Physical Change vs Chemical Change
In 2023, Australian households threw away about 7.6 million tonnes of food — but frying an egg creates something that can never go back to raw, which is very different from freezing water.
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● Know
- Two big families of change exist: physical and chemical.
- Physical change keeps the substance the same; chemical change makes a brand-new substance.
- Everyday examples: melting, dissolving, snapping, burning, rusting, cooking.
● Understand
- The reliable test is the question "did a new substance form?" — not "could you undo it?"
- Reversibility is a useful hint but isn't a perfect rule.
- Both kinds of change can release or absorb energy — heat alone doesn't prove anything.
● Can do
- Classify a change with reasoning that names the evidence you used.
- Spot a wrong classification and explain the missing test.
- Defend a call by listing the observed clues.
Snap a block of chocolate into 10 squares and then melt all of it in a bowl — the chocolate looks completely different, but let it cool and it hardens back into solid chocolate again. Now bite into one square and chew: that chocolate is gone for good, converted into new substances by your digestive system. Those two experiences put your finger on the key test: every change you observe fits into one of two families. The difference comes down to a single question: Did a new substance form? If the answer is no — if the material is still the same stuff, just looking different — then you have a physical change. If the answer is yes, and a brand-new material with new properties has appeared, then you have a chemical change.
Physical changes include melting, freezing, dissolving, bending, stretching and breaking. Chemical changes include burning, rusting, cooking, fermentation and exploding. Do not be fooled by dramatic appearances. Boiling water looks spectacular, but it is still H₂O — physical. Cutting paper looks trivial, but it is still physical because the paper remains paper.
Crushing a can is a physical change because the aluminium is still aluminium — just a different shape. Burning magnesium ribbon in a Bunsen flame is a chemical change because the shiny metal reacts with oxygen to form white magnesium oxide, a completely new substance.
BlueScope Steel operates one of Australia's largest steelworks at Port Kembla. When they roll hot steel into sheets, that shaping is a physical change. When they convert iron ore into iron in a blast furnace, that reduction is a chemical change. Workers must know which family each process belongs to for safety and quality control.
A common mistake is thinking that cutting or breaking something is a chemical change because the object is "destroyed." In fact, cutting paper, snapping a stick or crushing a can are all physical changes. The pieces are still the same substance; they have just been divided into smaller parts. No new material has formed.
In a chemical change, the substances you start with are called reactants. The new substances that form are called products. Reactants go IN to the reaction; products come OUT. You will see this written as reactants → products, where the arrow means "react to make."
This language helps scientists communicate precisely. When methane burns, methane and oxygen are the reactants. Carbon dioxide and water vapour are the products. Even if you do not know the exact chemistry, you should be able to name what went in and what came out. That is the first step to understanding any reaction.
When you bake a cake, the flour, sugar, eggs and baking powder are the reactants. The risen, browned cake is the product. You cannot un-mix the ingredients because chemical reactions have created new flavour and texture compounds that did not exist in the original bowl.
At CSIRO, food scientists study how reactants become products during cooking. By understanding the chemical reactions that create flavour and colour when bread browns (the Maillard reaction), they help Australian food manufacturers produce healthier and tastier products with less waste.
Students sometimes think that all colour changes prove a chemical reaction. This is false. Mixing red and blue paint makes purple, but both paints are still there unchanged — it is a physical mixture. A chemical colour change, like iron rusting from silver to brown, happens because a new substance with different light-absorbing properties has formed.
Classifying changes accurately takes practice. The key is to ignore how dramatic or boring a change looks and focus on whether new substances form. A quiet change like iron rusting is chemical. A noisy change like tearing cardboard is physical. The evidence matters more than the spectacle.
When in doubt, ask three questions. Can I reverse it easily? Did the chemical properties change? Can I see multiple clues of reaction? If any answer points to new substances, you are dealing with a chemical change. If everything stayed the same, it is physical.
