Water, H2O is a polar molecule (not symmetric) and has Hydrogen bonding (due to OH). There is a rule that says like dissolves like. Therefore, molecules that are polar will dissolve in water, and those that are not polar, will not. For example, O2 is a non polar molecule and will not dissolve in water. CH3OH is a polar molecule and will therefore dissolve in water.
Ionic compounds (metal+nonmetal) will also dissolve in water and dissociate into ions.
For example, NaCl is very soluble in water.
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