Investigating Physical and Chemical Changes
Melt ice and you still have water. Burn magnesium and you have something that is neither magnesium nor oxygen.
That is the whole distinction between a physical and a chemical change, and once you can see it at particle level the rest of the topic follows, including the reason a chemical reaction can never change the total mass of anything.
The difference, and it is about the particles
| What happens to the particles | What has to be overcome | |
|---|---|---|
| Physical change | The particles are rearranged. The particles themselves do not change | Weak intermolecular forces |
| Chemical change | The particles themselves change. New substances form | Strong chemical bonds |
Water evaporating is physical: the molecules move further apart, but every one of them is still H2O. Magnesium burning is chemical: magnesium atoms and oxygen atoms bond into a white powder that is neither.
The energy difference gives it away. Melting ice takes very little energy, because intermolecular forces are weak. Splitting water into hydrogen and oxygen takes an enormous amount, because covalent bonds are strong.
How to spot a chemical change
- A new substance with different properties
- A colour change that is not just mixing
- A gas given off
- A precipitate forming
- Energy taken in or given out, usually heat or light
- It is usually hard to reverse
None of these is proof on its own. Dissolving copper sulfate gives a dramatic colour change and it is physical. Boiling water gives off a gas and it is physical.
Dissolving salt is the one that catches everybody. It looks irreversible and it feels like something has been destroyed. Evaporate the water and the salt is all still there, so it was physical all along.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 2 |
| Topic | Chemical change |
| Status | Class experiments. Not a formal assessment |
| Marks | 40 on our worksheet. None is prescribed |
Four changes to test, and two of each
| What you do | What you see | Which kind |
|---|---|---|
| Melt ice | Solid becomes liquid | Physical. Still H2O |
| Dissolve salt in water | The salt disappears | Physical. Evaporate and it comes back |
| Vinegar and bicarbonate of soda | Vigorous fizzing | Chemical. A gas is a new substance |
| Burn magnesium ribbon | Brilliant white light, white powder left | Chemical. The powder is not magnesium |
Do not look directly at burning magnesium. Use tongs, never fingers.
Decomposition and synthesis: opposites, with the same two substances
| What it does | Example | |
|---|---|---|
| Decomposition | One substance breaks down into simpler ones | 2H2O2 → 2H2O + O2 |
| Synthesis | Two or more substances combine into something more complex | 2H2 + O2 → 2H2O |
Water is taken apart in one and built in the other, which is a neat piece of design in the syllabus and worth pointing out.
Decomposing hydrogen peroxide
- Set up a test tube with a stopper and a bent delivery tube, the far end under water in a bowl
- Invert a water-filled test tube over the end of the delivery tube to collect the gas
- Put dilute hydrogen peroxide in the first tube
- Add a spatula tip of manganese dioxide and stopper it at once
- Gas bubbles across and pushes the water out of the collecting tube
Test it with a GLOWING splint. It relights. That is oxygen.
A glowing splint, not a lit one. A lit splint is the hydrogen test. Getting the two the wrong way round costs a mark every year.
What a catalyst actually is
A catalyst speeds up a reaction by giving it an alternative route with a lower activation energy, and it is not used up.
The manganese dioxide makes the peroxide decompose fast enough to watch. Weigh it before and after and it is unchanged. That is what "not used up" means, and it takes thirty seconds to prove.
Synthesising water, and the best moment in the topic
- Same apparatus. Zinc granules in the first tube
- Add dilute hydrochloric acid and stopper immediately. Zn + 2HCl → ZnCl2 + H2
- Collect the hydrogen over water
- Close the tube under water with your thumb, turn it upright, stopper it
- Light a splint, remove the stopper, put the splint in
A squeaky pop. The hydrogen reacts with oxygen from the air, with a small burst of light and sound.
And the product is water. 2H2 + O2 → 2H2O. Learners watch this happen and almost none of them notice what has just been made, which makes it a very good exam question.
One test tube at a time, and nothing flammable nearby. The pop is meant to be a pop.
Why mass never changes
In a chemical reaction the atoms are conserved. The molecules are not.
Take the peroxide decomposition. Two molecules of H2O2 go in and three molecules come out: two of water and one of oxygen.
| Before | After | |
|---|---|---|
| Molecules | 2 | 3 |
| Hydrogen atoms | 4 | 4 |
| Oxygen atoms | 4 | 4 |
The molecules were taken apart and rebuilt. Every single atom is still there.
And because atoms have mass, mass is conserved. The law of conservation of mass is not a separate rule to memorise; it follows from the atoms.
