Pure Substances and Mixtures: a full term of science

Ten ready-to-teach lessons for Year 7 Chemical sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards, an evidence sheet or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards, an evidence sheet or tickets.
3. Gather the few items under “You need” on the plan. Most come from the science prep room, the kitchen or the garden.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 7 science teacher

The pack suits a science specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: a pure substance is made of one kind of particle only, while a mixture holds two or more substances whose particles are mixed but not joined, each keeping its own properties. Because the substances in a mixture differ, in grain size, magnetism, whether they dissolve, their boiling point, how far water carries them up paper or whether they float, each difference can be used to take one part out, one step at a time and in the right order. The everyday “pure” on a juice carton is not the scientist’s.

Two ways to run each lesson

Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 5 needs rock salt made up beforehand, and Lesson 6 a sunny day.

Safety and care in one look

Nothing is tasted in the science room, not even the water a still collects. Magnets go inside plastic bags, warm water comes from the hot tap, any heating is a teacher demonstration, and hands are washed after handling soil.

The term at a glance

One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.

How the sequence builds

Lessons 1 and 2 use the particle model to tell pure substances from mixtures, and solutions from suspensions. Lessons 3 to 7 each add one way of separating, and the property it uses: grain size, magnetism, being insoluble, evaporating, boiling and condensing, and how far a dye travels. Lesson 8 puts the methods together in the right order, Lesson 9 adds floating and sinking, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S7U06 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:

Assessment in this pack

• Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 7 voice, with working.
• The assessment sheet near the front has a class observation checklist and a three-level rubric.
• Lesson 10 is the summative pair: a separation plan for a beach sample, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons. Much of the kit is in a science prep room: balances, funnels, filter paper, beakers and magnets. The rest comes from the kitchen, the garden and a supermarket trip, and two things are worth doing ahead: make up the rock salt for Lesson 5, and save a bag of sawdust from the technology room for Lesson 9.

The one-trip list

• From the prep room: balances reading to 0.1 g, funnels, filter paper, beakers, stirring rods, droppers, measuring cylinders, magnets and iron filings.
• From the supermarket: salt, sugar, flour, bicarbonate of soda, dried beans, rice, cooking oil, coffee filters, cotton wool, cling film, food colouring, water-based black markers and distilled water.
• From home or the garden: sand, garden soil (not potting mix), plastic bowls, heavy mugs, pebbles, black plastic lids, a colander and a kitchen sieve.
• From the school: chalk powder or old chalk, sawdust from the technology room, and sealable plastic bags.

Safety and care in one look

• Nothing is tasted in the science room: not the salt water, not the sugar water, and not the water a still collects.
• Magnets go inside plastic bags; iron filings are kept away from eyes and mouths.
• Students wash their hands after handling soil; use garden soil, not potting mix.
• Warm water comes from the hot tap; any heating is a teacher demonstration. Spills of oil are wiped up at once.

Assessment without extra work

The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the beach sample task, in which students plan how to separate every part of a messy mixture and explain each step, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.

Word wall cards

Cut out the cards and build the wall as the words arrive: pure substance and mixture in Lesson 1; solution, solvent, soluble, insoluble, suspension and saturated in Lesson 2; decant in Lesson 3; filtrate and residue in Lesson 4; crystallisation in Lesson 5; distillation in Lesson 6; chromatography in Lesson 7; density in Lesson 9.

Dear families

This term in science, Year 7 studies pure substances and mixtures. Students learn that a scientist’s “pure” means one substance only, so tap water and even “100% pure” juice are mixtures. Then they use the ways the parts of a mixture differ to separate them: sieves, magnets, filters, evaporation, distillation and chromatography.

Along the way your student will purify rock salt, race filters with muddy water, build a solar still that turns coloured salt water into clear water, solve a mystery with the dyes in black ink, and separate sand, sawdust and salt with nothing more than water.

Try this at home

• Leave a few drops of tap water to dry on a dark plate. What is left behind, and where did it come from?
• Stir a spoon of salt into one glass of water and a spoon of flour into another. Leave both for ten minutes. What do you notice?
• Put a dot of water-based black marker near the bottom of a strip of paper towel and dip the end in water. What colours appear?
• Look at how your household rubbish and recycling are sorted. Which properties are used to separate them?

What to ask your scientist

• Is tap water a pure substance? How do you know?
• Why does a filter catch sand but not salt?
• How can you get both the salt and the water back from salt water?
• Why does the order of the steps matter when you separate a mixture?

A small note on care: students never taste anything in the science room, any heating is done by the teacher, and everyone washes their hands after handling soil.

Warm regards,

The Year 7 science team

Printed from the free seegongsik Pure Substances and Mixtures teaching pack · seegongsik.com/au/y7/chemical/AC9S7U06/pack

Pure or mixture?

In science a pure substance is made of one kind of particle only, the same all the way through. A mixture holds two or more substances whose particles are mixed but not joined, so each substance keeps its own properties. The everyday word is different: a carton of “100% pure orange juice” has nothing added, but to a scientist it is a mixture of water, sugars, acids and pulp. Groups dry a drop of four kinds of water on a black lid, where only a pure substance leaves nothing behind, and then sort fifteen everyday materials.

