Question

how do we know that a chemical reaction has taken place?

Answer 1

There is a change in texture, change in temperature, change in weight, evilution of gas or shape or the substances change to another substance then we told that the chemical reaction has taken place.

Answer 2

The product will be very different in weight and in properties from the original. So by this we will get to know.
Chemical reaction are of different types
Combination reaction
De composition reaction
Displacement reaction
Double displacement reaction.