Question

how do we know that a chemical reaction has taken place

Answer 1

The presence of any of the following changes helps us to determine that a chemical reaction has taken place.

Formation of new substance(s)

Change in state

Change in colour

Change in temperature

Formation of a precipitate

Evolution of a gas
For example, if on mixing two substances a gas is evolved, then we can say that a chemical reaction has taken place.

Answer 2

either there is any precipitate formation or some gas evolve or by the changement of colour of solution or by effervescence
all these are the signs for the taking place of a chemical reaction
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