We are learning to

• describe pure substances and mixtures with the particle model,
• use evidence to decide whether a sample is pure,
• tell the scientific meaning of pure from the everyday one.

Success criteria

• I can draw particle pictures of a pure substance and of a mixture, and explain the difference.
• I can use the dried drops as evidence that tap water is a mixture.

You need

• for each group: a black plastic lid, a dropper, and small labelled cups of distilled water, tap water, sea water (or salt water) and sugar water; an empty juice carton labelled “100% pure” for the opening,
• the worksheet (next sheet), one per student, and the material cards (third sheet), one set per pair.

Safety: nothing is tasted, even the sugar water. A hair dryer on low, used by the teacher, speeds up the drying.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the carton, the drops and the board in Session A, and the cards, the dried drops and Part C in Session B.

Watch for these ideas

• “Pure means natural, clean or healthy.” To a scientist it means one substance only: natural sea water is a mixture, and distilled water is pure.
• “Tap water is pure because it is clear.” Clear liquids can be mixtures; the dried drop shows what was dissolved in it.
• “In a mixture the particles join together.” They are mixed but not joined, which is why the parts can be separated again.

Make it easier, make it bigger

• Easier: sort the waters first, cards A, B, C, using the drops as evidence, then the rest.
• Bigger: a pure substance melts at one temperature, but a mixture melts over a range of temperatures. How could you use this to test whether a candle’s wax is pure?

Answers and look-fors

• Part A, example class: distilled water left nothing at all, so it is pure; tap water left a faint white ring, so it is a mixture; sea water left tiny white crystals, so it is a mixture; sugar water left a clear, sticky patch, so it is a mixture.
• Part B: pure substance C, F, G, J, K, M; mixture A, B, D, E, H, I, L, N, O.
• Part C: 1. Sea water is a mixture: as its water evaporates, the dissolved salt particles stay behind as crystals. Distilled water holds only water particles, so nothing is left. 2. No: it is a mixture of water, sugars, acids and pulp; “pure” on the label means nothing has been added. 3. The pure substance has one kind of particle only; the mixture has two kinds, jumbled together but not joined.
• Bigger: warm the wax slowly with a thermometer in it. A pure substance would melt at one fixed temperature; candle wax softens and melts over a range of temperatures, because it is a mixture.

Pure or mixture?

A. Four drops

B. Fifteen cards

C. Questions

Pure substance or mixture?

Cut out the cards. Lay down the two grey heading cards, read each card, and sort it.

Teacher note: 6 cards are pure substances and 9 are mixtures.

Dissolving and solubility

Some solids dissolve in water and some do not. A soluble substance breaks up into particles far too small to see, which spread evenly between the water particles: a solution, clear and never settling. An insoluble substance stays as grains, and stirred in it makes a cloudy suspension whose bits settle when it is left to stand. There is a limit to how much a given amount of water can dissolve; at that limit the solution is saturated, and anything more stays on the bottom. The limit, the solubility, differs from substance to substance: at 20 °C, 100 mL of water holds about 200 g of sugar but only 36 g of salt. Groups test six solids, find how much salt 20 mL of water holds, and sort twelve everyday mixtures.

We are learning to

• describe solutions and suspensions with the particle model,
• use the words solvent, soluble, insoluble and saturated,
• compare our own measurement with published data.

Success criteria

• I can explain, with particles, why a solution stays clear and a suspension settles.
• I can use a solubility table to predict how much of a substance will dissolve.

You need

• for each group: six cups of 100 mL of water and a spoonful each of salt, sand, sugar, flour, bicarbonate of soda and chalk powder; a cup of 20 mL of water, salt, a balance and stirring rods,
• the worksheet (next sheet), one per student, and the mixture cards (third sheet), one set per pair.

Safety: nothing is tasted; spills are wiped up at once.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the spoonfuls, the six solids and the salt in Session A, and the board, the cards and the questions in Session B.

Watch for these ideas

• “Dissolving is melting.” Melting needs heat and involves one substance; in dissolving, the particles of one substance spread out among the particles of a liquid.
• “The salt has gone.” Its particles are all still there, too small and too spread out to see; the dried drops in Lesson 1 showed them.
• “Cloudy means dissolved.” Cloudiness comes from undissolved bits. A solution is clear, even when it is coloured, like cordial.

Make it easier, make it bigger

• Easier: test four solids only, the salt, sugar, sand and flour.
• Bigger: at 100 °C, 100 mL of water dissolves about 39 g of salt but about 480 g of sugar. What does warming the water do to the solubility of each?

