Why does water change phase ?

 As seen practically in daily life, water can exist in form of solid, liquid or gas (vapour state). But as obvious at it seems, water does behave in some surprising ways, taking different amount of thermal energy for different phase change applications, or even for increase in temperature for the same phase.

Now why it really happens, the physics behind it on a molecular scale we will try to figure out in this blog, without sounding too technical and trying to remain at a layman level , which is principally my purpose of this blog to help people understand concepts without burdening them with terms and formulae.

If we think of a constant pressure application.. like for example having pressure as atmospheric pressure at for example 40C, water exists in liquid phase. At this temperature water is just at ease with itself and not really looking forward to vaporisation. Now when we increase thermal energy input to water, like for example through adding heat energy through a burner, temperature of water keeps on increasing.

Now this temperature is nothing but increase in average kinetic energy of water molecules... As we add more heat , more energy is transferred to molecules and more they collide with each other like balls on billiard board... and hence more kinetic energy they will have, in the same sense of kinetic energy that we learned in school. With increase in this average kinetic energy, we see in our instruments that temperature is increasing.

This follows this logical pattern till water reaches temperature of around 100 C, when it seems it has made up its mind that it can vaporise now. But what we see here is that water starts vaporising, that is it changes state from liquid to vapour state, but somehow its temperature does not increase ! (remains fixed at 100C).

Logically thinking adding thermal energy into water should increase the temperature (average kinetic energy of molecules) too, but it seems all this additional thermal energy is going somewhere else rather than to change kinetic energy of molecules.

Now when we start to think of it, let's think what happens in water in vapour state, apart from more kinetic energy of molecules, the major difference against liquid state is that molecules are more far apart from each other. Its just like I am running a race with my Son, who is another molecule, and while our speed is increasing , distance between us is also increasing... which we can say we changed into gaseous state.

This state at 100C, at which energy goes into change to vapour state is called latent heat. Basically they called it latent heat was because it was "latent" in the sense this additional energy could not be detected by instruments through a temperature reading with the reasons explained above.

Now this temperature remains at 100C and our water will be in mix of two phases (liquid and vapour), till all liquid is converted into vapour state... or "steam" that is generally called in day to day life.

Then we have this steam (with molecules having high K.E. and long distances between molecules), is basically a high energy state, which in turn is used in daily life to drive many applications like turbines, power plants, heating applications etc.

In order to ensure that this steam does not condense back to vapour state, it is mostly "superheated" by adding more thermal energy to ensure it remains in steam/ vapour state and can be used to drive these real world applications.

Hope I could help to increase some of your understanding on this topic. Thank you.