Teaching notes
Read before the lesson. Hidden from students when teacher mode is off.
The number that decides whether this works
Three 65 mm nails weigh about 13.8 g. A realistic mass gain over five weeks is:
| Iron converted | Mass gained | On 0.01 g scales |
|---|---|---|
| 0.5% — light rust | 0.03 g | 3 divisions |
| 1% | 0.06 g | 6 divisions |
| 2% — typical Jar B | 0.12 g | 12 divisions |
| 5% — heavy rust | 0.30 g | 30 divisions |
Readable, and comfortably so. On 1 g scales every one of those rounds to zero, which is why the 0.01 g scales matter today and nowhere else in the unit.
Dry properly: rinse briefly under the tap, blot hard with paper towel, then spread on a tray in the sun or in the frypan on low for a few minutes. Leave them until they look and feel completely dry, then leave them a bit longer. Weigh the Jar A dry control alongside — it should read within a division or two of its setup mass. If it has gained, your drying has failed and every other number is wrong.
The two moments that matter
1 · When the first group reads a bigger number
Stop the room. Ask what they expected. Almost everyone predicts lighter, because rust flakes off and looks like decay. Heavier is genuinely surprising. Then: where did the extra mass come from? Nothing was added to a sealed jar. It came out of the air, and it is now part of the nail. That is Lesson 2's logic running in reverse — mass went up, so something joined.
2 · When they get the other class's data
Students will critique another class in ways they will never critique their friends, and they cannot fall back on "I know what we meant." Insist on written critiques before any discussion. The writing is the assessment evidence; the discussion afterwards is where they find out whether they were right.
Running the weigh-in
- Do the drying first, before anything else. Fifteen minutes of lesson time is the drying. Get nails out and onto trays in the first five minutes.
- Tongs or gloves only. Rusted nails are sharp and carry a tetanus risk. Count them out and count them back in.
- One weighing station. Two scales, six groups, three jars each — that is 18 weighings per class. Queue them; do not try to run it in parallel with other work.
- Record to 0.01 g. Two decimal places, every time, including trailing zeros. 13.80 g is a different statement from 13.8 g.
- Vinegar jars will misbehave. Jar E often loses mass, because acid dissolves iron away rather than just oxidising it. That is a real result and worth naming, not hiding.
What the data should show
| Tuesday — what causes rust | Expected |
|---|---|
| B — half-submerged | Most rust. Water and oxygen together at the waterline. |
| D — salt water | Close second. Salt speeds up the reaction. |
| C — fully submerged | Less than B. Limited oxygen. |
| E — vinegar | Erratic. May lose mass as iron dissolves. |
| A — dry sealed | Almost none. No water. |
| F — boiled water under oil | Almost none. No oxygen. |
B beating C is the result that teaches. Students predicted more water means more rust. A and F together establish that rusting needs both water and oxygen — remove either and it stops.
Running CP2
- Groups draft the plan together; each student submits their own.
- The peer critique happens before submission and is written on the proforma. That box is assessable — it shows whether the student could take a criticism and act on it.
- Approve every plan before Week 6. A plan you have not signed off is a group that cannot start.
- Fifteen minutes is enough to draft, not to polish. Finishing at home or in a spare slot is fine.
Misconceptions to expect
| Misconception | How it shows today | Response |
|---|---|---|
| Rusting makes metal lighter | Predicting a loss before the weigh-in | The scale. Rust is made from the iron plus something from the air, and it stays attached. |
| Rust is dirt sitting on the surface | "You could scrub it off" | The nail is heavier than the steel it started as. You cannot scrub something off and end up with more. |
| More water means more rust | Predicting C beats B | The data. Rusting needs water and oxygen together, and the waterline has both. |
What comes next
Lesson 6 is the assessment investigation. Every plan needs signing off before then, and the equipment for each group's chosen variable needs to be available. Check the approved list against what groups have actually asked for.
Overview
Five weeks of waiting ends today. The nails come out, they go on the good scales, and the number is not what anyone expects.
What we are doing today
Five weeks ago you sealed nails in jars. Today they come out and go on the scales.
They weigh more than when they went in. Rust did not take anything off the nail — it added something to it. Working out where the extra mass came from tells you what rust actually is.
Then you graph it. Your own three jars first, then all six conditions from the whole class on one graph, so you can see which ones did the damage. Those graphs go to the other class next week, so they have to make sense to somebody who was not here.
In the last 25 minutes you plan the investigation you run next week.
