Question

how do we know a chemical change has taken place​

Answer 1

it's physical properties are changed

it's irreversible

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Answer 2

(1) A change in colour often indicates a chemical change.

(2) changes in temperature might also indicate to chemical change.

(3) The formation of a precipitate is a sign that a chemical change has  

     occured.

(4) Combustion reactions are notorious for giving off light.

(5) Some chemical changes produce a gas as a product of a reaction.