514  Melting  Points  and  Solubility  of  Salts.        { Am-  ^'lm*^ 
The  temperatures  given  above,  at  which  incipient  fusion  occurs,  are 
in  many  cases  the  temperatures  at  which  the  turning  point  in  the  curve 
of  solubility  occurs.  It  appears  therefore  probable  that  the  crystal- 
lized hydrates,  even  when  in  solution,  are  completely  dissociated 
within  comparatively  narrow  limits  of  temperature.  One  of  the  most 
striking  examples  is  of  course  sodium  sulphate,  but  similar  phenomena 
are  exhibited  by  sodium  carbonate  and  chromate,  and  by  manganous, 
ferrous  and  other  sulphates.  This  dissociation  must  also  be  the  cause 
of  the  fact  that  the  solubility  at  or  about  the  melting  point  of  so  many 
salts  is  less  than  would  be  otherwise  expected.  It  must,  however,  be 
pointed  out  that  salts  like  sodium  sulphate  and  phosphate,  which  may 
be  melted  in  their  water  of  crystallization,  are  miscible  with  water 
at  the  same  temperature  in  all  proportions,  in  other  words  the  solu- 
bility of  the  crystallized  salt  is  infinite.  Hence  there  must  be  some- 
thing wrong  in  some  of  the  recorded  solubilities  of  such  salts  at  these 
temperatures. 
If  we  take  from  the  list  salts  which  are  isomorphous  with  one 
another  and  contain  the  same  amount  of  water  of  crystallization,  and 
which  alone  are  comparable  among  themselves,  we  find  that  the  solu- 
bility and  fusibility  stand  in  the  same  order  at  all  temperatures  below 
the  point  of  fusion  or  dissociation  of  the  hydrate. 
The  following  are  examples  : 
The  solubility  is  taken  to  be  the  weight  of  crystallized  salt  dissolved 
in  100  parts  of  water  : 
Zinc  and  Magnesium  Sulphates. 
m.p.   Solubility  at  0°.  40°.  50°. 
Zinc                                  50°                  115-2  224  263*8 
Magnesium                         70°                   72  4  178         212'6  (at  49°) 
Alums. 
m.p.   Solubility  at  0°.            17°,  20°.  50°. 
Sodium   61°  110  ....  •   
Potassium           84-5°                  3-9    15-13  44-11 
Rubidium..  =       89°                 ......         2*27  (Redtenbacher,  Watts' % 
Caesium             105—106°                           0-619  Diet.  [5],  580-583.) 
Sodium  Arsenate  and  Phosphate. 
m.p.        Solubility  at  0°.  20°. 
Arsenate                                28°  17-2  140'7  (at  21°) 
Phosphate                              35°  6-5  27'2 
