Solutions
235(2) We now take into account the compressi-
bility of water and ice. The change in the volume
of water and ice will be
AV'
Pi
10 -2Vo„ 10 -
P' 2p'^(^2) Votce 2 X 10-6 m 3 ,
where Vow, =10-3 m 3 and Voice = 1.1 x 10-3 m 3
are the initial volumes of water and ice.
The work A' done by the external force to com-
press the mixture is
A'= p
i Al 2.5 J.
2
The total work of the external force is
A tot = A + A' N3 J.
Obviously, since we again have Atot. AQ., the
mass of the ice that has melted will be the same
as in case (1).
2.31. Considering that the gas density is p = M/V,
we can write the equation of state for water vapour
in the form p = (p/p,) RT , where p and p, are the
density and molar mass of water vapour. Boiling
takes place when the saturated vapour pressure
becomes equal to the atmospheric pressure. If
the boiling point of the salted water has been raised
at a constant atmospheric pressure, it means that
the density of saturated water vapour must have
decreased.
2.32. Let us consider a cycle embracing the triple
point: A (Fig. 203). The following
phase transitions occur in turn: melting vapor-
ization condensation of gas directly into the
solid. Provided that the cycle infinitely converges
to the triple point, we obtain from the first law
of thermodynamics for the mass m of a substance
mX mg — my = 0
since the work of the system during a cycle is zero,
there is no inflow of heat from outside, and hence
the total change in the internal energy is also
equal to zero (the right-hand side of the equation).