Answers and look-fors

• Part A, example class: salt dissolves, clear; after standing, no change; sand does not dissolve, grains sink at once; after standing, a layer on the bottom; sugar dissolves, clear; after standing, no change; flour does not dissolve, cloudy white; after standing, most has settled; bicarbonate of soda dissolves, clear; after standing, no change; chalk powder does not dissolve, cloudy white; after standing, settled, water clearing. Solutions: salt, sugar, bicarbonate of soda.
• Part B: the example groups dissolved 7, 7, 6 g. 1. 36 × 20 ÷ 100 = 7.2 g, so about 7 g; a group that found fewer may have stirred for less time. 2. 200 × 20 ÷ 100 = 40 g. 3. In a solution the solute particles are spread evenly among the water particles and keep moving, so they never settle; the bits in a suspension are grains made of huge numbers of particles, too heavy to stay up, so they sink.
• Part C: solution A, B, E, F, I, K, L; suspension C, D, G, H, J.
• Bigger: warming hardly changes how much salt dissolves (36 to 39 g), but more than doubles the sugar (200 to about 480 g).

Dissolving and solubility

A. Six solids

B. How much salt?

Grams of salt that dissolved in 20 mL of water: ______   Example class groups: 7, 7, 6 g.

C. Twelve mixtures

Solution or suspension?

Cut out the cards. Lay down the two grey heading cards, read each card, and sort it.

Teacher note: 7 cards are solutions and 5 are suspensions.

Sieves, magnets and decanting

Every way of separating a mixture uses a property that one part has and the others lack. A sieve uses grain size: the mesh holds back grains bigger than its holes and lets smaller ones through. A magnet uses magnetism, and lifts out iron and steel while everything else stays put. Decanting uses settling: once the heavy bits have sunk, the liquid above them is gently poured off. Groups plan and carry out the separation of a messy mixture of dried beans, rice, iron filings and sand, then weigh each part to see how much came back.

We are learning to

• choose a separation method from a property of each part,
• plan the steps in a sensible order,
• measure what was recovered, and explain the losses.

Success criteria

• I can name the property each tool uses.
• I can work out the percentage of each part we recovered, and give a reason for each loss.

You need

• for each group: 100 g of the messy mixture (made up beforehand: 30 g of dried beans, 25 g of rice, 10 g of iron filings, 35 g of sand), a coarse sieve or colander, a fine kitchen sieve, a magnet sealed in a plastic bag, four containers and a balance,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four; a jar of muddy water left to settle since the morning, for the decanting demonstration.

Safety: the magnet stays in its bag, so the filings never touch it; keep iron filings away from eyes, and wash hands afterwards.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Plan and separate in Session A, and weigh, decant, show the board and answer in Session B.

Watch for these ideas

• “A magnet attracts all metals.” It attracts iron and steel (and nickel and cobalt), not aluminium, copper or gold.
• “The sieve with the smallest holes should go first.” It would hold back the beans and the rice together; the coarse sieve goes first.
• “Our results should add up to exactly 100 g.” Some grains stick to the sieves, spill or stay in the bag; measuring what came back shows how careful the method was.

Make it easier, make it bigger

• Easier: give out a mixture of three parts only, the beans, the sand and the iron filings.
• Bigger: salt has been added to the sand. Plan how to get the salt back too.

Answers and look-fors

• Part A, one good order: the coarse sieve (grain size: beans stay), then the fine sieve (grain size: rice stays), then the magnet over what fell through (magnetism: iron filings), leaving the sand. Using the magnet first also works.
• Part B, example group: dried beans 30 of 30 g (100%); rice 24.8 of 25 g (99%); iron filings 9.4 of 10 g (94%); sand 33.6 of 35 g (96%); 97.8 of 100 g came back in all.
• Part C: 1. Filings cling to a bare magnet and are hard to pull off; in a bag, they drop off as soon as the magnet is taken out. 2. Iron filings: the grains are so fine that some stick to the sieve, the containers and the bag, and a few blow or spill away. 3. No: salt grains and sand grains are about the same size, so a sieve cannot sort them; adding water dissolves the salt, and then the sand can be filtered out.
• Exit ticket: 1. Grain size. 2. Magnetism. 3. Leave it to settle, then gently pour the clear water off the mud: decanting.
• Bigger: stir the sand and salt into water so the salt dissolves, filter out the sand, then let the water evaporate to leave the salt.

Sieves, magnets and decanting

A. Our plan

The mixture: dried beans, rice, iron filings and sand. In each box write the tool, what it takes out, and the property it uses.

B. What came back

Percentage recovered = mass got back ÷ mass in the mixture × 100.

C. Questions

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Filtering

A filter is a sieve with very fine holes. Undissolved grains bigger than the holes stay on the filter as the residue, while the liquid, and anything dissolved in it, runs through as the filtrate. A filter can only catch what is insoluble: dissolved particles are far smaller than any hole in the paper, so a clear filtrate may still be a mixture. Finer filters catch smaller bits but let the liquid through more slowly, so choosing a filter means weighing clarity against speed. Groups race four filters with muddy water, score each filtrate with a cross test, and put the steps of a water treatment plant in order.

We are learning to

• explain filtering with grain size and solubility, using the words residue and filtrate,
• compare filters fairly on speed and clarity,
• describe how a water treatment plant uses several separations in turn.

