This chemical reaction is very dangerous.

He always helps me when I'm in trouble.

Whenever he is feeling down, I always help him.

He found out that there was a foreign object inside his body.

This enzyme helps with chemical reactions in the body.

This chemical reaction is very dangerous.

This chemical reacts when heat is added.

He is majoring in chemistry at university.

He is an expert in chemistry.

My biological clock is completely messed up.