In case water has gathered enough heat to evaporate and expand to its ambient conditions, without extraneous heat addition, I call this spontaneous water evaporation (a.k.a steam explosion).
Water evaporation takes heat, dependent from its temperature as shown in
https://nl.wikipedia.org/wiki/Verdampingswarmte
(of which samples taken in
column empirische delta H the blue dots in the diagram).
An approximation for the relation I found (check
column "lin comb", the red dots in the diagram):
ΔH(T) = 0,5 × 2324√(648-T) + 0,5 × 10226∜(648-T)
with ΔH: enthalpy in J/mole
and T: temperature in Kelvin
Up to around and about 300 °C water's
specific heat varies isochorically (ΔV = 0) approximately as in
column s.w(T):
s(T) = -0,004232 × T + 5,369 J/gK
Extrapolating water heating behaviour from T=273 K and including heating (from 0 K) and melting of ice, the sponaneous evaporation is expected between
530 K (257 °C) and
540 K (267 °C).
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