Success criteria

• I can explain why the clearest filter was also the slowest.
• I can explain why a clear filtrate is not always pure water.

You need

• for each group: muddy water (a heaped spoon of garden soil stirred into 250 mL of water), four funnels in four cups, filter paper, a coffee filter, paper towel and cotton wool, a measuring cylinder and a stopwatch; a black cross drawn on white paper,
• the worksheet (next sheet), one per student, and the step cards (third sheet), one set per pair.

Safety: use garden soil, not potting mix, and wash hands after handling it. Nobody drinks the filtrate.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the coffee question and the filter race in Session A, and the board, the step cards and the questions in Session B.

Watch for these ideas

• “A clear filtrate is pure water.” Dissolved substances pass straight through a filter; dry a drop of the filtrate on a black lid and see.
• “The fastest filter is the best.” It depends on the job: the fast filters let more mud through.
• “Filtering makes the mud disappear.” The mud is all on the paper as the residue; nothing is destroyed.

Make it easier, make it bigger

• Easier: race two filters only, the filter paper and the paper towel.
• Bigger: design a better filter by stacking two of today’s filters. Predict its speed and clarity, then test it.

Answers and look-fors

• Part A, example class averages: filter paper 242 s, clarity 3; coffee filter 150 s, clarity 2; paper towel, two layers 90 s, clarity 1; cotton wool plug 45 s, clarity 1.
• Part B: 1. The filter paper; its holes are the finest, so it caught the most mud, and for the same reason the water took longest to squeeze through. 2. The filtrate: the brown colour is dissolved in the water, and dissolved particles pass through any filter. 3. No: substances dissolved from the soil pass through; a dried drop of filtrate leaves a mark. 4. The mud on the paper: it is the residue, and it was all still there.
• Part C: 1 screening, 2 clumping, 3 settling, 4 filtering, 5 disinfecting, 6 storing. Screening and filtering use grain size; clumping makes the specks big enough to settle and be filtered; settling uses the clumps sinking; disinfecting and storing do not separate anything, they keep the water safe.
• Bigger: two layers catch more but run even more slowly; look for groups that predicted before testing.

Filtering

A. Filter race

Stand each cup of filtrate on the black cross, look straight down through it, and score it:

B. Questions

C. A water treatment plant

From river to tap

Cut out the six cards. A water treatment plant takes in river or dam water and sends clean water to our taps. Put the steps in the order the water meets them.

Teacher note: the order is on this lesson’s plan. Plants differ in their details; these are the main steps most of them share.

Evaporating and crystallising

A filter cannot catch a dissolved substance, but evaporation can. As the water of a salt solution evaporates, the salt particles are left behind and gather into crystals: crystallisation. Put together with dissolving and filtering, it purifies rock salt: dissolve the salt, filter out the insoluble sand, then let the water evaporate. Slow evaporation grows larger crystals and quick evaporation many small ones. In north-west Western Australia, sea water is let into huge shallow ponds where the sun and wind evaporate it, leaving salt. Groups purify 20 g of made-up rock salt and weigh the salt and the sand they get back.

We are learning to

• use dissolving, filtering and evaporating together to purify a substance,
• explain crystallisation with particles,
• calculate a percentage from our results and explain a result that does not fit.

Success criteria

• I can explain why each step of the method is needed, in its order.
• I can use the results to say how much of the rock salt was salt, and explain an odd result.

You need

• for each group: 20 g of rock salt (made up beforehand: 12 g of salt mixed with 8 g of sand), a beaker of 50 mL of warm tap water, a stirring rod, a funnel and filter paper, a shallow dish, a wash bottle, a balance and a magnifying glass,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Safety: warm water comes from the hot tap. The dishes dry on a sunny windowsill over a few days; if the teacher speeds them up over a water bath, students stand back. Nothing is tasted.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Dissolve, filter and set the dishes in Session A, and do the board, the results and the crystals in Session B.

Watch for these ideas

• “The salt evaporates too.” Only the water leaves as vapour; the salt stays behind, which is why the dish turns white.
• “Filtering takes out the salt.” Dissolved salt passes through the paper; only the sand stays.
• “Group 3 made extra sand.” Their sand weighed 8.4 g, more than the 8 g put in, because it was still damp or held dried salt; rinsing and drying matter.

Make it easier, make it bigger

• Easier: skip the rinse, and compare only the salt recovered.
• Bigger: hang a thread in a saturated salt solution and leave it for a week. Explain, with particles, why slow evaporation grows bigger crystals.

Answers and look-fors

• Part A: 1. Dissolve: salt is soluble, sand is not. 2. Filter: the sand grains are held back, the salt solution runs through. 3. Rinse: a little water washes the last salt solution off the sand into the filtrate. 4. Evaporate: the water leaves as vapour; the salt stays and forms crystals.
• Part B, example class: Group 1 11.2 g of salt (56% of the rock salt) and 7.7 g of sand; Group 2 11.6 g of salt (58% of the rock salt) and 7.9 g of sand; Group 3 10.9 g of salt (55% of the rock salt) and 8.4 g of sand. The rock salt was really 60% salt; every group got a little less back.
• Part C: 1. As the water particles evaporate, the salt particles left behind come together and line up in a regular pattern: a crystal. 2. Some salt solution soaks into the filter paper, some crystals stick to the dish and the rod, and a little splashes or spills. 3. Under the glass the crystals are tiny cubes. Slow evaporation gives the particles time to line up neatly, so the crystals grow bigger.
• Exit ticket: 1. Filtering. 2. Evaporating, which leaves the salt as crystals. 3. Sand is insoluble, so its grains stay on the paper; dissolved salt particles are far too small to be caught.

