Chemical vs Physical Change
In 1864, French chemist Marcellin Berthelot showed that burning just 12 g of pure carbon always produced exactly 44 g of carbon dioxide — the original carbon was gone forever, transformed into an entirely new substance.
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Q1 · A chocolate bar melts in your hand on a hot day. When it cools, it hardens again. Is that a physical or chemical change? How do you know?
Q2 · When you burn a piece of toast, can you get the bread back? What does that tell you?
● Know
- The definition of a physical change and a chemical change
- That chemical changes produce new substances; physical changes do not
- Five signs that indicate a chemical change has occurred
● Understand
- Why physical changes are usually reversible but chemical changes are usually irreversible
- How the particle model explains the difference between the two types of change
- Why dissolving is a physical change, even though it looks like a chemical one
● Can do
- Classify everyday changes as physical or chemical and justify your answer
- Identify the signs of chemical change in a described scenario
- Use particle-level language to explain what happens during each type of change
- Physical change
- Chemical change
- Reversible
- Irreversible
- Products
- Makes new substances; bonds break and re-form
- New substances formed in a chemical change
- Can be undone to get the original substance back
- Form changes but no new substance is made
- Cannot easily be undone — original substance is gone
You can tear a piece of paper into a thousand pieces. Every tiny piece is still paper — still the same substance, just in a different shape. That's what makes it a physical change: the substance itself doesn't change.
A physical change alters the form, shape or state of a substance but does NOT change what it's made of. The same particles are still present — they've just moved, rearranged or separated.
Physical changes are usually reversible. Common examples:
- Cutting paper — still paper, just smaller pieces.
- Melting ice — H₂O changes from solid to liquid, but it's still H₂O. Freeze it again and you have ice back.
- Dissolving salt in water — the salt seems to disappear, but it's still there. Evaporate the water and you get the salt back. (A common trap: dissolving looks like a chemical change but it isn't.)
- Folding metal — bent but still the same metal.
- Breaking glass — smaller pieces of glass, not a new substance.
At the particle level: In a physical change, the same types of particles are still present. They may be in different positions or arrangements, but no bonds between atoms have been broken and re-formed to make new particle types.
A chemical change (also called a chemical reaction) produces one or more new substances that have different properties from the original. This happens because bonds between atoms break and new bonds form, creating entirely different types of particles — the products.
Chemical changes are usually irreversible (or very hard to reverse). You can't "unburn" wood back into its original form. Common examples:
- Burning wood → ash + carbon dioxide + water vapour. The wood is permanently gone.
- Cooking an egg → the egg proteins change permanently. You can't "uncook" an egg.
- Rusting iron → iron (Fe) + oxygen (O₂) → iron oxide (rust, Fe₂O₃). A completely new substance.
- Acid + bicarb soda fizz (vinegar + bicarb in a volcano) → CO₂ gas produced. New substance formed.
- Burning magnesium ribbon → bright white light + white powder (magnesium oxide). Completely new substance.
At the particle level: In a chemical change, the bonds holding atoms together in the original particles break, and atoms re-join in new combinations to form completely different particles (products). The identity of the atoms themselves doesn't change — only how they are bonded.
| Original particles | Process | New particles (products) |
|---|---|---|
| Iron atoms + oxygen molecules | Rusting | Iron oxide (rust) |
| Carbon compounds in wood | Burning | CO₂ + H₂O + ash |
| Proteins in egg white | Cooking (heat) | Denatured proteins (permanently changed shape) |
When you observe a change, look for these five signs. If you can spot any one of them, a chemical change has very likely occurred:
| Sign | Example | Why it matters |
|---|---|---|
| 1. Unexpected colour change | Iron going brown (rust); copper going green | A new substance with different properties has formed |
| 2. Gas produced | Bubbles when acid meets bicarb; CO₂ when wood burns | A new gaseous substance has been created |
| 3. Precipitate formed | Sudden cloudiness when two clear solutions are mixed | A solid new substance has appeared in a liquid |
| 4. Unexpected temperature change | Hand warmers getting hot; some reactions going cold | Energy being released or absorbed as new bonds form |
| 5. Light or sound produced | Burning magnesium producing bright white light; explosions | Large energy release during the reaction |
Important trap — dissolving: When you dissolve salt in water, no gas is produced, no colour changes, and no precipitate forms. The salt particles just spread out between the water molecules. You can get the salt back by evaporating the water. This is a physical change even though the salt seems to "disappear".
