why are ions and polar molecules soluble in water?

Answer 1

An excellent guideline for selecting a solvent is that "Like dissolves Like". Polar molecules tend to dissolve best in polar solvents, non-polar molecules prefer non-polar solvents. Water is a polar solvent, and ions are the extreme case of polar species. So ions and polar molecules will, in general, dissolve well in water.

Answer 2

Because H2O is itself a polar molecule that is slightly positive on one end, and slightly negative on the other.