Shape of the lesson
| Phase | Time | What happens |
|---|---|---|
| The weigh-in | 15 min | Nails out, rinsed, dried, weighed to 0.01 g against the setup mass. |
| Graph it | 30 min | Each group graphs its three jars, then the class pools all six conditions. These graphs are handed to the other class next week, so they have to be readable by someone who was not here. |
| The cross explained | 20 min | Why the nails gained mass. Then why somebody wanted the cross to do this. |
| ★ CP2 | 25 min | Investigation plan drafted in groups, peer-critiqued, submitted individually. |
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. The weigh-in 15 min |
Rusted nails are sharp, so tongs or gloves, counted out and counted back. Have drying trays and the frypan on low ready. Drying is the experiment — one drop of water weighs 0.05 g, which is bigger than the whole result. | Nails out, rinsed, blotted, dried properly, then weighed to 0.01 g against their Week 1 mass. The nails are heavier. |
| 2. Graph it 30 min |
Pool all six conditions on the board once groups have their own three. These graphs go to the other class next week, so push for labels, units and a title. | Graph their three jars, then help build the class graph. Make it good enough for someone who was not here to read. |
| 3. The cross explained 20 min |
Iron plus oxygen makes iron oxide, and the oxygen came out of the air in the jar — that is why the nail is heavier. Then answer the question left hanging since Week 1. | Work out why rust on ordinary steel never stops, and why the cross was built from steel chosen to rust. |
| 4. CP2 25 min |
Summative. Proforma and marking guide come from the Assessment Pack, not this site. Every plan needs approving before next week. | Draft the investigation plan in groups, get one hard question from another group, then write and submit their own. |
Everything you need
Prepare before the lesson
- Check the 0.01 g scales work and read zero
- Set out drying trays lined with paper towel, one per group
- Frypan on low, or a sunny windowsill spot cleared
- Print the graph frames, critique proformas and CP2 proformas
- Have each class's Week 1 setup masses ready to hand back
- Print or display the other class's raw data — with no method attached
- Photograph of the cross, and the client's asset, ready
For the weigh-in
- 2 × scales, 0.01 g resolution
- Tongs or disposable gloves — no bare hands
- Drying trays and paper towel
- Frypan on low (teacher-operated) or windowsill
- Rigid sealed container for nail disposal
- Safety glasses
Per student
- Their group's rust log from Weeks 1–4
- Graph frame (pre-scaled version available)
- Method critique proforma
- CP2 Investigation Plan proforma — from your Assessment Pack
- Ruler, pencil, coloured pencils
Whole class
- Criteria chart from Lessons 1, 2 and 4
- Board space for the pooled six-condition data
- The other class's raw dataset
- Week 1 setup masses for every jar
Tidying up
- Liquids down the sink with running water
- Nails into a rigid sealed container, not loose waste
- Jars washed, inspected, returned or discarded if chipped
- Benches and sink cleaned after disposal
Not needed today
- Frypan for cooking — only for drying, on low
- Funnels, basins, thermometers
- Any consumables: this is a data lesson
What we are learning
Learning intention
We are learning to turn our measurements into a graph that shows what the data actually says.
I will be successful when I can…
- construct a graph that shows what my data actually says
- describe a pattern or trend in the results
- plan an investigation that another group could run
Stretch
- I can tell a real difference from ordinary variation between my three jars.
- I can say which of my six conditions the client actually needs to know about, and why.
AC9S6I05 — recognise possible sources of error, pose questions for further investigation and select evidence to support reasoned conclusions.
AC9S6I03 — use equipment to observe, measure and record data with reasonable precision, using digital tools as appropriate.
Your goal for today
The weigh-in 15 min
Five weeks. Let us find out.
Getting them dry
- Lift your nails out with tongs. Keep each jar's three together and labelled.
- Rinse briefly under the tap.
- Blot hard with paper towel. Then blot again.
- Spread them on your tray and leave them to dry properly.
Why we are being this fussy about drying.
One drop of water weighs 0.05 g. The mass your nails have actually gained might only be 0.06 g. So a single drop left behind could double your result and you would never know. Water is not rust, and the scale cannot tell the difference.
Weighing
- Wait until the nails look and feel completely dry.
- Weigh all three together, exactly as you did in Week 1.
- Record to two decimal places, always — 13.80 g, not 13.8 g.
- Subtract the setup mass. That difference is your result.
What happened?
- Did your nails get heavier, lighter, or stay the same?
- Is that what you expected five weeks ago?
- Nothing was added to your jar. So where did the extra mass come from?
Show the answers
1. Heavier — most conditions gain between 0.03 g and 0.30 g. The vinegar jar may lose mass, because acid dissolves iron away rather than just rusting it.
