What Is Change in Matter?
In 2022, Whyalla's steelworks processed over 1 million tonnes of iron — but once that metal rusts orange, has it changed or become something brand new?
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● Know
- Matter can change in lots of observable ways.
- Scientists split change into two big families: physical and chemical.
- Physical change alters how matter looks; chemical change produces a brand-new substance.
● Understand
- The key question is: did a new substance form? — not how dramatic the change looked.
- Several clues hint at chemical change: new colour, gas bubbles, temperature change, precipitate, light or sound.
- Mass is conserved in every change — it never just disappears.
● Can do
- Classify everyday changes as physical or chemical and back it up with evidence.
- Spot when one clue alone isn't enough to call something a chemical change.
- Explain why classifying change matters when scientists design experiments.
Hold a piece of iron wire in a flame and watch it glow red, then leave a nail outside in the rain for a week — the nail turns orange and crumbles while the wire just gets hot. Those two experiments reveal something important: one change creates a new substance and one does not. Scientists sort every change into two big families. Physical changes alter the form, size or state of a substance, but the material underneath stays exactly the same. Ice melting into water, paper being torn, and salt dissolving are all physical. Chemical changes are different: they create one or more brand-new substances with new properties. Burning wood, rusting iron and cooking an egg are chemical because the original material is no longer what it was.
The single most reliable test is to ask: "Can I get the original substance back easily?" If yes, it is probably physical. If no, it is probably chemical. This framework helps scientists predict what will happen next and design safe experiments in labs around the world.
Ice melting in your drink is a physical change because the water can be refrozen into ice. Burning a marshmallow over a campfire is a chemical change because the sugar turns into carbon, water vapour and other new compounds that you cannot turn back into a marshmallow.
CSIRO researchers studying Australian bushfires must distinguish physical changes (heat evaporating water from soil) from chemical changes (vegetation burning and releasing new gases). Understanding this difference helps them model fire behaviour and predict how ecosystems will recover after a blaze.
Some students think matter disappears when wood burns because the pile of ash is so small. In reality, the total mass stays the same. Most of the mass escapes as invisible gases such as carbon dioxide and water vapour. If you collected every atom, the mass before and after burning would be identical.
Most physical changes are reversible. An ice cube melts into water, but you can refreeze it. Sugar dissolves in tea, but evaporate the water and the sugar crystals return. The substance itself never changed identity — only its arrangement or state did.
Most chemical changes are not easily reversible. Once toast burns, you cannot un-burn it. Once iron rusts, you cannot simply peel the rust away to reveal shiny metal underneath. Reversibility is a strong hint, but it is not the final test. Some physical changes are hard to undo too — smashing glass is physical, yet good luck putting the pieces back together perfectly.
Freezing juice into an ice-block is a reversible physical change because the juice returns to liquid when it warms. Frying an egg is a chemical change because the proteins permanently rearrange into new substances with a different colour, texture and taste. No amount of cooling will turn it back into raw egg.
At BlueScope Steel in Port Kembla, engineers recycle scrap steel by melting it down — a physical change that reshapes the metal without altering its chemical identity. In contrast, producing steel from iron ore involves chemical reactions that remove oxygen and add carbon, creating an entirely new material.
Many students believe that any change releasing heat must be chemical. This is wrong. When steam condenses on a cold mirror, it releases plenty of heat — yet it is a physical change because the water was H₂O before and remains H₂O after. Heat alone is not evidence of a chemical reaction.
Scientists do not guess whether a change is physical or chemical — they look for evidence. The most reliable clues include a colour change that cannot be explained by mixing, gas bubbles forming in a liquid, a sudden temperature change, a solid appearing where there was none before (a precipitate), or light and sound being produced.
One clue alone is rarely enough. A single observation might have a physical explanation. The golden rule is to collect multiple pieces of evidence before concluding that a chemical change has occurred. This careful, evidence-based approach is exactly how professional scientists work in research laboratories.
When you dissolve sugar in water, the sugar seems to vanish — but this is a physical change. The sugar crystals break into smaller particles that spread through the water. You can prove this by evaporating the water: the sugar reappears exactly as before. No new substance formed, so it is not chemical.
At ANSTO (Australian Nuclear Science and Technology Organisation), scientists use precise temperature and gas monitoring to track chemical reactions in nuclear medicine production. By measuring multiple clues simultaneously, they ensure that each batch of medical isotopes forms correctly and safely.
Students often think that dissolving sugar is a chemical change because the solid "disappears." In fact, dissolving is physical. The sugar molecules are still there, just spread out among water molecules. You can recover the sugar by evaporating the water, which proves no new substance was created.
Even experienced students fall into the same traps when classifying changes. The first trap is assuming that every colour change means a chemical reaction. Mixing blue and yellow paint makes green, but no new substance forms — it is still just paint. Only a colour change caused by a reaction (such as iron rusting from silver to reddish-brown) counts as chemical evidence.
The second trap is thinking that mass disappears during burning. It does not. The missing mass escapes as gases. The third trap is confusing dramatic appearance with chemical identity: boiling water looks exciting, but it is still H₂O, just in a different state.
A copper pipe left outside slowly turns green. This is a chemical change because the copper reacts with oxygen, water and carbon dioxide to form a new substance called verdigris. Mixing green and blue food colouring to make turquoise is a physical change because both dyes are still present and unchanged.
The Great Barrier Reef Marine Park Authority monitors coral colour changes carefully. Coral bleaching is a physical stress response where corals expel algae, but some colour changes involve chemical reactions in the surrounding water. Distinguishing the two helps scientists track reef health accurately.
Many students believe that a single clue is enough to prove a chemical change. This is false. Boiling water produces bubbles, which looks like gas formation, but it is just a physical change of state. Always look for multiple clues before deciding that a new substance has formed.
Here's a student's working. One line has an error — click it.
- A student says: 'When ice melts into water, the particles break apart into different atoms, so melting is a chemical change.'
Physical and chemical changes surround us every moment. A physical change alters the shape, size or state of a substance without creating anything new. Melting, freezing, dissolving, bending and tearing are all physical. Most can be reversed, though some (like shattering glass) are practically irreversible.
A chemical change produces at least one new substance with different chemical and physical properties. Burning, rusting, cooking and digesting food are all chemical. These changes usually cannot be undone by simple physical means. Recognising the difference is one of the most important skills in science.
Stretching a spring is a physical change because the metal returns to its original shape when released. Leaving an iron nail in salt water creates rust — a chemical change because iron oxide is a new substance with different properties from pure iron.
Indigenous Australians have used controlled burning for tens of thousands of years to manage landscapes. Understanding that burning is a chemical change that alters soil chemistry and plant communities helped them develop sophisticated fire regimes that promote biodiversity and reduce the risk of catastrophic bushfires.
Some students think that all irreversible changes must be chemical. This is not true. Smashing a glass bottle is physical and essentially irreversible, yet no new substance forms. The reliable test is always whether a new substance was created, not whether the change can be undone.
At the start of this lesson, you thought about whether rusting iron really "changes" into something new — or whether it just looks different.
Now that you know the difference between physical and chemical change, look back at your first thoughts. Did you use the idea of "new substance" to decide? How has your thinking about the two big categories of change shifted?
1. Which of the following is an example of matter?
2. Which statement best describes a physical change?
3. What happens to matter during most changes?
4. Which of these is a clue that a chemical change has occurred?
5. Why is understanding change important in science?