Teaching notes
Read before the lesson. Hidden from students when teacher mode is off.
The one thing to protect
Do not give them the method. The challenge is stated, the equipment is on the bench, and they work it out. Filtering then evaporating is not a difficult sequence to reason toward if you have seen sand and seen a kettle — but it has to be theirs, because in Week 5 they plan an investigation with nobody holding their hand and this is the rehearsal.
Why 50 mL and 8 g
- 50 mL is the most that will evaporate inside the lesson. The original plan said 100 mL — that would take the best part of an hour and the lesson would die waiting.
- 8 g is enough that a realistic loss of half a gram to a gram is actually readable on 1 g scales. Any less and the losses vanish into rounding, and the yield discussion has nothing to work with.
- 8 g in 50 mL is comfortably below saturation, so nothing crystallises out on the filter paper while they work.
The two moments that matter
1 · When the first basin comes off dry
There is white solid in the bottom of a dish that had clear liquid in it. Hold it up. Ask what it is and how they could check. Taste is not available — but they can look at the crystals, and they can weigh them. This is the answer to the question they were left with last week.
2 · When nobody gets 8 g back
Every group comes in under. The instinct is to feel they did it wrong. The move is to ask whether the salt was destroyed — no, it was measured badly or left behind. That distinction, between a chemistry problem and a measurement problem, is the whole of the error criterion in the assessment.
Method notes that decide whether this works
- Weigh the empty basin first. Recovered salt = final mass minus empty mass. Groups that forget this have no result.
- Use a sand bath. The basins are round-bottomed, so on the flat frypan they touch at one point, heat poorly and rock. Put 1.5–2 cm of dry sand in the pan and twist each basin down into it. That gives contact across the whole curve, holds the basin steady, and buffers the heat so the porcelain is not shocked and the salt does not spit. Not a water bath — that caps at 100 °C and roughly doubles the time.
- Take the basin off while a damp crust remains. Residual heat finishes it. Left too long, the salt spits and you lose some over the side.
- Let the basin cool before weighing. A hot dish sets up convection over the pan and gives a false low reading. Five minutes on a heat mat. This is worth naming out loud as a source of error rather than just enforcing.
- You are the only person who touches the frypan. Basins placed and removed by you with tongs.
What to expect
| Recovered | Yield | What it means |
|---|---|---|
| 7 g | about 88% | Typical good result. Losses on the filter paper and in transfer. |
| 6 g | about 75% | Common. Solution left in the beaker or soaked into the filter paper. |
| 5 g or less | under 63% | Something substantial was left behind. Ask where. |
| 9 g or more | over 100% | Not dry. Water is being weighed as salt. Back on the heat. |
The over-100% result is the most useful one if it happens. It cannot be right, everyone can see it cannot be right, and working out why is better thinking than any correct answer on the sheet.
On the resin content
Misconceptions to expect
| Misconception | How it shows today | Response |
|---|---|---|
| Filtering separates dissolved salt | Expecting the salt to be caught in the filter paper with the sand | Look at the filtrate — it is clear but it is not water. The salt went straight through because it is in pieces far smaller than the holes. |
| A low yield means the salt was destroyed | "We lost some salt" | It is not lost, it is somewhere. On the paper, in the beaker, spat over the side. Name the place. |
| Evaporating is the same as boiling away the salt | Expecting the basin to end up empty | Only the water leaves. The salt cannot become a gas at these temperatures. |
| The white solid might be something new | Reasonable question, actually | Good question. How would you check? It weighs about what went in, it looks like salt, and nothing was added. Weak evidence individually, strong together. |
What comes next
Lesson 4 turns the heat up. Same equipment, but this time the heating goes past the point where anything can be recovered. Carry forward the yield numbers — they are referred to in CP1 as evidence students collected themselves.
Overview
Last week you said the salt was still in there. Today you have to prove it — and you will be weighed.
What we are doing today
You get a beaker with 8 grams of salt, 50 millilitres of water and a spoonful of sand, all stirred together. Your job is to get the salt back — all of it, dry, in the dish. It gets weighed at the end.
Nobody tells you how. Half the groups use filter paper in a funnel, half tip the liquid off the sand carefully. Then we find out which method lost less salt.
Nobody gets all 8 grams back. The interesting part is working out exactly where yours went, and being able to name the step it happened at.
