Teaching notes
Read before the lesson. Hidden from students when teacher mode is off.
The one thing to protect
Students must commit a number in writing before anything is weighed. Not a guess said aloud, not a show of hands — a number on paper with their name on it. The learning comes from the gap between what they wrote and what the scale says, and if they never committed, there is no gap.
The corollary: do not tell them mass is conserved. That sentence is the answer to today's question and it is theirs to reach.
The two moments that matter
1 · When the salt result comes in
Most of the class will have predicted lighter. The scale says the same. Do not resolve it quickly — sit in it. Ask: "so where is the salt?" Someone will say it is still in the water. Ask how they could prove that. Park the answer; Lesson 3 is them proving it.
2 · When the two containers disagree
Same reaction, same ingredients, two containers. The sealed bag holds its mass; the open beaker loses about three grams. Students have to account for the difference themselves. The answer — that something left the open one — is only available to a class that has already accepted the gas is a real substance with real mass.
Method that decides whether this works
- Weigh the whole system, including the containers. Beaker of water AND the cup of salt both go on the scale together. Tip the salt in, then put the empty cup back on. Same total. This is the same logic as the sealed bag, so use it in both — consistency between the two activities is what lets students transfer the idea.
- Cover the beaker. Warm water in an open container loses measurable mass to evaporation inside twenty minutes. A drifting reading in a lesson about conserved mass is the worst possible confound. Cling film or a lid.
- Keep everything under 400 g. The scales max out at 500 g including the container.
- Let the fizzing finish. The visible reaction stops in about a minute but degassing continues. Swirl gently, wait five minutes, then reweigh.
Reading the results
| What they do | Expect | What it tells you |
|---|---|---|
| Salt into water | No change | Nothing was destroyed. Dissolving is a reversible change. |
| Sealed bag fizzing | No change, bag inflates | The gas is inside and it weighs something. Nothing was created either. |
| Open beaker fizzing | About 3 g lighter | Something left. It went into the room. |
Groups will not all land on exactly 3 g — expect 2.2 to 2.9. Some carbon dioxide stays dissolved in the liquid, and a little is lost as spray. That spread is useful: it is the first time this unit that students see real measurements disagreeing, and it previews the error discussion in Week 5.
On precision
Misconceptions to expect
| Misconception | How it shows today | Response |
|---|---|---|
| Dissolving destroys the substance | Predicting the salt solution will weigh less than the total | The scale. Then ask how they would prove the salt is still there — and let Lesson 3 answer it. |
| Gas is nothing | "It turned into nothing" when the bag is discussed | The inflated bag is clearly full of something, and the sealed mass has not dropped. It is in there and it weighs something. |
| Mass is created by the reaction | Predicting the fizzing container will get heavier | Neither container gains. One holds, one loses. |
| Bubbles always mean a new substance | Carried over from Lesson 1's boiling water card | Boiling makes bubbles of the same stuff. This makes bubbles of something that was not there before — and you can tell because the mass drops. |
| More decimal places is more accurate | Writing 118.00 g from a whole-gram scale | Precision belongs to the instrument. Record what it actually gives you. |
What comes next
Lesson 3 answers the question this lesson opens. Students are told the salt is still in the water; next week they have to get it back out and weigh it. Carry forward the class's own sentence about where the mass went, and the criteria chart.
Overview
The scales come out. Two things that look like disappearing turn out to be nothing of the sort.
What we are doing today
Stir salt into water and it looks like the salt has gone. Mix vinegar and bicarbonate of soda and the bubbles look like they came from nowhere.
Today the scales settle both. You weigh everything before, then after, and find out that nothing vanishes and nothing appears out of thin air.
Then you do the same fizzing reaction twice. Once sealed in a bag, once in an open beaker. Same two ingredients, but the scales say something different each time.
Working out why is the whole lesson.
