ACARA v9 CONTENT DESCRIPTION “compare physical and chemical changes and identify indicators of energy change in chemical reactions”
Builds on the particle model of matter and on changes of state. Here that model draws a clear line between two kinds of change: physical changes, which only rearrange the same particles, and chemical changes, which make an entirely new substance and come with signs of an energy change.
Two kinds of change
A physical change alters the form of a substance without making anything new. Melting, dissolving and tearing all leave you with the same substance, so they can usually be undone. A chemical change makes a brand-new substance with new properties. Burning, rusting and cooking produce something that cannot easily be turned back.
Physical or chemical?
Pick an everyday change. The test is simple: does a brand-new substance appear, or is it the same stuff in a new form?
Solid water becomes liquid water. It is the same substance, just in a new state, and freezing turns it straight back to ice.
A physical change at the particle level
When a solid melts or a liquid boils, the particles themselves do not change. Adding energy moves them further apart and lets them move more freely; removing it packs them back together. The count and the kind of particle stay exactly the same, which is why a physical change can be reversed.
A physical change, particle by particle
Watch the very same particles as you add and remove energy. Count them: the number never changes.
In a solid the particles are packed close in a fixed pattern, only vibrating on the spot. They are the same particles you will see in every other state.
A chemical change at the particle level
In a chemical change the bonds between atoms break and the atoms re-join in a new pattern. The product is a new substance with its own properties. No atoms are created or destroyed, so the totals are conserved, but the new arrangement is not easily taken apart, which is why chemical changes are hard to reverse.
A chemical change, particle by particle
Here the bonds themselves change. Step forward and watch old bonds break and new ones form into something different.
We begin with the reactants: pairs of joined atoms. Nothing has reacted yet, so each pair is still its starting substance.
Clues that a reaction has happened
We cannot see bonds breaking, so we look for indicators: a colour change, bubbles of a new gas, a solid precipitate forming in a clear liquid, a change in temperature, or the giving off of light. No single clue is absolute proof, because a physical change can mimic one of them, but seeing several together is strong evidence that a chemical reaction has occurred.
Clues that a reaction has happened
Toggle the signs you might observe. No single one is proof, so watch how the evidence stacks up.
No clues are showing yet. On their own, none of these signs is absolute proof, but together they build a strong case that a chemical reaction has happened.
Every reaction is an energy change
Breaking and forming bonds always involves energy, so every chemical reaction takes energy in or gives energy out. An exothermic reaction releases energy to its surroundings and the temperature rises; an endothermic reaction absorbs energy and the temperature falls. A change in temperature is therefore one of the clearest indicators of a reaction.
Energy change: heat given out or taken in
Both reactions start at the same temperature. Run each one and read the thermometer to see which way the energy flows.
Both reactions begin at the same 20 degrees. A change in temperature is one of the clearest signs that energy is flowing, so run each type and compare.
Why this matters
Telling a physical change from a chemical one helps you read the world: knowing whether melted chocolate can be set again, why iron rusts, or why a cold pack goes cold. Spotting the indicators and the direction of the energy change is the start of understanding the reactions behind cooking, fuels, batteries and living cells.
Quick self-check
1. Which one of these is a physical change?
2. What is the surest sign that a chemical change has happened?
3. At the particle level, why is a state change a physical change?
4. Which observation would be evidence of a chemical reaction?
5. A reaction makes the surroundings warmer. This reaction is...