TITLE.PM5

PROPERTIES OF PURE SUBSTANCES 97

Dharm
\M-therm/th3-2.p65

8. Mollier chart/diagram is more widely used than any other entropy diagram, since the work done on vapour
   cycles can be scaled from this diagram directly as a length, whereas on T-s diagram it is represented by an
   area.


9. Different processes :
   (i) Constant volume heating or cooling
   xv 12 g 12 =x vg

xv v v

T

1 g 122 g Ts

2

2

==sup. sup

(ii) Constant pressure heating or cooling

Q = h 2 – h 1

(iii) Isentropic expansion (non-flow process)

W = (u 1 – u 2 ) and s 1 = s 2

(iv) Throttling

hxh hxhf112 2+=+ (^12) fg f fg ...(For wet condition)
=+ +hh cT Tf 22 fg pss()sup− 2 ...(For superheated condition)

10. Dryness fraction of steam can determined by the following methods :
    (i) Bucket calorimeter
    (ii) Throttling calorimeter
    (iii) Separating and throttling calorimeter.

Objective Type Questions

Choose the correct answer :

1. (a) Specific volume of water decreases on freezing
   (b) Boiling point of water decreases with increasing pressure
   (c) Specific volume of CO 2 increases on freezing
   (d) Freezing temperature of water decreases with increasing pressure.


2. (a) The slope of vapourisation curve is always negative
   (b) The slope of vapourisation curve is always positive
   (c) The slope of sublimation curve in negative for all pure substances
   (d) The slope of fusion curve is positive for all pure substances.


3. (a) The process of passing from liquid to vapour is condensation
   (b) An isothermal line is also a constant pressure line during wet region
   (c) Pressure and temperature are independent during phase change
   (d) The term dryness fraction is used to describe the fraction by mass of liquid in the mixture of liquid
   water and water vapour.


4. The latent heat of vapourisation at critical point is
   (a) less than zero (b) greater than zero
   (c) equal to zero (d) none of the above.


5. (a) Critical point involves equilibrium of solid and vapour phases
   (b) Critical point involves equilibrium of solid and liquid phases
   (c) Critical point involves equilibrium of solid, liquid and vapour phases
   (d) Triple point involves equilibrium of solid, liquid and vapour phases.