He is an expert in petrochemistry.

He is an expert in chemical analysis.

It's difficult to decipher this chemical equation.

Understanding chemical equations is difficult.

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

Understanding chemical equilibrium is very important in understanding chemical reactions.

This substance undergoes a chemical change when heat is applied.

In this experiment, you can observe a chemical reaction.

He is conducting research on natural phenomena.

He has a deep interest in natural history.