He is an expert in chemistry.

He has a very passive personality.

His body showed a rejection reaction to the transplanted organ.

His single word triggered a chain reaction, and everything changed.

I was surprised by his overreaction.

She overreacted to his words.

Nuclear reactions are used for mass production of energy.

He is an excellent chemist who made many discoveries.

He is an expert in petrochemistry.

He is an expert in chemical analysis.