The shape of the lesson
By the end of the lesson, learners can
- Two kinds of change: Ask one question: is there a new substance at the end?
- Signs of a chemical change: In practice you infer a new substance from what you observe.
- New substance formed? Yes: chemical. No: physical.
- Physical: changes of state, dissolving, separating mixtures.
- Chemical: burning, rusting, neutralisation, decomposition.
- Evidence: gas, precipitate, lasting colour change, temperature change.
One picture every idea returns to
A physical change does not produce a new substance: the particles are only rearranged or moved apart, as in melting, boiling, dissolving or separating a mixture, and it is usually easy to reverse. A chemical change produces one or more new substances with different properties, as in burning, rusting or neutralisation. Chemical changes are often accompanied by an energy change and are usually difficult to reverse.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, New substance or not?. Melting ice and burning hydrogen both change what you see. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Two kinds of change | Ask one question: is there a new substance at the end? | Whole class, then pairs |
| 17 to 30 min | Teach and check: Signs of a chemical change | In practice you infer a new substance from what you observe. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Signs of a chemical change, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Dissolving is a physical change: evaporate the water from salt solution and the salt is recovered unchanged. | Do not confuse "it disappeared" with "it reacted". And a colour change alone is not proof: heating anhydrous copper(II) sulfate is a chemical change, but a solution simply getting paler when diluted is not. |
Physical and chemical changes: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State two differences between a physical change and a chemical change.
Classify each change as physical or chemical: (a) wax melting, (b) wax burning, (c) iron rusting, (d) water evaporating from a puddle.
Explain why dissolving sugar in water is a physical change.
Give three observations that could show a chemical change has taken place.
When magnesium ribbon is heated it burns with a bright white flame and leaves a white powder. Explain why this is a chemical change.
A student says that heating ice until it melts is a chemical change because energy is taken in. Explain why the student is wrong.
Answer key and marking notes
- A chemical change forms a new substance; a physical change does not.
- A physical change is usually easy to reverse; a chemical change usually is not (and often has a larger energy change).
- (a) physical
- (b) chemical
- (c) chemical
- (d) physical
- No new substance is formed; the sugar molecules are only spread out among the water molecules.
- The sugar can be recovered unchanged by evaporating the water.
- A gas is given off (fizzing).
- A precipitate forms, or there is a colour change that does not reverse.
- The temperature changes without external heating or cooling.
- A new substance, magnesium oxide (the white powder), is formed.
- Energy is given out as heat and light, and the change cannot easily be reversed.
- Energy changes happen in physical changes too, so an energy change alone does not show a chemical change.
- Liquid water is still H2O; no new substance forms, so melting is a physical change.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which of these is a physical change?
- Aiodine crystals turning to a purple vapour when warmed
- Ba candle wick burning
- Cmilk turning sour
- Dan iron nail rusting in damp air
Which process is a chemical change?
- Aheating calcium carbonate strongly to form calcium oxide and carbon dioxide
- Bseparating sand from water by filtration
- Ccondensing steam on a cold window
- Dgrinding rock salt into a fine powder
A student heats some solid wax until it melts, then lets it cool. The solid that forms has the same melting point as the original wax. What does this show?
- AMelting was a physical change, because the same substance was recovered
- BMelting was a chemical change, because energy was taken in
- CMelting was a chemical change, because the wax changed shape
- DMelting was a physical change, because the wax changed colour
Four observations are made during experiments. Which one is the best evidence that a chemical change has happened?
- AA mixture of two solutions gets warmer on its own and a solid appears
- BA solid disappears when stirred into water
- CA liquid turns into a gas when it is heated
- DA coloured solution becomes paler when water is added
Hydrogen burns in oxygen with a squeaky pop and water is formed. Why is this a chemical change?
- AA new substance, water, with different properties from hydrogen and oxygen is formed
- BA gas is used up
- CIt makes a noise
- DThe water can be boiled back into steam
Which statement about chemical changes is correct?
- AThey form new substances and are often difficult to reverse
- BThey never involve an energy change
- CThey only happen when substances are heated
- DThey change the atoms of one element into atoms of another
A cook makes toffee by heating sugar. The sugar first melts to a clear liquid, then turns brown and gives off a burnt smell. Which describes these two stages?
- Amelting is physical; turning brown with a smell is chemical
- Bmelting is chemical; turning brown is physical
- Cboth stages are physical
- Dboth stages are chemical
Which pair of processes are BOTH physical changes?
- Adistilling sea water and freezing orange juice
- Bdissolving sugar and burning sugar
- Ccrystallising copper(II) sulfate and rusting iron
- Dneutralising an acid and evaporating water
Quiz key
Iodine turning to a vapour (subliming) is a change of state: the vapour is still iodine molecules, and it turns back to crystals on cooling, so no new substance forms. Burning a wick makes carbon dioxide and water, souring milk makes lactic acid, and rusting makes hydrated iron(III) oxide; each of those forms a new substance, so they are chemical changes.
Heating calcium carbonate is thermal decomposition: it forms calcium oxide and carbon dioxide, which are new substances, so it is a chemical change. Filtering only separates a mixture, condensing is a change of state, and grinding only changes the size of the pieces; in each case the substances are the same at the end.
The solid that forms has the same melting point, so it is the same substance: no new substance was made, and the change reversed easily. That makes it a physical change. Taking in energy happens in physical changes too, so it does not prove a chemical change; a change of shape is also not evidence of a new substance, and colour was not the reason given.
A temperature rise with no heating from outside, together with a new solid (a precipitate), shows a new substance forming, so it is a chemical change. A solid disappearing in water is dissolving, boiling is a change of state, and a solution getting paler on dilution is the same solute spread through more water; all three are physical.
Hydrogen and oxygen are gases; water is a new compound with very different properties, so a chemical change has taken place. A gas being used up and a noise are observations that can accompany a reaction, but they are not the defining feature. Boiling water into steam is a separate physical change and says nothing about how the water was made.
In a chemical change atoms are rearranged into new substances, and the change is usually hard to reverse. Most chemical changes do involve an energy change (heat, light or sound), many happen without heating (for example mixing two solutions), and the atoms themselves are not turned into different elements.
Melting sugar is a change of state: cooling it straight away gives solid sugar again, so it is physical. When it turns brown and smells, new substances are being formed (the sugar is decomposing), which is a chemical change that cooling will not reverse. Calling both stages the same misses that only the second makes new substances.
Distilling only separates water from the dissolved salts, and freezing is a change of state, so both are physical. Burning sugar, rusting iron and neutralising an acid each make new substances, so each of the other pairs contains one chemical change alongside a physical one.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.