Dissolving salt in water is physical because evaporating the water recovers the salt unchanged. Baking bread is chemical because the yeast produces carbon dioxide gas and new flavour compounds that cannot be separated back into flour and water.
The Bureau of Meteorology monitors both physical and chemical changes in the atmosphere. Water evaporating from the ocean is a physical change that forms clouds. Lightning creating nitrogen oxides is a chemical change that affects air quality. Distinguishing the two helps meteorologists predict weather patterns across Australia.
Many students believe that bubbles always mean a chemical reaction. This is not true. Boiling water produces bubbles of water vapour, but no new substance forms — it is a physical change. In a chemical reaction, bubbles might be a new gas such as carbon dioxide or hydrogen, but you need other clues to be certain.
- Melting butter
- Burning wood
- Dissolving salt
- Baking a cake
- Crushing a can
- Physical change
- Physical change
- Chemical change
- Chemical change
- Physical change
Three traps catch students again and again. First, confusing a change of state with a chemical reaction. Ice, water and steam are all H₂O — the particles are simply arranged differently. Melting and boiling are physical, no matter how dramatic they look.
Second, assuming that all irreversible changes are chemical. Shattering glass is physical and irreversible, yet no new substance forms. Third, thinking that all heat release means combustion. Neutralising an acid with a base releases heat, but there is no fire involved. Combustion is just one type of exothermic reaction.
A broken ceramic plate cannot be fixed perfectly, yet breaking it is physical because the ceramic is still ceramic. A copper roof turning green after years of rain is chemical because verdigris (copper carbonate) is a new substance with different properties from pure copper.
Researchers at the Australian Synchrotron in Melbourne use powerful X-rays to study chemical reactions in real time. By watching reactants turn into products at the atomic level, they develop better catalysts for Australian industries, from agriculture to pharmaceuticals.
A widespread misconception is that change of state is chemical because the substance "looks completely different." Steam looks nothing like ice, but both are H₂O. The particles themselves do not change during a physical state change — only their spacing and movement change. Ask yourself: is it still the same substance? If yes, it is physical.
You leave a copper roof outside for 10 years. It turns green. Predict: is this a physical or chemical change?
How close was your prediction?
Nice calibration — your intuition is good for this kind of problem.
Good — being surprised is the point. This answer is worth remembering.
Physical changes reshape, resize or re-state a substance without altering its chemical identity. Melting wax, dissolving cordial, stretching a rubber band and crushing a can are all physical. The key signature is that the original substance could, in principle, be recovered.
Chemical changes create new substances called products from starting materials called reactants. Burning, rusting, cooking, digesting and photosynthesising are all chemical. The products have different properties from the reactants, and simple physical methods cannot reverse the process. This distinction is fundamental to all chemistry.
Sculpting clay into a pot is physical because the clay remains clay. Firing the pot in a kiln is chemical because the intense heat drives reactions that create new mineral compounds, making the ceramic hard and durable. The fired pot can never return to soft clay.
Indigenous Australian fire management involves understanding that burning vegetation is a chemical change that transforms plant matter into ash, charcoal and gases. This knowledge, refined over tens of thousands of years, helps modern land managers conduct controlled burns that protect communities and maintain ecological health.
Some students believe that physical changes are always reversible. While many are, some physical changes are practically irreversible. Breaking glass, shredding paper and grinding wheat into flour are all physical, yet you cannot easily restore the original object. Reversibility is a useful clue, but the defining test is always whether a new substance formed.
At the start of this lesson, you considered why a cooked egg can never go back to being raw — even though a melted ice cube can.
Now that you've studied reactants and products and the rule that "new substance = chemical change," reconsider your original answer. What key idea were you missing — or did you already have it?
1. Which of the following is a physical change?
2. What is produced during a chemical change?
3. Which observation is the best evidence of chemical change?
4. Why is burning a chemical change?
5. Which of these is NOT a reliable clue for chemical change?