What is the sign of the heat change q when 1.0 mol of water vapor at 100∘C condenses to liquid water at the same temperature?
a) q is zero because the temperature is constant during condensation.
b) q is negative because energy is released during condensation.
c) q is positive because the enthalpy of vaporization of water is positive, so the same sign is kept for condensation.
d) q can be positive or negative depending on the external pressure of the system.