= Co-distillation
<Co-distillation> is the vaporization and collection of multiple components together during <distillation>. A component can enter the collected vapor alongside another volatile substance instead of being isolated as a pure vapor. <Steam distillation> is the case in which water supplies a volatile carrier; co-distillation is not restricted to water.
The result depends on <vapor-liquid equilibrium>. In an ideal, fully mixed binary liquid at a specified temperature, <Raoult's law> gives $p_A=x_Ap_A^{\ast}$ and $p_B=x_Bp_B^{\ast}$. The vapor fraction is $y_A=p_A/(p_A+p_B)$. The McMaster distillation manual uses liquid/vapor composition diagrams to explain why the vapor can be enriched in one component while still containing both.<Chem2O06 Laboratory Manual by McMaster University/Distillation introduction>[General Principles; A Mixture of Ideal Liquids; composition diagrams]{ref}
For original hypothetical data, let $x_A=0.25$, $p_A^{\ast}=80\,\mathrm{kPa}$ and $p_B^{\ast}=20\,\mathrm{kPa}$. Then $p_A=20$ and $p_B=15\,\mathrm{kPa}$, giving $y_A=20/35\approx0.571$. Both components co-distil, but the vapor is richer in $A$ than the starting liquid. These are equilibrium pressures at the stated temperature; boiling additionally requires their sum to match the external pressure.
With two persistent immiscible liquid phases, a different limiting model uses the pure-component vapor pressures directly, rather than multiplying each by its fraction of a homogeneous mixture. An <azeotrope> is another special equilibrium situation. Specify the phase behavior before assigning a distillate composition, and use a <distillation material balance> to distinguish enrichment from total recovery. Collecting a component together with a carrier does not itself establish complete separation from every impurity.
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