Shape of the lesson
| Phase | Time | What happens |
|---|---|---|
| Rust check | 5 min | Week 2 reading. Photograph, rate, record. |
| The challenge | 8 min | 8 g of salt, 50 mL of water, a spoon of sand. Get the salt back. No method supplied. |
| Plan and critique | 12 min | Groups draft a method, then read someone else's and find the flaw. |
| Separate and heat | 15 min | Sand out by filtering or decanting, liquid onto the frypan. Must happen by the 40-minute mark. |
| Resin | 12 min | While it evaporates — a reversible change chosen deliberately, for thousands of years. |
| Weigh and work out the yield | 18 min | Basins cooled, weighed. How much came back? |
| Which method won? | 20 min | Filtering against decanting. Both lost salt — in different places. |
If you are teaching this cold
The whole lesson in order. Each step has its own tab above with the detail.
| Step | You do | Students do |
|---|---|---|
| 1. Rust check 5 min |
Logs out. Five minutes. Let them notice that jar B is ahead, but do not start explaining it — that is Week 5. | Photograph, rate all three jars, top up to the marked line, record. |
| 2. The challenge 8 min |
Give each group 8.0 g of salt dissolved in 50 mL of warm water with a spoonful of sand in it. Tell them to get the salt back, dry, in the dish, and that it will be weighed. Supply no method. | Work out what they are actually being asked to separate, and what is dissolved and what is not. |
| 3. Plan and critique 12 min |
Assign half the groups filtering and half decanting — they do not choose. Watch for the group that forgets to weigh the empty basin. Keep this to twelve minutes or the basins will not dry in time. | Draft a numbered method on a whiteboard, swap with another group, write one question about a step that would not work, then fix their own. |
| 4. Separate and heat 15 min |
Sand bath in the frypan, on before the lesson. Place and remove basins yourself with tongs. Heating has to start by the 40-minute mark. | Weigh the empty basin, separate the sand out by their method, and get the liquid into the basin. |
| 5. Resin 12 min |
Runs while the basins evaporate. Watch the pans while you talk — pull any basin that is down to a damp crust. | Learn how First Nations peoples used plant resin as a glue that can be softened and reset, and answer the four questions on section 7 of their sheet. |
| 6. Weigh and work out the yield 18 min |
Basins off the heat and cooling for five minutes before they go near the scales. A hot basin gives a false reading. | Weigh the basin with the salt in it, subtract the empty mass, and compare with the 8.0 g they started with. Nobody gets it all back. |
| 7. Which method won? 20 min |
Both columns on the board. Filtering usually beats decanting, which surprises them. Push for the exact place each method lost its salt. | Name where their own method lost salt. Not "we were not careful" — which step, and why. |
Everything you need
Prepare before the lesson
- Weigh 8.0 g of salt per group into labelled cups — do this accurately, it is the number everything is measured against
- Make up the mixtures: 8 g salt + 50 mL warm water + 1 tsp sand per group, stirred
- Put 1.5–2 cm of dry sand in the frypan and turn it to medium ten minutes before the lesson
- Fold filter papers into cones ready to go
- Print the recovery record sheets, both versions
- Clear a heat-resistant cooling area away from the bench traffic
Per group of four
- Porcelain evaporating basin
- Filter funnel and two filter papers (filtering groups)
- Three beakers — mixture, separated liquid, and a spare (decanting groups need the extra)
- Stirring rod
- Scales — 1 g resolution is fine
- Whiteboard and marker for the method draft
- Safety glasses for every student
Teacher bench
- Electric frypan with a 1.5–2 cm sand bath, on medium — you operate it
- Tongs for placing and removing basins
- Heat-resistant mat for cooling
- Heat-resistant gloves
- Marked exclusion line, one metre
- Cold running water available
Consumables per group
- Salt — 8.0 g, accurately weighed
- Warm water — 50 mL
- Sand — about a teaspoon
- Filter papers — two, filtering groups only
- Paper towel
Printed sheets
- Recovery record sheet — now includes the plant resin questions (section 7) — one per student
- Supported version — method steps to sequence
- Exit ticket slips
- Rust log (already with the jars)
Not needed today
- The 0.01 g scales — Week 5
- Thermometers — Lesson 4
- Zip-lock bags, bicarbonate, vinegar — finished with
- Test tubes and racks
What we are learning
Learning intention
We are learning to recover a dissolved substance, and to account for how much of it we get back.
I will be successful when I can…
- separate an insoluble solid from a mixture by filtering
- recover a dissolved solid by evaporating the water
- compare what I started with to what I recovered
- give a reason for the difference between the two
Stretch
- I can work out my recovery as a percentage of what I started with.
- I can propose one specific change to the method that would improve it, and say why.
AC9S6I02 — plan and conduct repeatable investigations, including describing potential risks and planning for the safe use of equipment.
