60-minute lesson
Year 8 Science: Separating mixtures
Lesson objective
- Distinguish between pure substances and mixtures.
- Choose a suitable technique to separate a given mixture based on the properties of its parts.
- Separate a sand, salt and water mixture.
Success criteria:
- I can explain the difference between a pure substance and a mixture.
- I can match filtration, evaporation, decanting and magnetic separation to suitable mixtures.
- I can describe the steps to separate sand and salt.
Materials
- Sand and table salt
- Warm tap water
- Two clear jars or glasses
- A funnel (or cut-off top of a plastic bottle) and coffee filter paper or paper towel
- A shallow dish or saucer
- A magnet and a few steel paper clips
- Water-based felt-tip pens and strips of paper towel or coffee filter
Introduction
6 minIntroduce today's words:
- pure substance: A material made of only one kind of particle, such as distilled water or pure salt.
- mixture: Two or more substances physically combined; each keeps its own properties.
- filtration: Separating an insoluble solid from a liquid by passing it through a filter.
- filtrate: The liquid that passes through a filter.
- residue: The solid left behind in the filter paper.
- chromatography: Separating dissolved substances, such as inks, by how far they travel through paper with a solvent.
Ask your child what they already know about separating mixtures.
Explanation
12 minA pure substance contains only one kind of particle. A mixture contains two or more substances that are not chemically joined, so each keeps its own properties. Sea water, air and soil are mixtures.
Because the parts of a mixture keep their own properties, we can use those properties to separate them.
- Filtration separates an insoluble solid (like sand) from a liquid. The liquid passes through as filtrate; the solid stays behind as residue.
- Evaporation separates a dissolved solid (like salt) from a liquid by letting the liquid turn into a gas. Salt is harvested this way from sea water in evaporation ponds in parts of Australia.
- Decanting is carefully pouring off a liquid and leaving heavier solids that have settled at the bottom. Magnetic separation pulls out magnetic materials like iron and steel.
- Paper chromatography separates the different dyes in an ink. Dyes that dissolve well and stick less to the paper travel further.
Water treatment plants that supply Australian cities use similar steps: letting particles settle, then filtering, then disinfecting the water.
Worked examples
9 minSeparating sand, salt and water
You have a mixture of sand and salt. How can you get both back separately?
Step 1: Add warm tap water and stir. The salt dissolves; the sand does not.
Step 2: Filter the mixture. The sand is the residue in the filter paper.
Step 3: Pour the salty filtrate into a shallow dish and leave it in a warm, sunny spot.
Step 4: The water evaporates over a few days, leaving salt crystals.
Step 5: Let the sand dry in the filter paper.
Answer: Dissolve, filter (sand stays as residue), then evaporate the filtrate (salt remains).
Choosing a method
Which technique would separate steel paper clips from sand?
Step 1: Look for a property that differs: steel is magnetic, sand is not.
Step 2: Run a magnet over the mixture.
Answer: Magnetic separation.
Reading a chromatogram
A black felt-tip ink separates into blue, yellow and purple bands. What does this show?
Step 1: Each band is a different dye.
Step 2: The ink is made of at least three dyes, so it is a mixture, not a pure substance.
Answer: The black ink is a mixture of at least three dyes.
Guided practice (do together)
12 min1. Which technique separates sand from water?
(a) Evaporation (b) Filtration (c) Chromatography (d) Magnetic separation
2. How is salt obtained from sea water in salt ponds?
(a) Filtration (b) Decanting (c) Evaporation (d) Using a magnet
3. Which of these is a mixture?
(a) Sea water (b) Distilled water (c) Pure gold (d) Oxygen gas
4. In filtration, what is the solid left in the filter paper called?
(a) Filtrate (b) Solvent (c) Solution (d) Residue
Independent practice
15 min5. Which technique would separate the dyes in a felt-tip ink?
(a) Decanting (b) Paper chromatography (c) Magnetic separation (d) Filtration
6. What is the liquid that passes through a filter called?
7. Carefully pouring a liquid off a settled solid is called d________.
8. Can filtration remove dissolved salt from water? Answer yes or no.
Questions to check understanding
- Can you explain the difference between a pure substance and a mixture?
- Can you match filtration, evaporation, decanting and magnetic separation to suitable mixtures?
- Can you describe the steps to separate sand and salt?
- What was the trickiest part today?
Answer guide
1. Filtration — Sand is insoluble, so filtering traps it while water passes through.
2. Evaporation — The Sun evaporates the water and leaves salt behind.
3. Sea water — Sea water contains water, salts and other dissolved substances.
4. Residue — The solid caught by the filter is the residue.
5. Paper chromatography — Chromatography separates dissolved dyes as they travel up paper.
6. filtrate — The liquid that passes through is the filtrate.
7. decanting — This technique is decanting.
8. no — Dissolved particles pass straight through filter paper; evaporation is needed.
Review
6 minRecap the success criteria together. Watch for these common misconceptions:
- Students often think filtering removes salt. Dissolved substances pass through filter paper.
- Some think 'pure' means 'natural' or 'clean'. In science, pure means one substance only; fresh spring water is a mixture.
- Some believe dissolved substances are destroyed. Evaporation shows they were there all along.
Extension activities
- Run chromatography on several black or brown felt-tips and compare their dye patterns.
- Design a water filter using gravel, sand and cotton wool, and compare cloudy water before and after. (Filtered water is still not safe to drink.)
- Research how desalination plants in Perth, Sydney or Adelaide produce drinking water.
Suggested follow-up
- Revisit separating mixtures tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
Tips for parents
- Cone a coffee filter in a funnel or the cut-off top of a plastic bottle; it filters better than a single paper towel.
- Use the sun or a warm windowsill for evaporation rather than a stove.
- Safety: wear eye protection, take care with the cut edge of a plastic bottle (adult to cut), and do not taste any mixtures.