# Why is the latent heat of vaporization 0 at the critical point?

This is not a homework question.

The critical point of water is $$\mathrm{374 \ C}$$ and $$\mathrm{22.06 \ MPa}$$. At the critical point the latent heat of vaporization is $$0$$ - why?

Assume the water starts at $$\mathrm{100 \ C}$$.

If you add the heat required to raise the temperature of water by $$\mathrm{274 \ degrees}$$ - using $$\mathrm{4200 \ J / kg}$$, it doesn't add up to the heat required to vaporize water. Each degree of temperature requires $$\mathrm{4200 \ J/kg}$$ so $$374 - 100 = 274 × 4200 = 1$$, $$150$$ , $$\mathrm{800 \ Joules}$$. This is roughly half the latent heat of vaporization required to vaporize a kilogram of water - $$\mathrm{2260 \ kJ/kg}$$.

Even if I use values from an Isobaric specific heat chart which shows the specific heat increasing substantially at higher temperatures, it still doesn't add up. There's still $$\mathrm{~608,000 \ joules}$$ short.

I've read in several studies that higher temperatures decrease the strength of hydrogen bonds. Inversely, low pressures also decrease the strength of hydrogen bonds. Could this decreased strength in Van Der Waals forces be the reason less heat is required to render the heat of vaporization zero at the critical point?

• At the critical point the liquid and the gas phases are identical states of matter, so no external heat is required to turn one into the other. Commented Dec 9, 2021 at 1:01
• Do you want to take a stab at the second part of the question? You are correct in your answer, but I want to understand why that is the case if there isn't enough heat to vaporize the water. Commented Dec 9, 2021 at 1:16
• Related post by OP: physics.stackexchange.com/q/681567/2451 Commented Dec 9, 2021 at 11:46
• Both the latent heat and the specific heat are temperature dependent, so the 4,200 J/Kg is not valid at the higher temperatures. Commented Dec 9, 2021 at 12:43
• Mike Stone - So, the strength of hydrogen bonds decrease as temperature increases? Do you know where I can find that documented? Also, if possible, where can I find documentation or studies that show this and / or that hydrogen bonds weaken with decreased pressure. Commented Dec 9, 2021 at 16:19