Distinguish reaction indicators from physical changes that can look similar.
Method and worked examples
Possible reaction indicators include gas formation, a new solid, unexpected colour change and energy transfer. A precipitate is a new insoluble solid formed by a reaction in solution. An identified new product is stronger evidence of chemical change than appearance alone.
Similar observations can have physical causes. Boiling gives water-vapour bubbles; a dissolved gas can escape without being newly formed. Mixing dyes can change colour without reaction. Existing sand settling from water is not a precipitate. Read whether the gas or solid formed through reaction or was already present. A visible clue alone does not classify every possible change.
Example 1
Two clear solutions react and a new insoluble solid appears. Name the indicator.
Precipitate formation. The new solid is a reaction product rather than a solid already suspended in the starting mixture.
Example 2
Bubbles form throughout boiling pure water. Do they alone prove a chemical reaction?
No. They are water vapour formed by a physical change.
Watch out:Bubbles can be a reaction gas, but boiling and release of dissolved gas can also produce bubbles physically.
Recognising reaction evidence | Year 8 Science | Ascent Arcade