Shape of the lesson
| Phase | Time | What happens |
|---|---|---|
| Rust check | 5 min | Week 1 reading. Photograph, rate, record. No discussion. |
| Predict | 10 min | Salt into water. Everyone commits a number in writing. |
| Dissolving test | 20 min | Weigh the whole system, dissolve, weigh again. |
| Fizzing test | 25 min | Two containers, same reaction. One sealed, one open. |
| Summarise | 20 min | Where did the mass go? Amend the criteria chart. |
| Review | 10 min | Exit ticket. |
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 |
Hand out the coverage logs. Keep it to five minutes and do not let the discussion start — one week of data is noise. | Find their jars, photograph all three, rate each one 0 to 4, top up to the marked water line, record. |
| 2. Predict 10 min |
Show 20 g of salt and 100 g of water. Ask what the whole lot will weigh once the salt has dissolved. Every student writes a number before anything is weighed. | Commit a number in writing. No hands up, no discussion first. |
| 3. Dissolving test 20 min |
Weigh the whole system — beaker, water, and the cup of salt. Tip the salt in, stir, cover, then weigh again with the empty cup back on the scales. The mass is the same. | Weigh, dissolve, weigh again. Compare against their prediction and write down whether they were right. |
| 4. Fizzing test 25 min |
Two containers. A: 50 mL vinegar and 2 g bicarbonate sealed in a 2 L freezer bag. B: 100 mL vinegar and 6 g bicarbonate in an open beaker. Weigh each before and after. | Run both. The sealed bag stays the same mass and blows up. The open beaker loses about 3 g. |
| 5. Summarise 20 min |
Put every group’s open-beaker loss on the board. Expect a spread of about 2.2 to 2.9 g, not one clean number. Add "the mass changed" to the criteria chart. | Explain where the missing mass went. Nothing was destroyed — a gas left the beaker. |
| 6. Review 10 min |
Collect the exit tickets. | Answer: a sealed jar of fizzing tablets weighs the same before and after, and a student says nothing happened. Are they right? |
Everything you need
The full kit, so you are not going back to the plan.
Prepare before the lesson
- Decant vinegar into labelled bench bottles — 150 mL per group
- Pre-weigh bicarbonate into small cups: 2 g and 6 g, one pair per group
- Fill jugs with warm water
- Check every scale switches on and reads zero
- Print the prediction sheets, both versions
- Have the criteria chart from Lesson 1 on the wall
Per group of four
- Scales — 1 g resolution is fine today
- 250 mL beaker, plus cling film or a lid
- Tall beaker or conical flask for the open fizz (limits spray)
- 2-litre freezer zip-lock bag — not a sandwich bag
- Two small cups for pre-weighed bicarbonate
- Measuring cylinder, stirring rod, tray
- Safety glasses for every student
Consumables per group
- Salt — 20 g
- Warm water — 100 mL
- White vinegar — 150 mL total (50 sealed, 100 open)
- Bicarbonate of soda — 8 g total (2 g sealed, 6 g open)
- Paper towel
Whole class
- Criteria chart from Lesson 1 — on display
- Rust jars, accessible for the weekly reading
- Laminated rust coverage scale
- Board space for pooling group results
- Spill kit at each bench
Printed sheets
- Mass prediction and result sheet — one per student
- Supported version — tick-box predictions, pre-filled table
- Exit ticket slips — six to a page
- Rust log (already with the jars from Lesson 1)
Not needed today
- The 0.01 g scales — Week 5 only
- Frypan, kettle, thermometers — Lessons 3 and 4
- Funnels and evaporating basins — Lesson 3
- Any heat source at all
What we are learning
Learning intention
We are learning to use mass to work out whether anything new has been made.
I will be successful when I can…
- use the scales accurately and record to the precision they actually allow
- make a prediction, then say honestly whether my evidence supported it
- explain why the mass stays the same when something dissolves
- explain why the mass drops when a gas escapes from an open container
Stretch
- I can predict the mass loss in grams before it happens, and account for the difference.
- I can explain why writing more decimal places than my instrument gives is an error, not extra care.
AC9S6I03 — use equipment to observe, measure and record data with reasonable precision, using digital tools as appropriate.
AC9S6I01 — pose investigable questions to identify patterns and test relationships and make reasoned predictions.
Your goal for today
One thing you want to get better at. Nothing here is saved or sent anywhere.
Rust check 5 min
Week 1 reading. Five minutes, then we move on.
Photograph your three jars. Rate each one 0 to 4 against the scale. Write it in the log. Rate all three separately — do not average them in your head.
- Collect your log sheet and find your jars. Do not move them.
- Photograph all three from the same angle as last week.
- Rate each jar against the laminated scale.
- Top the open jars back up to the marked water line, and tick the log to say you did.
- Write anything you notice in the last column.
Predict 10 min
On the scale: a beaker holding 100 g of warm water, and next to it a cup holding 20 g of salt. Together they read a number.
Now the salt gets tipped in and stirred until it vanishes. The empty cup goes back on the scale.
What will the scale read now?
- Heavier, lighter, or the same?
- Write the actual number you expect.
- Write why. One sentence.
- Choose from heavier, lighter or the same.
- Then finish: "I think this because…"
- Write a number in grams.
- Write one sentence of reasoning.
- Predict the fizzing results too, both containers, before you see either.
Dissolving test 20 min
Safety glasses on. Everything stays on the bench.
