# How is the temperature constant during evaporation but it has different values depending on the location within the liquid?

During phase change such as water evaporation the temperature is supposed to be constant but it varies within the liquid itself i.e. the highest temperature in heterogenous evaporation is at the bottom, convection currents make the middle part slightly lower in temperature and there's temperature discontinuity near the interface with the vapour. Is it constant overall or after a certain duration perhaps?

• "During phase change such as water evaporation the temperature is supposed to be constant" Who supposes this? Evaporation occurs at all temperatures until the relative humidity above is 100% (or the material turns entirely into a gas). In addition, evaporation results in cooling, so by Fourier's law, a temperature gradient has to exist to conduct heat toward the surface. Commented Apr 18 at 21:28
• Okay let me add some details: If we heat water until it reaches 100 Celsius at 1 atm a phase change will start (evaporation) during this change the temperature will remain constant throughout the process as the heat is latent not sensible. but if we zoom in on the liquid being heated we'll notice the temperature gradient caused by the reasons stated above in the question. So is this statement "temperature is constant during phase change" (you can google it of course) talking about the system overall or after a certain duration? Commented Apr 18 at 22:11
• A constant phase-change temperature is just an idealization, as I discuss here. But even the approximation of a constant temperature where the phase change is occurring doesn’t imply a uniform temperature everywhere else. Commented Apr 18 at 23:01
• "a constant temperature where the phase change is occurring doesn’t imply a uniform temperature everywhere else." This! I get it now, thanks a lot! Commented Apr 19 at 9:43