The idea that water ice does not change phase ie thaw until +4C is simply wrong.
This is basic Second Law of Thermodynamics stuff.
Water has an equal probability of solid and liquid phases at 0 degrees C under standard (we can argue) conditions of T&P or, more accurately, the Entropy Change ("Delta S Total") is zero at 0 degrees C under standard conditions of T&P say 101.3 kPa et al.
Yes yes that means the Gibbs Free Energy Change Delta G is zero too.
Shifting the parameters of T & P will simply shift the position of the equilibrium.
At any temperature > 0 degrees C, including +4C, the probability of liquid phase water existing represents a positive entropy change over that of solid phase water. ie Liquid is more probable.
In simple language ice melts at > 0C
and it's crucial to grasp that there is no single "switch temperature", its a continuous shift in equilibrium position and probability that's going on.
Do the sums, try different values.
Delta G = Delta H minus [T(in Kelvin) x Delta S of the particles]
Or if you prefer
Delta S Total = Delta S of the Particles plus Delta S of the surroundings.
Delta H Fusion water is +6 kJ/mol
Entropy of Liquid phase water is 63 Jk1 mol 1
Entropy of Solid phase water is 41 Jk1 mol 1
CW 