Why does atmospheric pressure not affect SVP?

[Roshan Patel]

I'm an anaesthetist... I have exams coming up... I use lots of gases and vapours and need to appreciate the associated physics.

My question relates to saturated vapour pressure and ambient pressure...

SVP increases with temperature - more molecules have sufficient energy to escape the surface/increased temp increases rate of endothermic reactions.

When a substance is below its SVP it behaves more or less as an ideal gas such that reducing volume/increasing pressure increases the partial pressure of the vapour within the mixture in accordance to Dalton's law.

What I do not understand is why at SVP pressure has no effect. I have seen this time and again but no explanation is given..

I would kindly appreciate clarification on this.

Thanks

 

[Chestermiller]

SVP comes into play when the partial pressure in the gas phase reaches the SVP, in which case liquid can begin forming. Otherwise, the gas phase behaves just like an ideal gas mixture. For the SVP to be involved, you have to have liquid present or you have to have liquid on the verge of being present.

Chet

 

[Roshan Patel]

Hi Chet,

Thanks for your response.

I think I get it - at SVP any further increase in pressure will cause liquification of some of the gaseous phase but the vapour remains saturated. This ambient pressure has no effect...

Thanks again...

I forgot how cool physics is - once you get it, it's a great feeling..