Why boiling occurs at fix temperature?

[Arvind22]

Can anyone explain to me why does temperature of water remains constant while boiling in an open vessel or in general sense why phase transition is isothermal in nature?please give me answers from thermodynamic point of view also.

 

[Bystander]

Boiling in an open vessel is ordinarily at constant pressure. This is the thermodynamic constraint for constant temperature boiling of a single component system.

 

[russ_watters]

How could it be anything but fixed? Suppose you boil away half of the water in a pot. What is different about the remaining water from what you started with, that would cause the boiling temperature to be different?

 

[mrspeedybob]

How could it be anything but fixed?

I suppose one could ask why phase transition is a discrete transition. The OP asked why phase transition is isothermal. I could re-state that question to ask why does a liquid absorb heat and convert to a gas at the same temperature? Why does the liquid not simply absorb more and more heat, and become less and less dense, and gradually change properties from those of a liquid to those of a gas as it's temperature rises?

 

[russ_watters]

I suppose one could ask why phase transition is a discrete transition. The OP asked why phase transition is isothermal. I could re-state that question to ask why does a liquid absorb heat and convert to a gas at the same temperature? Why does the liquid not simply absorb more and more heat, and become less and less dense, and gradually change properties from those of a liquid to those of a gas as it's temperature rises?

So, that question boils down to what is a liquid/what is a gas/why are they different states of matter?

They are different because in a liquid the molecules are loosely chemically bonded together and in a gas, they are not. Chemical bonds carry a specific amount of energy and given the same conditions always require the same energy to break.

 

[Arvind22]

I suppose one could ask why phase transition is a discrete transition. The OP asked why phase transition is isothermal. I could re-state that question to ask why does a liquid absorb heat and convert to a gas at the same temperature? Why does the liquid not simply absorb more and more heat, and become less and less dense, and gradually change properties from those of a liquid to those of a gas as it's temperature rises?

Perhaps i could not phrase my question well so i am writing the exact statement of a textbook which is like this
An equilibrium process in the course of which the system's temperature remains constant is known as an isothermal (constant-temperature) process. An example of an isothermal process is the boiling of pure water in an open vessel: until all the water boils out of the vessel, its temperature remains practically constant (provided atmospheric pressure does not change in the process of boiling).now my question is why in this case temperature remains constant.

 

[russ_watters]

What have you learned from the responses you have gotten so far?

 

[Arvind22]

Ok guys.i got the point.thank you all of you for helping me.