AC9S6I05 — recognise possible sources of error and select evidence to support reasoned conclusions.
Your goal for today
Rust check 5 min
Week 2 reading. Photograph, rate, record. Five minutes.
Rate all three jars separately against the laminated scale. Top the open jars back up to the marked water line. Write anything you notice in the last column.
The challenge 8 min
In this beaker there is 8 grams of salt, 50 millilitres of water, and a spoonful of sand. They are all mixed together.
Get the salt back. All of it, dry, in the dish. You will be weighed.
Two ways to get the sand out
Half the class will use one, half the other. We are going to find out which one loses less salt.
Pour the whole mixture through a paper cone in a funnel. The sand stays on the paper, the liquid runs through.
Let the sand settle to the bottom, then carefully pour the liquid off it into a second beaker. No paper at all.
Before you plan
- Which of the three things in that beaker is dissolved, and which is not?
- Why does that difference matter for getting the salt out?
- A funnel on its own, with no paper in it — would that separate anything? Why not?
- Which method do you think will lose more salt, and where would it be lost?
- What is the one thing you have to do first, and why does the order matter?
Show the answers
1. The salt is dissolved. The sand is not — you can still see it sitting there.
2. Anything still in visible pieces can be caught by a filter or left behind when you pour. Anything dissolved travels with the water wherever the water goes.
3. No. A funnel is just a cone that directs liquid into a narrow opening. It has nothing in it to hold anything back, so the sand would pour straight through with the water. The paper is what does the separating.
4. Open question — that is what we are finding out. Filtering loses salt in the solution soaked into the paper. Decanting loses salt in the liquid you have to leave behind so the sand does not come with it.
5. Get the sand out first, then evaporate. The other way round and you end up with salt and sand mixed in the dish, and no way to separate them.
Plan and critique 12 min
Draft it — 6 minutes
On your whiteboard, number the steps for your method. Someone from another group has to be able to follow it.
Swap — 6 minutes
Pass your whiteboard to another group. Read their method as if you had to carry it out.
Write one question about a step that would not work, or that you could not follow. Not a compliment. A question.
- Is there a step missing?
- Is anything in the wrong order?
- Is there a point where salt could get left behind without them noticing?
Get your board back. Fix what they found. Then you get your equipment.
Support sheet — correct order
The steps are printed scrambled. The numbers below are the rows as they appear on the sheet, top to bottom.
Filtering — 5, 3, 12, 9, 11, 1, 6, 2, 8.
Weigh the empty basin · fold the filter paper · pour the mixture through the filter · pour the liquid into the basin · hand it to the teacher · wait for it to evaporate · cool for five minutes · weigh the basin with the salt · subtract the empty basin mass.
Decanting — 5, 7, 9, 11, 1, 6, 2, 8. The same, with row 7 in place of the two filtering steps.
The two that belong to no method — row 10, stir the sand until it dissolves (sand does not dissolve, which is the entire reason the method works) and row 4, pour the salt down the sink (the salt is what we are recovering). A student who crosses out anything else has given you the conversation worth having.
- Number the steps on the support sheet, in order for your method.
- Then check your order against a partner's.
- Draft the method from scratch.
- Write one critique question on another group's.
- Predict how much salt you will get back, in grams, before you start.
- Name the step where you expect to lose the most.
Separate and heat 15 min
Move quickly. The water needs as long as possible on the heat.
Before anything goes in it. Write the number down. Without this you have no result at all.
Filtering: fold the paper into a cone, sit it in the funnel, funnel over the empty beaker. Pour slowly and keep the liquid below the top of the paper — overfill it and liquid runs down the outside and misses the paper entirely.
Decanting: let the sand settle completely first. Then tip slowly and steadily, and stop the moment sand starts moving toward the lip. Do not chase the last drop.
Pour your separated liquid into the weighed basin. It should be clear.
Get every drop across. Anything left in the beaker is salt you will never see again.
Bring your basin to the line and hand it over. The teacher sets it into the sand and takes it out.
Then sit down. It takes about twenty-five minutes.
Before you sit down
- Where is the sand now?
- The liquid that came through is clear. Is it just water?
- Look at what you left behind. Is any of it wet? What is in that liquid?
- Why did the sand get separated out but the salt did not?
Show the answers
1. Filtering — on the paper. Decanting — in the bottom of the first beaker.
2. No. It is a salt solution. Clear does not mean pure, and you proved that last week when the mass did not change.
3. Yes — the paper is soaked, or there is liquid sitting on the sand. That liquid is salt solution, and every millilitre of it is salt you are not going to recover.
