Once upon a time, a group of friends lived happily. They all played together at recess. You could even say they were synthesized.

They were as close as

NaCl

But one day, the group broke up. Their friendship decomposed.

Just like

NaCl -> Na + Cl

Billy noticed the tension in the group and stayed away for a while.

Mr. Gnome and Blue fairy are a couple and this fight was no big deal. They got back together soon. But then Blue Fairy noticed Hansel. She left Mr. Gnome for Hansel. This is like a single replacement.

Mg +Zn(NO3) 2 ------> Mg(NO3)2 +Zn