Vapour, superheating pressures, curve

The effect of superheated steam may be illustrated by reference to baizaldehyde, which boils at 178° at 760 mm. It distils with steam at 97-9° (Pj = 703-5 mm. and pg = 56-5 mm.) and the distillate contains 32-1 per cent, of benzaldehyde by weight. If one employs steam superheated to 133°, the vapour pressure of benzaldehyde (extrapolated from the boUing point - pressure curve) is 220 mm. hence pj = 540 (water), Pg = 220 (benzaldehyde), and... 

Here, the region to the right of the vapour pressure curve represents the gaseous state. To the left side of the curve, the substance is liquid or solid. In the condition described by the curve liquid and saturated vapour coexist in equilibrium. When the temperature of an unsaturated (superheated) vapour is reduced, condensation starts at the saturation temperature (dew point temperature) corresponding to the actual vapour pressure. 

When the (P,T) state lies above flie saturation vapour pressure curve ABC, and to the right hand side of flie To isotherm. This is a stable superheated state . 

---