4. The sand is in pieces big enough to be caught by paper or to sink to the bottom. The dissolved salt is spread through the water in pieces far too small for either. Both methods sort by size — and dissolved things are below the cut.
While it evaporates 12 min
A reversible change that people chose on purpose, and used for thousands of years.
Across Australia, First Nations peoples made adhesives from plant resin — including resin from spinifex and from grass trees.
The resin was worked with heat. Warm it and it softens. Let it cool and it sets hard. It holds a stone point onto a spear shaft, a blade into a handle, a barb onto a hook.
The part that matters
Years later, out on Country, a long way from anywhere, the tool breaks.
You do not need a new one. You warm the resin again, it softens again, you reset the point, and it hardens again. The same material, as many times as you need.
- Is softening and resetting resin a reversible or an irreversible change? How do you know?
- Which change we have studied is it most like — melting chocolate, or toasting bread? Say why.
- Why would being reversible be exactly the property you would want in a glue?
- Superglue cannot be undone. When would that be better, and when would it be worse?
Show the answers
1. Reversible. Nothing new is made — the same resin goes soft and hard, over and over.
2. Melting chocolate. A change of state driven by heat, with the substance unchanged.
3. Because it can be repaired anywhere, with a fire, using the glue that is already on the tool. An irreversible glue means a broken tool is finished, and you need new materials to replace it.
4. Irreversible is better when you never want it to come apart — a boat, a building. It is worse when the thing will need maintaining, or when you are a long way from replacements.
Weigh it 18 min
Work it out
| Mass (g) | |
|---|---|
| Basin with dry salt in it | |
| Empty basin (from step 1) | |
| Salt recovered | |
| Salt we started with | 8 |
| Difference | |
| Our method (filter or decant) |
- How much salt did you get back?
- How much is missing?
- Look at what is in the dish. What is it, and how do you know?
- Did the missing salt get destroyed?
Show the answers
1–2. Most groups recover 6 to 7 grams. Everybody is under 8.
3. Salt. It looks like salt, it appeared where a salt solution used to be, nothing was added, and it weighs roughly what went in. No single one of those settles it — together they are strong.
4. No. Mass does not vanish — you proved that last week. It is somewhere in this room: on the filter paper, in a beaker, spat over the side of a basin.
- Fill in the table row by row.
- Subtract: what was in the dish, minus the empty dish.
- Complete the table and answer all four questions.
- Work out your recovery as a percentage of 8 g.
- Whose method got the highest percentage, and what did they do differently?
Which method won? 20 min
Both columns go on the board
Filtering groups on one side, decanting groups on the other. Nobody got 8 grams.
| Method | Typical recovery | Where the salt went |
|---|---|---|
| Filtering | 7.4 – 7.8 g | Soaked into the paper and the wet sand on it. |
| Decanting | 6.1 – 7.4 g | Left behind in the liquid sitting on the sand. |
- Which method got more salt back?
- Was that what you predicted? Why did you think the other one?
- Name the exact place each method loses its salt.
- Two groups used the same method and got different numbers. Does that mean one did it wrong?
- What one change would improve either method?
Show the answers
1. Filtering, usually by half a gram to a gram.
2. Most people predict the paper loses more, because you can see it getting wet. But a filter paper only holds a millilitre or two. Decanting makes you abandon five to ten millilitres, or the sand comes across with the liquid.
3. Filtering — in the paper and the damp sand on it. Decanting — in the liquid you deliberately left in the first beaker.
4. No. Different hands, different pouring, different patience. A different number is a reason to ask what they did differently, not to assume somebody failed.
5. Rinse. Pour a little clean water through the paper, or over the settled sand, and add that to the basin too. It sounds like cheating. It is not — the salt in that rinse was always yours, and you are only adding water, which is going to evaporate anyway.
Words we used today
Exit ticket
Two groups both started with 8 g of salt. The group that filtered got 7.5 g back. The group that decanted got 6.5 g. Neither group did anything wrong. Explain how that is possible.
Sorting the exit tickets into piles
Ready to move on — names where each method loses salt and says mass is not destroyed. Bonus for noting the decanting group had to leave more liquid behind.
Watch in Lesson 4 — knows nothing was destroyed but cannot name a place, or says one method is "just better" without a mechanism.
Target directly — says the decanting group must have made a mistake, or that a gram of salt was destroyed. Both mean the Lesson 2 idea has not carried, and CP1 is two weeks away.
Next week
Same frypan, more heat. This time the heating goes past the point where anything can be brought back — and your job is to say exactly when that happened.
Resources
Print before the lesson. Everything here also lives in the repository as a separate file.
PDF All student sheets Every sheet below in one file.