Answer:
The presence of
any of the following changes helps us to determine that a chemical reaction has
taken place.
(i) Formation
of new substance(s)
(ii) Change in state
(iii)
Change in colour
(iv)
Change temperature
(v)
Formation of a precipitate
(vi)
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.
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