Evaporating and crystallising

A. The method, and why

B. Results

Each group started with 20 g of rock salt. Salt as a percentage = salt back ÷ 20 × 100.

C. Crystals

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Distilling

Evaporation gets the salt back from salt water but loses the water to the air. Distillation keeps both: the liquid is evaporated or boiled, and the vapour is cooled on a cold surface so that it condenses and can be collected. Only the water turns to vapour, so the collected water is pure, while the salt, the colour and anything else dissolved stay behind. A solar still does this with sunlight and a sheet of cling film. Several Australian cities, Perth among them, get part of their drinking water from desalination plants that take the salt out of sea water; most of them force sea water through very fine membranes rather than boiling it, and all of them use a great deal of energy. Groups build solar stills in the sun and the shade, test what they collect, and weigh up desalination.

We are learning to

• explain distillation as evaporating and condensing,
• run a fair test and check the purity of a product without tasting it,
• weigh up the benefits and costs of a separation a community depends on.

Success criteria

• I can explain where the water in the mug came from, and why it is clear.
• I can give one benefit and one cost of desalination, and say which I think matters more and why.

You need

• for each group: two large bowls, two heavy mugs, cling film, two pebbles, 1000 mL of water with two tablespoons of salt and a drop of food colouring, a measuring cylinder and a black lid; a sunny spot and a shady one,
• the worksheet (next sheet), one per student, and the build card (third sheet), one per group.

Safety: nobody drinks or tastes the collected water; it is tested by drying a drop. Stills sit on the ground or a low bench where they cannot be knocked off.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Build the stills and show the board in Session A, and weigh up desalination, collect and test in Session B.

Watch for these ideas

• “The salt water splashed into the mug.” The collected water is clear and leaves nothing when dried, so it arrived as vapour, not as splashes.
• “The sun boils the water.” The water in the bowl never boils; it evaporates faster in the warmth and condenses on the cooler film.
• “Desalination plants just boil sea water.” Most force it through very fine membranes; either way, getting fresh water from salt water takes a lot of energy.

Make it easier, make it bigger

• Easier: build one still for the sun only, and test its water.
• Bigger: suggest two changes that would make a still collect more water in the same time, and explain each with particles.

Answers and look-fors

• Part A, example class: in full sun 15, 17, 14 mL (average 15.3); in the shade 3, 2, 4 mL (average 3). Changed: sun or shade; measured: the water collected; kept the same: the bowl, the mug, the 500 mL of salt water, the time.
• Part B: the collected water is clear and colourless, and its dried drop leaves nothing, while the salt water’s drop leaves white crystals. Only water particles evaporated; the salt and the colour stayed in the bowl.
• Part C, look for one benefit (water in droughts that does not depend on rain; a supply for dry places) and one cost (much energy, often from fossil fuels; very salty water returned to the sea; building and running costs), and a reasoned opinion.
• Questions: 1. Water evaporated from the warm salt water, the vapour condensed on the cooler film, and the drops ran down to the low point and dripped into the mug. 2. In the sun the water was warmer, so more of its particles had the energy to escape each minute. 3. No: it is clear and leaves no crystals, but without tasting we can only say that the salt and colour stayed behind; a bowl that is not perfectly clean could add other substances.
• Board: without the stopper the vapour would escape out of the top of the flask; without the iced water much of it would drift out of the beaker before it could condense.
• Bigger: a wider bowl (more surface to evaporate from), a darker bowl or more sun (warmer water), a cooler film (more condensing), and sealing the edges so no vapour escapes.

Distilling

A. Sun or shade?

B. Test the water, without tasting it

Dry a drop of the collected water and a drop of the salt water side by side on the black lid.

C. Weighing up desalination

Several Australian cities, Perth among them, get part of their drinking water from desalination plants, which take the salt out of sea water. They keep working through droughts, when dams run low. But they use a great deal of electricity, they return very salty water to the sea, and they cost a lot to build and run.

D. Questions

Build a solar still

1. Pour 500 mL of the coloured salt water into the bowl.
2. Stand the clean, empty mug in the middle of the bowl. Its rim must be above the water but below the bowl’s rim.
3. Stretch cling film tightly over the bowl and press it down round the edge so no vapour can escape.
4. Put a pebble on the film right above the mug, so the film dips to its lowest point there. The film must not touch the mug.
5. Set one still in full sun and the other in the shade, at the same time.
6. After 4 hours, lift the film carefully, take out the mug and measure the water with a measuring cylinder.
7. Test the water by drying a drop on the black lid. Never drink or taste it.

