358  COLLEGE   CHEMISTRY

The  sulphides  are  formed  by  the  direct  union  of  the  metal  with
sulphur,  or  by  the  action  of  hydrogen  sulphide  or  of  some  soluble
sulphide  upon  a  solution  of  a  salt  (cf.  p.  254).  In  one  or  two  cases
they  are  made  by  the  reduction  of  the  sulphate  with  carbon.  The
sulphides,  except  those  of  the  alkali  metals,  are  but  little  soluble  in
water.  The  sulphides  of  aluminium  and  chromium  are  hydrolyzed
completely  by  water,  giving  the  hydroxides,  and  those  of  the  metals
of  the  alkaline  earths  are  partially  hydrolyzed  (cf.  p.  254).
The  carbides  are  usually  formed  in  the  electric  furnace  by  inter-
action of  an  oxide  with  carbon  (cf.  p.  321).  Some  'of  them  are
decomposed  by  contact  with  water,  after  the  manner  of  calcium
carbide,  giving  a  hydroxide  and  a  hydrocarbon.  Of  this  class  are
lithium  carbide  Li2C2,  barium  and  strontium  carbides  BaC2  and  SrC2,
aluminium  carbide  A14C3,  manganese  carbide  MnC,  and  the  carbides
of  potassium  and  glucinum.  Others,  such  as  those  of  molybdenum
CMo2  and  chromium  Cr3C2,  are  not  affected  by  water.
The  nitrates  may  be  made  by  any  of  the  methods  used  for  prepar-
ing salts.  They  are  all  at  least  fairly  soluble  in  water.
The  sulphates  are  made  by  the  methods  used  for  making  salts,  and
in  some  cases  by  the  oxidation  of  sulphides.  They  are  all  soluble
in  water,  with  the  exception  of  those  of  lead,  barium,  and  strontium.
Calcium  sulphate  is  meagerly  soluble.
The  carbonates  are  prepared  by  the  methods  used  for  making  salts.
They  are  all  insoluble  in  water,  with  the  exception  of  those  of  sodium
and  potassium.  The  hydroxides  of  aluminium  and  tin  are  so  feebly
basic  that  these  metals  do  not  form  stable  carbonates  (cf.  pp.  347,
354).
