Question

how do you decide whether a change is physical or chemical

Answer 1

if the change is reversible, then it is a physical change and on the other hand if the change cannot be reversed, it is a chemical change.

eg: melting of a wax candle - this is a physical change as the candle again be made melting the wax and moulding it into any certain shape you want it to be in.

:burning of paper - this is a chemical change because the paper cannot be made again with its ashes.

Answer 2

Physical-- reversible Chemical--- irreversible