Teacher note: the colour in the salt water is the quickest evidence; if the mug water is clear, the dye stayed behind with the salt.

Chromatography

Many inks that look like one colour are mixtures of several dyes, and paper chromatography separates them. A dot of ink sits on a pencil line near the bottom of a paper strip, and the end of the strip stands in water. As the water soaks up the paper it carries the dyes along, but each dye dissolves in the water and clings to the paper to a different degree, so each one travels a different distance and the dot spreads into separate bands. The distance a dye travels divided by the distance the water travels, its Rf value, stays the same for that dye on the same paper with the same liquid, so it can help to identify the dye. Groups run strips of black markers, then measure an evidence sheet to find which of four pens wrote a mystery note.

We are learning to

• explain how chromatography separates the dyes in an ink,
• measure distances carefully and calculate Rf values,
• use evidence to reach a conclusion, and say how sure we can be.

Success criteria

• I can explain why the dyes in one ink travel different distances.
• I can calculate an Rf value and use the whole pattern to match the note to a pen.

You need

• for each group: strips of filter paper about 2 cm by 12 cm, a pencil, a ruler, water-based black markers of three or four kinds, cups holding 1 cm of water, and paper clips to hang each strip from a pencil laid across its cup,
• the worksheet (next sheet) and the evidence sheet (third sheet), one each per student.

Safety: no special hazards; use water-based markers only, and keep the cups away from the evidence sheets.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Run the strips and show the board in Session A, and solve the note and answer Part C in Session B.

Watch for these ideas

• “Black ink is one substance.” Most black inks are mixtures of dyes; the strips show them.
• Measuring to the top or bottom of a spot. Measure from the pencil line to the middle of each spot, and from the pencil line to the water front.
• “Two pens that share a colour must be the same.” The whole pattern must match: every dye, at the same Rf, and no extra dyes.

Make it easier, make it bigger

• Easier: measure the distances, and match the patterns by eye without Rf values.
• Bigger: another class ran the note on a longer strip of the same paper. The water front reached 7.5 cm and the blue dye 3 cm. Does the blue still match pen D? Why do scientists use Rf values rather than distances?

Answers and look-fors

• Part A: results depend on the markers. Look for a pencil line, a dot above the water, the water front marked, and distances measured to the middle of each spot.
• Part B: the water front is at 6 cm on every strip. Pen A: blue 2.4 cm (Rf 0.40), red 4.2 cm (Rf 0.70); Pen B: blue 2.4 cm (Rf 0.40), pink 3.6 cm (Rf 0.60), yellow 5.1 cm (Rf 0.85); Pen C: brown 1.2 cm (Rf 0.20), green 4.8 cm (Rf 0.80); Pen D: blue 2.4 cm (Rf 0.40), yellow 5.1 cm (Rf 0.85); The note: blue 2.4 cm (Rf 0.40), yellow 5.1 cm (Rf 0.85). The note matches pen D exactly; pen B has the same two dyes but an extra one, so it does not match.
• Part C: 1. Each dye dissolves in water and clings to the paper to a different degree; a dye that dissolves well and clings little is carried furthest. 2. Pencil does not dissolve in water, so the line stays put; an ink line would run and mix with the dots. 3. Under the water, the dot would dissolve into the water in the cup instead of being carried up the paper. 4. The pattern shows which kind of pen wrote the note, not who used it; every pen of that kind would give the same pattern.
• Bigger: 3 ÷ 7.5 = 0.40, the same as pen D’s blue. Distances change with how far the water runs; Rf values do not.

Chromatography

A. Our strips

B. The mystery note

A note was left on the classroom door, written in black pen. Four black pens were found nearby. Use the evidence sheet: measure each spot from the pencil line to its middle, then work out Rf = distance of the dye ÷ distance of the water front.

C. Questions

The evidence: five chromatograms

A dot from each of the four pens and from the note was run on the same paper, in water, for the same time. Read each distance from the scale on the left, measured from the pencil line to the middle of the spot. The water front reached the dashed blue line on every strip.

Teacher note: the answers are on this lesson’s plan. The scale is part of the drawing, so the sheet can be printed at any size.

Choosing a method

Real mixtures rarely give way to a single method. Separating one takes several steps, each using one property, in an order that works: the magnet before the filter, so the iron is not caught with the sand, and the filter before the evaporating, so the salt is not left mixed with the sand. Pairs plan three separations with nine tool cards, choosing each step from a property and saying why the order matters. Then they read how Aboriginal people separate seeds from husks and grit with a shallow wooden dish, knowledge built up and tested over many generations.

We are learning to

• choose a method for each part of a mixture from its properties,
• put the steps of a separation in an order that works, and explain the order,
• recognise knowledge of separating that First Nations Australians have built and passed on.

Success criteria

• My three plans name the tool and the property for every step, in an order that works.
• I can explain what would go wrong if two of the steps swapped places.

You need

• the tool cards (third sheet), one set per pair, and the worksheet (next sheet), one per student.

