Question

What is "the thing" in the structure of a compound that decides if it is going to be soluble in water or not?

Answer 1

It must be Ionic, or have a metal and a non-metal

Explanation:

Here is the way I learned it:
There are 4 types of bonds:
Ionic
Metallic
Network Covalent
Molecular Covalent

Although a few molecular covalent compounds can dissolve, all Ionic bonds can dissolve, and are therefore soluble in water. :)

But as Truong-Son pointed out, there are some exceptions. Example: `AgCl` is a metal and nonmetal- but is is poorly soluable. So always double check!

I hope I answered you question, but if not, feel free to ask more!