CHAPTER  XII
ACIDS,  ALKALIES,  AND  SALTS

143.  Classes  of  Compounds. — Compounds  may  be  di-
vided roughly  into  four  classes  of  substances:  water,  acids,
bases  or  alkalies,  and  salts.
144.  Properties  of  Water. — Pure  water  is  the  commonest
compound  that  exists.     The  water  we  use  comes  to  us  either
in  the  form  of  rain  or  of  melted  snow  from  the  mountains.
Part  of  it  trickles  or  percolates  through  the  ground  and
dissolves  any  soluble  material  or  gases  with  which  it  comes
in  contact.     When  the  water  has  passed  into  the  ground  and
comes  in  contact  with  limestone  and  magnesium  compounds,
some  of  the  substances  are  dissolved  and  the  water  becomes
hard.     This  kind  of  water  appears  when  an  artesian  well  is
drilled.     Water  that  flows  over  the  surface  of  the  earth
contains  suspended  matter  or  dirt  and  is  generally  called
soft  water.
Thus  the  distinction  between  hard  and  soft  water  depends
upon  the  substances  which  they  carry,  and  especially  upon
their  chemical  action.  In  soft  water,  soap  readily  lathers
and  the  suds  thus  formed  exert  a  rapid  cleansing  action.  In
hard  water,  soap  lathers  only  with  difficulty  and  often  will
not  lather  satisfactorily  at  all,  because  of  the  formation  of
lime  soap,  which  is  insoluble.
If  hard  water  is  boiled  the  hardness  often  disappears,  and
soap  then  acts  as  in  soft  water,  but  in  some  cases  boiling

135


136  APPLIED  SCIENCE

has  no  effect.  Such  a  condition  is  wholly  due  to  the  action
of  the  dissolved  solids  upon  the  soap.  Hard  water  contains
either  magnesium  or  calcium  sulphates  or  carbonates.  If
carbonates  only  are  present  in  the  water,  it  is  likely  to  become
soft  when  it  is  boiled,  for  boiling  drives  out  carbonic  acid  gas
(CO  2)  which  holds  the  carbonates  in  solution.