2. Almost nobody expects heavier. Rust flakes and looks like decay, so lighter feels obvious.
3. Out of the air. Rust is iron joined to oxygen, and the oxygen came from inside the jar. The nail is now iron plus something it collected — which is exactly why it weighs more. In Lesson 2 the mass dropped when a gas escaped. Here it rose because a gas joined.
Graph it 20 min
Your three jars first
Plot your own condition. Three bars, one per jar.
- Are your three bars the same height?
- Should they be? You set all three up identically.
- How different would two jars have to be before you would say it was a real difference rather than just wobble?
Then the whole class
Six conditions on one graph. Use the average of each group's three jars.
- Which condition rusted the most?
- Which two conditions barely rusted at all? What do they have in common?
- Compare the half-submerged jar with the fully submerged one. Which won?
- Is that what you predicted in Week 1?
Show the answers — Tuesday, what causes rust
1. Jar B, half-submerged, usually by a clear margin. Salt water (D) is often close behind.
2. Jar A (dry, sealed) and Jar F (boiled water under oil). A had air but no water. F had water but no oxygen. Neither rusted, and between them they prove you need both.
3. Half-submerged beats fully submerged — which surprises nearly everyone.
4. Most people predicted more water means more rust. The waterline is where water and oxygen meet, and rusting needs both at once. Under the surface there is far less oxygen available.
Show the answers — Thursday, what stops rust
1. The bare control, by a long way. That is the point — everything else was trying to beat it.
2. The galvanised nail and whichever coating stayed intact. A tape or polish coating with a gap in it often does worse than expected, because water gets underneath and cannot dry out.
3–4. The useful surprise is that a damaged barrier can be worse than no barrier at all. That matters for the client's recommendation: a coating only works if it is complete and stays complete.
- Use the pre-scaled graph frame.
- Plot your three jars, then the class averages.
- Build and scale both graphs yourself.
- Write the pattern in words underneath.
- Express the gain as a percentage of the starting mass. Does that change which condition won?
- Is the gap between conditions bigger than the gap between your own three jars? What does your answer mean?
The cross explained 20 min
First — what actually happened to your nails
Iron + oxygen → iron oxide. That is rust.
The oxygen came out of the air in your jar. It is now locked into the surface of the nail, which is why the nail weighs more than the steel you started with. A new substance was made, and it was made from the nail.
- Why can you not scrub rust off and get your nail back to new?
- Rust flakes off and exposes fresh metal underneath. What happens next?
Show the answers
1. Because the rust is not sitting on the nail — it is made of the nail. Scrub it off and you take some of the nail with it. The iron that turned into rust is not coming back.
2. It rusts too. Then that flakes off, and the metal underneath rusts. On ordinary steel this never stops. Given enough time, there is no steel left.
Now — the question from Week 1
The cross at our entrance has been rusting for years. Nobody has ever tried to stop it.
Somebody chose that. Why?
It is made from weathering steel, which has other metals mixed into it. It rusts like any steel — but then something different happens.
Rust forms. It is flaky and loose.
It falls off, exposing clean metal.
That rusts too. It never stops.
Rust forms. It is tight and it sticks.
It seals the surface instead of falling off.
Water and air cannot get past it, so it stops.
- Both are irreversible changes. Why is one a problem and the other a solution?
- So does knowing a change is irreversible tell you whether it is bad?
- What would you need to know to decide?
Show the answers
1. Identical chemistry — iron and oxygen making iron oxide. The difference is what the rust does once it has formed. Loose rust keeps the reaction going. Tight rust shuts it down.
2. No. Irreversible means it cannot be undone. It says nothing about whether you wanted it.
3. What the change is for. Cooking, setting concrete and curing glue are all irreversible and all deliberate. The cross was designed to do this. The gate was not.
Words we have used
★ Investigation Plan 15 min
Assessment. Drafted in your group, critiqued by another, submitted individually.
Next week you run your own investigation, and the results go to [CLIENT NAME].
The shared question is how to protect metal from rusting. Your group picks what to change. Your plan has to be good enough that another group could run it without asking you anything.
What the plan needs
- Your question — naming what you will change and what you will measure
- Your prediction, and the evidence from the last five weeks that led you to it
- Your variables: changed, measured, and at least three controlled
- Your method, numbered
- How many times you will run it, and why that many
- Your risks and what you will do about them
- Your equipment
- What another group asked, and what you changed because of it
Swap your plan
Draft it, hand it to another group, and get one hard question back. Write down what they asked and what you changed. That box is part of the assessment — it shows whether you can take a criticism and use it.
Next week
You run it. Bring your plan — you will be following it, and you will be watched while you do.
Resources
Print before the lesson.
PDF All student sheets Every sheet below in one file.