Signs of a chemical change
"Has a new substance formed?" is the definition. But how do we see that one has? There are several pieces of evidence.
Burning with a flame
The magnesium ribbon burned with a brilliant white flame — that was the observation in activity 16.1 (ii).
Becoming hot
The beaker became hot and the thermometer reading rose when NaOH was added to the acid — activity 16.1 (iii).
Gas bubbles given off
Gas bubbles appeared when magnesium was dropped into dilute acid — activity 16.1 (iv).
A change of colour
The blue of a copper sulphate solution fading is an example — we meet it in section 16.3.
And a fifth — a precipitate forming
When two solutions are mixed and an insoluble solid appears in the solution, that solid is called a precipitate. The formation of barium sulphate is an example.
Evidence is not proof
These are signs, not definitions. Boiling water gives off bubbles too — but they are air and water vapour, not a new substance. Boiling water is a physical change.
Remember
The evidence that a chemical change has taken place: burning with a flame, becoming hot, gas bubbles being given off, a change of colour, and the formation of a precipitate.