Question

what are the chemical reactions, what are they?

Answer 1

Hii friend

when the chemical change takes place in any substance which changes it's chemical properties, then this is know as chemical reaction.
Thanks

Answer 2

chemical reactions are the reactions in which the chemical properties of a substance get changed
A new substance is formed by the reaction has different chemical properties than the one from which it was made
there characteristics are
1) chemical reactions are usually irreversible
2) a new substance is formed with different chemical properties
3) reaction involves a lots of energy
For example-
burning of wood
burning of wood is irreversible(the burnt wood cannot be reversed into the original wood) the burnt wood has different chemical properties like it is weaker, black and different molecular formula.