Proving it: three reactions in sealed containers
Put the two reactants in separate containers inside a sealed bottle or a bag with the air pressed out. Weigh the lot. Tip them together without opening it. Weigh again.
| Reaction | What you see | Mass change |
|---|---|---|
| Copper(II) sulfate + sodium carbonate | Blue-green precipitate | None |
| Sodium hydroxide + hydrochloric acid | Nothing visible, but the tube warms | None |
| Effervescent tablet + water | Vigorous fizzing | None |
"None" means within the readability of your balance. On a 0,1 g balance, a reading that moves by one increment has not moved.
Now do the tablet again with the lid off
| Mass after | |
|---|---|
| Sealed | Unchanged |
| Open | Lower, typically by 0,2 to 0,5 g |
The open one has not broken the law. The carbon dioxide left the container and took its mass with it. The system was not closed.
That contrast is the whole lesson. A class that only ever sees the sealed version has been told the law. A class that sees both has understood the condition attached to it.
A note on lead, because the standard method uses it
The usual version of this experiment starts by reacting lead(II) nitrate with sodium iodide. It is the golden rain demonstration and the yellow precipitate is genuinely spectacular.
Do not use it. Soluble lead salts are cumulative toxins and reproductive toxicants, they are restricted in school laboratories across most of the world, and the waste cannot go down a drain.
It is also completely unnecessary. The experiment is proving that mass does not change. Any reaction that visibly does something inside a sealed container proves it.
| Instead of | Use | You get |
|---|---|---|
| Pb(NO3)2 + NaI | Copper(II) sulfate + sodium carbonate | A blue-green precipitate. Non-toxic and cheap |
| or calcium chloride + sodium carbonate | A white precipitate. Both food-grade |
If it does not work
| What you see | What caused it |
|---|---|
| No gas collects | The stopper is not sealing. The commonest failure in both gas experiments |
| The glowing splint does not relight | Too little oxygen collected, or the splint was properly alight rather than just glowing |
| No pop | Air got into the collecting tube, or it stood open too long and the hydrogen escaped upwards |
| The pop is a bang | Too much hydrogen. One test tube at a time |
| The peroxide barely fizzes | Old peroxide. It decomposes on the shelf, which is worth pointing out |
| The sealed mass drops | The container is not sealed. Check the gasket, or use a bag with the air pressed out |
| The mass changes by 0,1 g | Within the readability. That is not a change |
| Nothing visible in the acid and base tube | Correct. Feel the tube, it warms |
| Learners say dissolving salt is chemical | Ask them how to get the salt back. They can |
The failure worth keeping
A group whose bottle is not properly sealed will watch the mass fall and conclude that mass is not conserved.
Let them present that. Then ask where the gas went. The mass did not vanish, it left the container, and a wrong result explained properly teaches more than a right one that agreed.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Physical versus chemical, with reasons | 8 |
| Decomposition and the catalyst | 6 |
| Synthesis and the pop test | 6 |
| Conservation of mass, sealed versus open | 12 |
| Atoms conserved, molecules not | 5 |
| Conclusion | 3 |
No mark allocation is prescribed. The worksheet and this split are ours.
The mark most often dropped is explaining the open-container loss. "The mass disappeared" earns nothing. "The carbon dioxide escaped and took its mass with it" earns both.
Close behind: watching hydrogen and oxygen combine with a pop and not writing the equation for what was just made.
If you have time
Weigh the manganese dioxide before and after. Unchanged. That is what a catalyst means, proved in thirty seconds.
Burn steel wool on a balance, in the open. The mass goes up. Oxygen from the air has joined it, so the system was open in the other direction. It is the exact opposite of the tablet and it catches everybody out.
Ask why a rusting car gets heavier. Same reason, and nobody expects it.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the classification table, both gas experiments, the sealed and open mass readings and the atom count
- Marking memorandum, with expected mass changes, how to mark a leaking container, and the six places learners most often drop marks
Related practicals
- Heating and cooling curves, Grade 10. Physical change, measured
- Separating mixtures, Grade 10. Physical separation, same term
- Percentage yield, Grade 11. Where conservation of mass goes next
Buy this experiment
We are putting together a Physical and Chemical Change Kit for Grade 10 with the test tubes, rack, beakers, stoppers, delivery tubes, retort stand, sealed bottles and goggles in one box, plus a printed teacher guide and the marking memo. Coming shortly.
The balance is the item this practical cannot do without. Proving conservation of mass needs 0,1 g readability. A 2 g balance cannot show it, because the change being disproved is smaller than the smallest division. They are in the balances and scales range.
Every chemical in this practical is ordinary store cupboard stock, and the contents card says exactly which ones you need.