Why do phase changes have to be isothermal processes?

Question

I was wondering if someone can explain simply why phase changes are isothermal processes?

I am studying hydrology and some of our material about snowmelt has to do with phase changes and in this example, water retains a constant temperature when melting/freezing, or boiling/vaporizing. I was wondering why, as the ice melts, why does the temperature not increase as it is changing phase? I am just wondering if there is a chemical/molecular reason that better explains why the heat energy is no longer warming the ice as it changes phase. Like, is all the energy directed to separating the bonds and cannot go to heating?

My background is aerospace/mechanical engineering so I have some thermo experience but the more simple the explanation the better. :)

Answer 1

I was wondering why, as the ice melts, why does the temperature not increase as it is changing phase?

There are two components of the internal molecular energy of a substance: Kinetic energy and potential energy.

Kinetic energy is the energy associated with the motion of the molecules and thus the temperature of a substance.

Potential energy is the energy associated with the separation of molecules and intermolecular forces. (It is also the potential energy associated with chemical bonds).

During phase changes (not involving chemical reactions) heat that is added or removed increases or decreases the separation between molecules and thus increases or decreases molecular potential energy. This occurs without changing molecular kinetic energy. Thus the temperature remains constant. That heat is referred to as "latent heat".

Once the phase change is completed, any heat added or removed results in an increase or decrease in molecular motion, and thus temperature. Those temperature changes are governed by the specific heat of the substance.

Hope this helps.

Answer 2

It is better to say: phase changes can be isothermal. But it is not always the case.

Most steels for example have a crystalline structure called austenite at high temperatures, and ferrite at low temperatures. The process of changing phase is isothermal for a controlled cooling in laboratory, keeping cooling slowly. The temperature of transformation is a function of the chemical composition.

But it is not what happens normally in the industry. When a bar cools after being rolled or forged at high temperatures (in the austenite phase), it is possible that the effect of the phase transformation be only a decrease of the cooling speed (because the transformation is exothermic) at a range of temperatures. During that period the material recrystallize, with ferrite crystals growing until replacing the austenite ones.