Question

how can you say that chemical reaction has taken place

Answer 1

The following can indicate that a chemical change has taken place, although this evidence is not conclusive:

Change of odor.

Change of color (for example, silver to reddish-brown when iron rusts).

Change in temperature or energy, such as the production (exothermic) or loss (endothermic) of heat.

Answer 2

Chemical reaction, when it takes place their is either evolution of some games or some smell comes out. Even colour change, precipitation, takes place. Sometimes even sound is heard.. All these happen during chemical reaction(because of formation of new substance).. Thus, chemical reaction has occurred can be identified.