A change produces new substances and is usually . Signs include a sudden change, a being produced, or an unexpected temperature change. A physical change only alters the of the substance.
A student mixes two clear colourless solutions together. Immediately, a bright yellow solid appears in the liquid and the test tube feels slightly warm. Is this a physical or chemical change? List all the evidence that supports your answer.
How close was your prediction?
Great — you spotted the signs of chemical change correctly.
Remember: precipitate + temperature change = strong evidence for chemical change.
For each change below, write whether it is physical or chemical, then give one reason using particle language.
| Change | Type | Particle-level reason |
|---|---|---|
| Crushing an aluminium can | ||
| Burning petrol in a car engine | ||
| Dissolving sugar in your tea | ||
| Milk going sour in the fridge | ||
| Melting butter in a hot pan | ||
| Fireworks exploding in the sky |
Read the following scenario, then answer the questions below.
Hamish is doing a science experiment. He mixes two clear solutions in a beaker. Within seconds, bright orange bubbles start rising and the mixture changes from colourless to dark brown. The beaker also becomes warm to touch.
- List ALL the signs of chemical change visible in Hamish's experiment.
- Is this a physical or chemical change? Explain why in 2–3 sentences using particle language.
- Could Hamish get the original two clear solutions back? Explain why or why not.
At the start of this lesson you were asked: When you cook an egg, why can't you "uncook" it? What has actually changed at the particle level? Think about what you said before the lesson.
Now write a proper scientific explanation of why you can't uncook an egg. Use the words chemical change, irreversible and products — and explain what's happening to the particles inside the egg.
Q1. State whether each of the following is a physical or chemical change and give a reason: (a) bending a wire (b) cooking a sausage (c) mixing sand and water. (3 marks)
Q2. A student mixes two clear solutions and a yellow precipitate forms immediately. Explain whether this is a physical or chemical change and give two reasons. (3 marks)
Q3. Using particle-level language, explain the difference between dissolving salt in water (physical) and burning magnesium (chemical). Describe what happens to the particles in each case. (4 marks)
Answers
▾MCQ 1
C — Melting ice is a state change. The H₂O molecules are still H₂O — only their arrangement and movement changes. No new substance is formed. Freeze it again and you have ice back.
MCQ 2
B — An unexpected gas being produced is a strong sign of a chemical change — a new substance (gas) has been created. Melting, cutting and dissolving are all physical changes.
MCQ 3
C — When iron rusts, iron atoms bond with oxygen atoms to form iron oxide (Fe₂O₃) — a completely new substance with different properties (brown, brittle, flaky). This is a chemical change.
MCQ 4
C — Dissolving sugar in water is a physical change. The sugar molecules spread between water molecules but remain unchanged. You can recover the sugar by evaporating the water. Burning, digesting and souring all produce new substances.
MCQ 5
B — Burning wood is irreversible because the products — ash, carbon dioxide and water vapour — are completely new substances that have different properties from wood. You cannot combine ash + CO₂ + water and get wood back.
Short Answer 1
Model answer: (a) Bending a wire = physical change — the metal atoms are just pushed into a new arrangement; no new substance is formed and the wire is still metal. (b) Cooking a sausage = chemical change — heat causes proteins and fats to react and form new substances; you can't "uncook" a sausage. (c) Mixing sand and water = physical change — the sand particles and water molecules remain unchanged; the sand can be filtered back out.
Short Answer 2
Model answer: This is a chemical change. Reason 1: A yellow precipitate (solid) formed — this is a new substance that was not present in either original solution, proving new particles have been created. Reason 2: The precipitate appeared instantly in a liquid, which is a recognised sign of a chemical change (precipitate formation).
Short Answer 3
Model answer: When salt dissolves in water, the sodium and chloride ions (particles that make up salt) spread out into the spaces between water molecules. No bonds are broken and re-formed to make new types of particles — the ions are still there, just spread out. This is why evaporating the water gets the salt back. When magnesium burns, magnesium atoms react with oxygen molecules: the bonds in O₂ break and new bonds form between magnesium and oxygen atoms, creating magnesium oxide — a completely new substance with different properties (white powder). This is irreversible because the new substance cannot easily be converted back into magnesium and oxygen.
- Physical change — form changes, same substance. Usually reversible. Particles rearrange but no new types form.
- Chemical change — new substances with different properties are made. Usually irreversible. Particle bonds break and re-form.
- Five signs of a chemical change: colour change, gas produced, precipitate, temperature change, light/sound.