Freezing of water at 273K (1atm) and boiling of water at 373K (1atm) - Are they spontaneous processes? If yes, then why? Cuz their S>0? How?

The entropy change of the system does not determine spontaneity, but the entropy change of the universe does. The concept of Free energy change of the system allows you to determine spontaneity.

Delta G = Delta H - T Delta S

For the freezing of water at 273K, the process is exothermic (Delta H < 0), and the entropy of the system decreases (Delta S<0). At 273, Delta G = 0, and there is an equilibrium between solid and liquid water. So, the process is neither spontaneous nor non-spontaneous at 273K. At 272K, the reaction is spontaneous, but at 274K, it is not.

In the same way, at 373 K, the free energy change for H2O(l) <--> H2O(g) is 0. In this case, Delta H >0 and Delta S>0. But again, AT the boiling point, you have an equilibrium. Now, at 374K, the reaction is spontaneous.

Hope this helps.