Method
- Put the beaker of warm water on the scale. Put the cup of salt on beside it. Record the total.
- Take both off. Tip the salt into the water and stir until you cannot see it.
- Cover the beaker.
- Put the beaker back on the scale — and the empty cup with it. Record the total.
Then answer
- What did the scale read before? What did it read after?
- Was your prediction right? Write what actually happened.
- You cannot see the salt any more. So where is it?
- How could you prove it is still there?
Show the answers
1. The same number, both times. Small differences of a gram or two come from splashes and from the scale rounding.
2. Most predictions will have been lighter. The honest answer is "I thought it would be lighter and it was not."
3. Still in the water, in pieces far too small to see. Nothing was destroyed — it was spread out.
4. Get it back out. Take the water away and see whether the salt is still there. That is next week.
Fizzing test 25 min
Same reaction, two containers. Watch what the scale does differently.
50 mL vinegar in one corner of the freezer bag. 2 g bicarbonate in the other corner. Seal it, keeping them apart.
Weigh the whole sealed bag. Record.
Tip it so they mix — without opening it. Let it finish.
Weigh the whole sealed bag again. Record.
100 mL vinegar in the tall beaker. 6 g bicarbonate in its cup beside it, on the scale.
Weigh both together. Record.
Tip the bicarbonate in. Put the empty cup back on the scale. Leave it open.
Swirl gently. Wait five minutes. Weigh again. Record.
What happened?
| Container | Mass before | Mass after | Difference |
|---|---|---|---|
| A — sealed | |||
| B — open |
- Which container changed mass?
- Both had exactly the same reaction happening. Why did only one change?
- The bag puffed up. What is inside it, and does it weigh anything?
- Where has the missing mass from container B actually gone?
Show the answers
1. Only B, the open one. It loses roughly 2 to 3 grams.
2. In A nothing could get out. In B the gas escaped into the room. The reaction was identical — the container was not.
3. A gas, made by the reaction. It was not there before. And it weighs something, which is why A did not get lighter — the gas is still on the scale.
4. Into the air in the classroom. It has not been destroyed and it has not become nothing. It has moved somewhere we cannot weigh.
- A partner reads the scale aloud while you write.
- Use the table with the steps already filled in.
- Run both containers, record all four masses.
- Answer all four questions.
- Predict B's loss in grams before you run it, then explain the difference from what you got.
- If you did it again with half the bicarbonate, what would change?
Summarise 20 min
Pool the numbers
Every group's container B loss goes on the board.
- Are they all the same? Should they be?
- What could make one group's number different from another's?
The two big ideas
Nothing disappears
When salt dissolves, the mass does not change. The salt is still there — spread through the water in pieces too small to see. Nothing was destroyed, so nothing new was made. That is a reversible change.
If it got lighter, something left
When the mass drops in an open container, a substance has escaped — usually as a gas. That gas was not there before, which means something new was made. That is an irreversible change.
Back to the chart
Last week you listed the evidence that something new has been made. Add the one nobody suggested:
The mass changed.
- Where does this belong on our list — near the top or near the bottom?
- Is it stronger evidence than colour change? Why?
Where this is going
Mass is stronger than colour, smell, or bubbles, for one reason: it is a number. Two people looking at the same brown liquid can disagree about how brown it is. Two people reading the same scale cannot.
It is not perfect — the spread on the board shows that. But it can be checked, repeated, and compared between groups in a way that "it looked different" never can.
Words we used today
Review 10 min
Three things to take away
- Dissolving does not destroy anything. The mass proves it.
- If a sealed container keeps its mass, nothing has left — even if the reaction looks dramatic.
- If an open container loses mass, something has escaped, and it was made by the reaction.
Exit ticket
A sealed jar of fizzing tablets weighs exactly the same before and after the fizzing stops.
A student says: "That proves nothing happened."
Are they right? How do you know?
Sorting the exit tickets into piles
Ready to move on — says the student is wrong, and explains that the gas is still inside the jar so it still counts on the scale. Bonus if they mention that opening it would drop the mass.
Watch in Lesson 3 — says the student is wrong but the reason is thin: "because something did happen" or "because we saw bubbles". Right conclusion, no mechanism.
Target directly — agrees with the student. These students have not yet accepted that a gas is a substance with mass, and Lesson 3's recovery work will not land until they do. Worth two minutes each next week.
Think back
You wrote a goal at the start. Did you get there? Nothing here is saved.
Next week
You said the salt is still in the water. Next week you have to prove it — get it back out, and weigh what you recover. You will be given eight grams. Nobody gets eight grams back.
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. 5 pages