The Phase Rule and Its Applications

by Alexander Findlay

On heating Na₂SO₄,10H₂O, a point is reached at which the dissociation pressure into anhydrous salt and water vapour becomes equal to the vapour pressure of the saturated solution of the anhydrous salt, as is apparent from the following measurements;[^[212]] the differences in pressure being expressed in millimetres of a particular oil.

Temperature:	29.0°	30.83°	31.79°	32.09°	32.35°	32.6°
Difference of pressure:	23.8	10.8	5.6	3.6	1.6	0

At 32.6°, therefore, the vapour pressures of the two systems

Na₂SO₄,10H₂O—Na₂SO₄—vapour

Na₂SO₄—solution—vapour

are equal; at this temperature the four phases, Na₂SO₄,10H₂O; Na₂SO₄; solution; vapour, can coexist. From this it is evident that when sodium sulphate decahydrate is heated to 32.6°, the two new phases anhydrous salt and solution will be formed (suspended transformation being supposed excluded), and the hydrate will appear to undergo partial fusion; and during the process of "melting" the vapour pressure and temperature will remain constant.[^[213]] This is, however, not a true but a so-called incongruent melting point; for the composition of the liquid phase is not the same as that of the solid. As has already been pointed out (p. [137]), we are dealing here with the transition point of the decahydrate and anhydrous salt, i.e. with the reaction Na₂SO₄,10H₂O

Since at the point of partial fusion of the decahydrate four

phases can coexist, the point is a quadruple point in a two-component system, and the system at this point is therefore invariant. The temperature of this point is therefore perfectly definite, and on this account the proposal has been made to adopt this as a fixed point in thermometry.[^[214]] The temperature is, of course, practically the same as that at which the two solubility curves intersect (p. [112]). If, however, the vapour phase disappears, the system becomes univariant, and the equilibrium temperature undergoes change with change of pressure. The transition curve has been determined by Tammann,[^[215]] and shown to pass through a point of maximum temperature.