# Homework Help: Question about Critical Temperature and Interface between Liquid and vapour

1. Aug 3, 2011

### Abir Sarkar

While Studying 'surface tension' I came across a statement that 'At critical Temperature the interface between liquid and its vapour disappears' without any explanation or logic. Can Anyone please tell me why such phenomenon is observed at critical temperature ?

Last edited: Aug 3, 2011
2. Aug 5, 2011

### ardie

not sure what you mean by that, emphasis on the word interface

3. Aug 5, 2011

### Abir Sarkar

As far as I know an interface is a tool and concept that refers to a point/layer of interaction between components/ Different substances. :)

4. Aug 5, 2011

### ehild

In case when some liquid is in a closed vessel and the volume above the surface is filled with the saturated vapour of the liquid, the physical properties (for example density) of the liquid are clearly different from that of vapour. Heating the system, the density of the liquid decreases, that of the vapour increases, together with the increasing pressure. There is less and less difference between liquid and vapour and the difference vanishes at the critical temperature: you get a single phase.

ehild

5. Aug 6, 2011

### Abir Sarkar

Mr.Ehild,
First of all thanks a lot for your reply. The Reference you have given in your reply helps a lot to understand the term "Critical Temperature". Ultimately I just reached to a conclusion that "at Critical temperature [ in general ] The distinction between liquid and vapor disappears as the boiling point is always less than that of critical temperature of that particular liquid" . But now the question arises that is there any possibilities to have any liquid with greater boiling point than that of critical temperature ?

6. Aug 6, 2011

### ehild

Look at the phase diagram: does liquid phase exist above the critical temperature? A gas does not boils.
When a bubble appears inside a liquid, its volume is filled with saturated vapour. The pressure of the saturated vapour depends on temperature. If it is less than the ambient pressure, the bubble collapses. At that temperature where the saturated vapour has the same pressure as the ambient, the bubbles do not collapse, but increase and raise up to the surface of the liquid and leave it. The boiling point of a liquid is defined as the temperature where the pressure of the saturated vapour is the same as the ambient pressure.
There is no liquid and vapour above the critical temperature. The substance does not have defined volume. How would you define boiling point for a gas?

ehild

7. Aug 6, 2011

### Abir Sarkar

oops .. now I just got it :)
Thanks again for your time & consideration and for such great explanation too :)

8. Aug 6, 2011

### ehild

You are welcome.

ehild