The normal dividing line for soluble/not soluble is 0.1 M.
If a substance, as a saturated solution at room temp and pressure, cannot form a solution concentration of 0.1 M, then it is considered insoluble.
The 0.1 M figure was selected because _most_ substances are either well above that value or well below. Some substances (calcium hydroxide is one? not sure) come close to the 0.1 M dividing line but there are only a very few. but most likely it wont mix
