When chemical x is added to a certain liquid, it breaks down into chemicals y and z. it is not possible to simplify chemicals y and z more. why are chemicals y and z elements?

Elements are the same type of atoms, so there is nothing else to separate. Hydrogen has hydrogen atoms, and no other kind. Therefore nothing can be taken out of it anymore. Chemical x is a mixture, made by y and z. Nothing can make up y and z but themselves.