# Reducing pressure in a binary liquid solution

Consider the following diagram for a binary liquid system comprised of A and B, where A is the more volatile component.

My professor says

Consider an isopleth from $$a$$ to $$a_4$$. At the start, the solution is in pure liquid phase at state $$a$$.

As we reduce the pressure, the solution starts to boil at $$a_1$$. The composition of the liquid solution is denoted by the point $$a_1$$ which is the same as the overall composition of the solution since the solution has only just started to boil. The composition of the first trace of vapor formed is $$a_1'$$.

As the we reduce the pressure, the solution continues to boil to reach point $$a_2''$$. We may draw a tie line to find the composition of the liquid $$a_2$$ and the composition of the vapor $$a_2'$$. At every pressure where the solution exists as a mixture of liquid and vapor, we may use a tie line to find out its composition in the liquid and vapor phase.

Upon reaching $$a_3'$$, the solution has just been completely changed into vapor phase and the last trace of liquid has the composition $$a_3$$. After all the solution is vaporized at $$a_4$$, the composition of the vapor is again the overall composition of the solution.

I am slightly confused about we are able to make the pressure drop from $$a_1$$ to $$a_3'$$. My reasoning is: since the liquid and vapor are in equilibrium, shouldn't enough liquid keep on evaporating to maintain a constant vapor pressure?