At 1 atm and a temperature of 0 celcius which phase of #H_2O# can exist? Why?
Not enough information to define state - can be solid, liquid or gas.
Indefinite answer, because...
The main point is the temperature. At
The change from a liquid to water state has relatively low pressure within the object. It's also possible to relate pressure to the amount of energy needed for water to go through the shift in state, because of specific heat capacity -> more energy means more pressure within the object BUT we also have to consider atmospheric pressure, not necessarily atomic pressure.
In addition, water and ice exerts an equilibrium vapour pressure. Thus, it can also exist as a gas at these lower temperatures.
Hope this helps a little :)