A system comprised of chloroform, 1,4-dioxane, and ethanol exists as a two-phase vapor/liquid system at 50°C and 55 kPa. After the addition of some pure ethanol, the system can be returned to two-phase equilibrium at the initial T and P. In what respect has the system changed, and in what respect has it not changed?

Question

What will be an ideal response?

Answer 1

The phase rule tells us that the degrees of freedom of the system are
F =2 -? +N =2 -2 +3 =3.
With only T and P fixed, one degree of freedom remains available. We can specify one more intensive variable, like the mole fraction of one of the components in one of the phases. Thus, changes in the phase compositions are possible for the given T and P. If ethanol is added in a quantity that allows T and P to be restored to their initial values while 2 phases remain present, the ethanol distributes itself between the phases so as to form new equilibrium phase compostions and different amounts of the vapor and liquid phases. Nothing remains the same except T and P. The phase compositions and the amount of each phase change.