A note on the reading: where you can, invite a local Aboriginal or Torres Strait Islander knowledge holder, or use resources made with community permission. Practices differ from place to place, and this knowledge belongs to the communities who hold it.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the board and the three plans in Session A, and the swap, the reading and the wrap-up in Session B.

Watch for these ideas

• One tool for everything, such as filtering salt water. Ask: what would the filter catch here?
• Evaporating before filtering. The salt is then left mixed with the sand, and with no liquid the filter cannot help.
• Distilling when only the salt is wanted. It works, but evaporating is simpler; distilling is chosen when the liquid itself is wanted.

Make it easier, make it bigger

• Easier: plan the first mixture together on the board, then pairs do the other two.
• Bigger: plan a fourth mixture, sand, salt and sawdust, and name the new property you need for it.

Answers and look-fors

• Part A, mixture 1 (salt, sand and iron filings, to get each of the three on its own): Magnet (card C): magnetism takes out the iron; then Add water and stir (card E): salt is soluble, sand is not; then Filter (card A): the sand grains are held back; then Evaporate (card F): the water leaves, the salt stays.
• Part A, mixture 2 (muddy sea water, to get clear water good enough to drink): Settle and decant (card H): the heavy mud sinks; then Filter (card A): the last fine mud is held back; then Distil (card B): only the water boils off and condenses; the salt stays behind.
• Part A, mixture 3 (grass seeds with husks and fine grit, to get clean seeds): Winnow (card D): the light husks are carried away; then Sieve (card G): grit smaller than the seeds falls through.
• Not needed: chromatography (card I). For mixture 2, distilling alone would also work, but settling and filtering first keep the flask clean.
• Part B: 1. How heavy and how big each part is: the light husks are carried off and the heavier grit settles apart from the seed. 2. By careful watching and practice, tried again and again and passed on by teaching across many generations. 3. The knowledge belongs to those communities; asking first respects that and keeps the description accurate.
• Bigger: stir it into water; the sawdust floats and is skimmed off, the sand sinks and is filtered out, and the salt is left when the water evaporates. The new property is whether each part floats or sinks, which Lesson 9 tries.

Choosing a method

A. Three plans

In each box write the tool and the property it uses. You may not need every box.

B. Seeds, husks and a wooden dish

For thousands of years, Aboriginal people in many parts of inland Australia have ground the seeds of native grasses and other plants into flour. First the seeds must be separated from their husks and from grit. One way uses a shallow wooden dish: the seeds are tossed and rocked in it with skilled movements, so that the light husks are carried away and the heavier grit settles apart, leaving clean seed. In the Pilbara this is called yandying. The skill was developed through careful observation and practice, and passed on from one generation to the next.

Nine tools

Cut out the cards. For each mixture, lay out the tools you need in order, then copy your plan onto the worksheet.

Teacher note: one card is not needed for the three mixtures; the plans are on this lesson’s plan.

Floating, sinking and settling

Density is how much mass is packed into a given space. In water, anything less dense than water floats and anything denser sinks, and that difference can separate a mixture. Sawdust floats and can be skimmed off the top; sand sinks, so the water above it can be decanted; and two liquids that do not mix, such as oil and water, settle into layers that can be taken apart. Gold panners use density too: gold is about 7 times as dense as sand, so swirling gravel and water in a pan washes the sand away and leaves the gold. Groups separate sand, sawdust and salt with nothing but water, take oil off water, and read about gold panning.

We are learning to

• use floating, sinking and settling to separate a mixture,
• describe density in simple terms,
• combine several methods in one plan, and measure how well it worked.

Success criteria

• I can explain why the sawdust floated and the sand sank.
• I can separate oil from water, and explain how the method works.

You need

• for each group: 30 g of the mixture (made up beforehand: 5 g of sawdust, 15 g of sand, 10 g of salt), a beaker of 200 mL of water, a tea strainer or spoon, a second beaker, a funnel and filter paper, a shallow dish and a balance; a clear cup, 50 mL of water, 20 mL of cooking oil and a dropper,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Safety: use sawdust from untreated pine, not from treated timber or particle board; wipe up oil at once and wash hands afterwards.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the cork, the three parts and the oil in Session A, and the board, the reading and the ticket in Session B.

Watch for these ideas

• “Heavy things sink and light things float.” A heavy log floats and a tiny pebble sinks: what matters is density, the mass packed into each bit of space.
• “Oil floats because there is less of it.” A cupful of oil has less mass than a cupful of water: oil is less dense, whatever the amount.
• “The salt sank to the bottom.” Dissolved salt is spread through the water; it goes through the filter with the water and is left when the water evaporates.

Make it easier, make it bigger

• Easier: separate only the sand and the sawdust.
• Bigger: why do panners still lose some of the finest flakes of gold? Suggest a change to the method that would keep more of them.

