Deviation (Non-Ideal)

Positive Deviation

$$P_{\text{total}} > (P^\circ_A \cdot x_A + P^\circ_B \cdot x_B)$$

Concept: Occurs when adhesive forces ($$\text{A-B}$$) are weaker than cohesive forces ($$\text{A-A}$$ and $$\text{B-B}$$), causing molecules to escape into the vapour phase more easily.

Negative Deviation 

$$P_{\text{total}} < (P^\circ_A \cdot x_A + P^\circ_B \cdot x_B)$$

Concept: Occurs when adhesive forces ($$\text{A-B}$$) are stronger than cohesive forces ($$\text{A-A}$$ and $$\text{B-B}$$), making it harder for molecules to escape into the vapour phase.

Notation:

• $$P_{\text{total}}$$ = Actual total vapour pressure;
• $$P^\circ_A, P^\circ_B$$ = Vapour pressures of pure components A and B;
• $$x_A, x_B$$= Mole fractions in liquid phase.