A chemical reference book says that substance x is not soluble in water

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