Answers and look-fors

• Part A, example group (weighed dry): sawdust 4.6 of 5 g (92%), floats, so it is skimmed off the top; sand 14.2 of 15 g (95%), sinks, so the water is decanted and the sand filtered; salt 9.3 of 10 g (93%), dissolves, so it is left when the water evaporates.
• Part B: the oil forms a layer on top of the water; it is less dense than water and does not mix with it, so the dropper can lift it off the top. The last thin film is spread so widely that the dropper cannot gather it.
• Part C: 1. 19.3 ÷ 2.65 = about 7.3, so gold is about 7 times as dense as sand. 2. The moving water carries the less dense sand and gravel over the edge, while the gold sinks to the bottom of the pan and stays. 3. Both use density: the dense gold and the dense sand both sink, while the less dense parts are carried away or float.
• Exit ticket: 1. Density. 2. It floats, so it can be skimmed off the top. 3. Let it settle into two layers, then take the oil off the top with a dropper.
• Bigger: very fine flakes are so small and light that moving water can carry them over the edge; swirling more gently, or catching the water that leaves the pan in a second pan, keeps more of them.

Floating, sinking and settling

A. Three parts, one liquid

B. Oil and water

C. Panning for gold

In the 1850s, gold rushes brought hundreds of thousands of people to the goldfields of New South Wales and Victoria. Many searched for gold with a pan, a wide, shallow dish. They scooped up gravel and water from a creek and swirled it round, tipping the pan a little, so that the moving water carried the sand and gravel over the edge. Gold is far denser than sand, so the flakes and nuggets sank to the bottom of the pan and stayed behind. Densities: gold 19.3 g in every cubic centimetre, sand 2.65 g.

Exit tickets

Cut into four. Students answer on their own.

Teacher note: the answers are on this lesson’s plan.

Show what we know

The last lesson brings the term together. Students plan how to take every part out of a jar of water from a beach: the floating seaweed, black iron sand, ordinary sand, the dissolved salt and the water itself. For each step they name the method and the property it uses, and they draw the particles before and after distilling. Then each student sits the final check on their own, and the class finishes with the unit quiz on the board.

We are learning to

• plan a separation of a new mixture, choosing each method from a property,
• put the steps in an order that works, and explain the order,
• show what we know about pure substances and mixtures on our own.

Success criteria

• My plan takes out every part, names the property for each step, and puts the steps in an order that works.
• I can back each answer on the final check with a property, a particle picture or a reason.

You need

• the beach sample task (next sheet) and the final check (third sheet), one each per student,
• the interactive unit open for the closing quiz.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the task and the partner check in Session A, and the final check and the class quiz in Session B.

Watch for these ideas

• A filter step for the salt. Ask: what would a filter catch, and what would pass through?
• The magnet after filtering. Ask: where would the iron sand be by then?
• Evaporating at the end. It gets the salt back, but the plan also asks for the water; ask how to keep both.

Make it easier, make it bigger

• Easier: give the four methods in a jumbled list, and ask students to put them in order and add the properties.
• Bigger: the jar also holds a film of oil from a boat. Add it to your plan, and say where it goes in the order.

Answers and marking guide

• Task: 1. skim or sieve off the seaweed (it floats, and its bits are big); 2. a magnet in a bag lifts out the iron sand (magnetism); 3. let the sand settle, decant, then filter (sand sinks and is insoluble); 4. distil the salty water (only the water boils off and condenses; the salt stays in the flask). Particle pictures: the salty water has water particles touching and jumbled, with salt particles scattered among them; the collected water has water particles only. Last question: the magnet must come before the filter, or the iron sand is caught with the ordinary sand; and distilling, not evaporating, keeps the water as well as the salt.
• Check: 1. distilled water. 2. A pure substance has only one kind of particle; a mixture has two or more kinds, mixed but not joined. 3. add water and stir. 4. Dissolved substances pass through a filter, so the clear filtrate may still hold dissolved salts; a dried drop would show them.
• Check: 5. travels a different distance up the paper. 6. Distil it: heat the salt water so the water boils off (boiling point) and cool the vapour so it condenses into a clean container, leaving the salt behind. 7. Filtered first, the iron filings are caught on the paper together with the wet sand, and would have to be dried and pulled out of the sand afterwards; the magnet first takes the iron out cleanly, in one step. 8. Stir the mixture into water: the sawdust is less dense than water and floats, so it can be skimmed off, while the denser sand sinks.
• Bigger: let the oil settle into a layer on top and lift it off with a dropper (density, and oil not mixing with water), before or after the seaweed but before anything is heated.
• Scoring: 1 mark each for 1, 3 and 5; 2 marks each for 2, 4, 6, 7 and 8. The rubric near the front of the pack turns the task and the check into a level.

A jar from the beach

A jar of water from a rock pool holds: floating bits of dried seaweed; black iron sand that a magnet picks up; ordinary sand; salt, dissolved in the water. Plan how to get each part out on its own, and the water as well.

A. The plan

In each box write the step, what it takes out, and the property it uses.

B. Particle pictures

C. The order

Show what we know

Show what you know about pure substances and mixtures. Read each item, then circle or write.

Teacher note: the marking guide is on this lesson’s plan; the rubric near the front of the pack turns the score into a level.