[automagp68]

So i came upon what i think is a classical thermo problem to ask students

hich process requires more energy: completely vaporizing 1 kg of saturated liquid water at 1 atm pressure or completely vaporizing 1 kg of saturated liquid water at 8 atm pressure?

My inclination was to say 8 atm given the fact that the boiling point would be much higher at 8 atm then at 1 atm requiring more energy in the form of heat to make the phase change

however i now know the answer is wrong

Can anyone explain to me why its 1 atm?

[mjc123]

What is the water saturated with? Or do you mean water in equilibrium with saturated vapour?
Are you talking about boiling? A liquid can vaporise below its boiling point.

My inclination was to say 8 atm given the fact that the boiling point would be much higher at 8 atm then at 1 atm requiring more energy in the form of heat to make the phase change

If you're talking about the heat of vaporisation at the boiling point, you don't take into account the energy used in heating the liquid to the boiling point in the first place. (The question as asked is ambiguous here.)
So, we're comparing ΔH_vap at 100°C with ΔH_vap at the boiling point at 8 atm. Do you know a formula for how ΔH varies with temperature?

[Enthalpy]

The latent heat of vaporisation is smaller at higher temperature.

You can remember this because it vanishes at the critical point. This implies that the liquid has a bigger heat capacity than the vapour.

You can understand the heat capacity of a liquid as the (whole?) capacity of the gas plus the heat absorbed in separating the molecules more often at higher temperature. Separating them only at some contact points and for a limited time, since they remain liquid - but if the pressure allows to reach the critical temperature as a liquid, then all the necessary heat has already been given to make a gas.