clean drinking water, extracting valuable minerals from rocks, or even just making a delicious cup of tea – all of these involve separation techniques! In South Africa, with our diverse industries and environmental challenges, understanding separation techniques is crucial for many jobs and for protecting our precious resources like water. This week, we'll explore different methods and understand the science behind them.

Lesson notes

What are Mixtures and Solutions?

Mixture: A combination of two or more substances that are physically combined but not chemically bonded. Each substance retains its own properties. Think of a mixture of sand and gravel. You can still see the individual grains of sand and pieces of gravel, and they haven't changed into something new. A very common South African example is "biltong spice" - a mixture of coriander, pepper, salt, and other spices.

Solution: A special type of mixture where one substance (the solute) dissolves completely into another substance (the solvent). The solute is distributed evenly throughout the solvent. The result is a homogeneous mixture, meaning it looks the same throughout. An example is sugar dissolved in water (sugar water). Here, sugar is the solute and water is the solvent. Another example, especially relevant in warmer parts of South Africa, is mixing cooldrink powder (like Oros) in water. The powder dissolves evenly.

Key Terms: Solute: The substance that dissolves in a solvent to form a solution (e.g., sugar in sugar water, salt in seawater).

Solvent: The substance that dissolves the solute (e.g., water in sugar water, water in seawater). Methods of Separating Mixtures and Solutions: Sieving: This method separates mixtures based on particle size. A sieve (or screen) has small holes that allow smaller particles to pass through while trapping larger particles.

Example:* Separating stones from sand. This is used in construction in South Africa to get clean sand for making concrete. Another example could be separating mealie meal (fine maize flour) from larger bran particles after milling.

Why it works:* The holes in the sieve are designed to be larger than the size of the smaller particles but smaller than the size of the larger particles.

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).

Example:* Filtering muddy water to remove dirt and sand. Think of using a cloth to strain pulp from fruit juice or filtering water using a sand filter. This is especially important in rural South African communities where access to clean water can be limited.

Why it works:* The filter paper has tiny pores that are smaller than the solid particles, so they cannot pass through.

Evaporation: This method separates a soluble solid from a liquid. The solution is heated, and the liquid evaporates, leaving the solid behind.

Example:* Obtaining salt from seawater. Large salt pans are used in South Africa where seawater is allowed to evaporate in the sun, leaving behind salt crystals.

Why it works:* Liquids have lower boiling points than solids. When heated, the liquid turns into a gas and escapes, while the solid remains.

Decantation: This method separates a liquid from a solid that has settled at the bottom of the container. The liquid is carefully poured off, leaving the solid behind.

Example:* Separating sand from water after the sand has settled. This is sometimes used in small-scale alluvial diamond mining in South Africa.

Why it works:* It relies on gravity causing the solid to settle, allowing the liquid to be poured off without disturbing the solid significantly.

Magnetism: This method separates magnetic materials from non-magnetic materials. A magnet is used to attract and remove the magnetic material.

Example:* Separating iron filings from sand. This is used in recycling plants in South Africa to separate magnetic metals from other waste materials.

Why it works:* Only magnetic materials are attracted to a magnet.

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

Example:* Separating alcohol from water in the production of spirits. This can be relevant to understanding the production of traditional South African alcoholic beverages.

Why it works:* Liquids boil at different temperatures. By carefully controlling the temperature, you can selectively evaporate and then condense each liquid individually.

Worked example

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

Solution:

Use a magnet to remove the iron filings.

Add water to dissolve the salt.

Filter the mixture to separate the sand from the saltwater.

Evaporate the water from the saltwater to obtain the salt.

Commentary: We used a combination of methods because the mixture contained components with different properties (magnetic, soluble, insoluble).

Problem: A farmer has a mixture of wheat and chaff (the outer husks of grains). Which method would be most suitable to separate them?

Solution: Sieving.

Commentary: Wheat grains are larger than the chaff. A sieve with appropriate sized holes would allow the chaff to pass through, leaving the wheat behind.

Problem: You have a solution of copper sulphate in water. How would you obtain copper sulphate crystals?

Solution: Evaporation.

Commentary: Heat the solution to evaporate the water. As the water evaporates, the copper sulphate will become more concentrated and eventually crystallize out.

Guided Practice (With Solutions)

Question: Identify the most suitable method to separate a mixture of rice and small pebbles. Explain why you chose that method.

Solution: Sieving. Rice grains are larger than pebbles. A sieve with appropriate sized holes will allow the pebbles to pass through, leaving the rice behind.

Commentary: We choose sieving because it separates based on particle size, and the rice and pebbles have significantly different sizes.

Question: You have a mixture of oil and water. Which separation method would be most appropriate, and why?

Solution: Decantation. Oil and water are immiscible (they don't mix). If left to stand, the oil will float on top of the water. You can then carefully pour off the oil, leaving the water behind. A separating funnel makes this process easier and more precise.

Commentary: Decantation works because the liquids are immiscible and have different densities, causing them to separate into layers.

Question: How could you separate a mixture of sand and salt? Provide a step-by-step explanation.

Solution:

Add water to the mixture. The salt will dissolve, and the sand will remain undissolved.

Stir the mixture to ensure the salt dissolves completely.

Filter the mixture. The sand will be trapped on the filter paper (residue), and the salt water will pass through (filtrate).

Evaporate the water from the salt water. The water will evaporate, leaving the salt behind.

Commentary: This combines filtration (to remove the insoluble sand) and evaporation (to recover the dissolved salt).

Independent Practice (Questions Only)

Classify each of the following as either a mixture or a solution: muddy water, air, sugar water, concrete, fruit salad.

You accidentally mix sugar and small iron nails. Describe how you would separate them.

Explain the difference between filtration and evaporation. Provide an example of when each method would be used.

Why is distillation a more complex process than evaporation?

Imagine you are working in a gold mine. Describe how you might use separation techniques to extract gold from the ore.

How could you separate a mixture of different-sized gravel pieces?

Design an experiment to determine which type of soil (sandy, clay, or loam) filters water the fastest.

Explain why you can't use filtration to separate a solution.

A farmer wants to separate pumpkin seeds from the pumpkin flesh after harvesting. Suggest the best method and explain why it would work.

Give an example of a separation technique used in a South African industry (other than mining or salt production), and explain its purpose.