Oxygen, which is one of the chemical elements, is necessary for us to breathe.

This chemical reaction is very dangerous.

There is a possibility that an earthquake may happen.

She had two apples.

Hydrogen is the most abundant element in the universe.

He is conducting an experiment to synthesize a new compound.

This chemical reaction occurs by the synthesis of two elements.

He is majoring in chemistry at university.

This chemical reacts when heat is added.

This chemical reaction is very dangerous.