Hone

Lessons · Teaching · physical change against chemical change

Can you get it back? That is the test

A physical change alters form but produces no new substance, and it can usually be reversed: melting, dissolving, cutting, changing state. A chemical change produces a new substance and is usually not reversible: burning, rusting, cooking, mixing acid with a carbonate.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.

What it is for

'Dissolving sugar is a chemical change' is the error, and it is fixed by one question: can you get the sugar back? Evaporate the water and you can. Burn a piece of paper and the ash will not go back to paper, however long you wait.

How to think about it

Ask whether a new substance exists afterwards, and look for the signals: a gas produced, a colour change that will not reverse, heat or light given off, a solid appearing from two liquids. Then apply the reversibility check as the confirmation.

Worked example

Physical: ice melting, sugar dissolving, paper being torn
The substance is unchanged, and the change can be undone.
Chemical: wood burning, iron rusting, an egg cooking
A new substance exists, and it cannot be undone.
Signals of a chemical change: gas bubbling off, light or heat, a permanent colour change
Any one of these is worth investigating; none is proof on its own.
Dissolving is physical, because evaporating the water returns the solid
The most commonly miscategorised example in the topic.

Your turn

Name the kind of change that produces a new substance.

A  change makes a new substance

The trap

Treating 'it looks different' as the test. A cut apple looks different and is still apple; a browning apple has actually reacted with the air. The test is whether a new substance exists, not whether the appearance moved.

Next in Teaching

Every Teaching lesson on one page

Practise physical change against chemical change on HoneA question on it now, a real problem where there is one, and it is remembered for review. Free, no email needed.