ACARA v9 CONTENT DESCRIPTION “compare reversible changes, including dissolving and changes of state, and irreversible changes, including cooking and rusting that produce new substances”
Builds on knowing that materials can change in many ways. Here we split changes into two groups: reversible changes, which can go back to the start, and irreversible changes, which make a new substance and cannot easily be undone.
Two kinds of change
When something changes, ask one question: can it go back? If the starting material can return, the change is reversible. Melting ice, freezing water and dissolving sugar are all reversible, because the same substance is still there and can be recovered. If a brand new substance forms and you cannot get the original back, the change is irreversible. Cooking an egg, burning toast and rusting iron are all irreversible. Telling these two groups apart is one of the most useful ideas in chemistry.
Sort the changes
Pick a change, then drop it into reversible or irreversible. A reversible change can go back to how it was.
A reversible change can be undone: the stuff you started with comes back, like melting ice that can freeze again, or sugar that dissolves and can be recovered. An irreversible change makes a new substance and cannot easily go back, like a fried egg, burnt toast or rusty iron. Sort each one and check whether it can return to the start.
Dissolving: it looks gone, but it is not
When sugar dissolves it seems to disappear, but the sugar is still in the water, spread out so finely that you cannot see it. Nothing new is made. The proof is simple: let the water evaporate and the sugar is left behind, exactly the same as before. Because you can get the starting material back, dissolving is a reversible change, not the making of something new.
Dissolving sugar is reversible
Step through dissolving sugar, then evaporate the water. The same sugar comes back.
When sugar dissolves it seems to vanish, but it is still there, spread out evenly through the water. No new substance is made. If you let the water evaporate, the sugar is left behind, exactly the same as before. Because you can get the sugar back, dissolving is a reversible change.
Changes of state go both ways
Melting, freezing, boiling and condensing are changes of state. Ice melts to water and water freezes back to ice; you can do this over and over. The substance is always water, just arranged differently as a solid or a liquid. Since no new substance is ever made and the change can swap back and forth, every change of state is reversible.
Changing state is reversible
Melt the ice and freeze it again. It is the same water (H2O) every time, both ways.
Melting and freezing are changes of state. Ice melts into water and water freezes back into ice as many times as you like. It is the very same substance, water, in both forms, so no new substance is ever made. Because it can swap back and forth, a change of state is reversible.
Cooking makes something new
Heating food can change it for good. A raw egg has a clear, runny white; cooking turns that white into a firm solid with completely new properties. This is a new substance, and no cooling will turn it back into a raw egg. Cooking, like burning, is an irreversible change because it produces new substances that cannot return to the original.
Cooking an egg is irreversible
Fry the egg and watch the white turn solid. Then try to undo it: you cannot get the raw egg back.
Heating an egg changes its clear, runny white into solid white. This is a brand new substance with new properties, and no amount of cooling will turn it back into a raw egg. Because a new substance forms and the change will not reverse, cooking is an irreversible change.
Rusting: a slow, one-way change
Iron left in damp air slowly combines with oxygen and water to make rust, a flaky, orange-brown material that is nothing like shiny iron. Rust is a new substance, and it will not turn back into iron by itself. Even though it happens slowly, rusting is an irreversible change, which is why we paint and oil iron to keep oxygen and water away.
Rusting is irreversible
Let time pass and watch iron, oxygen and water slowly make rust. Try to undo it: the rust stays.
When iron meets oxygen and water, it slowly combines with them to make rust, a flaky orange-brown substance that is completely different from shiny iron. Rust is a new substance with new properties, and it will not turn back into iron on its own. Because a new substance forms and the change does not reverse, rusting is irreversible.
Why this matters
Sorting changes into reversible and irreversible helps you understand the world and stay safe. You can refreeze melted ice or recover dissolved salt, but you cannot un-cook food or un-burn fuel, and you have to protect metal from rust because it will not undo itself. Asking can it go back is the key idea, and it sets you up for the chemistry of reactions in the years ahead.
Quick self-check
1. What is the difference between a reversible and an irreversible change?
2. Why is dissolving sugar in water a reversible change?
3. Ice melts into water and water freezes back into ice. What does this tell you?
4. Why is frying an egg an irreversible change?
5. Rusting happens when iron combines with which things to make a new substance?