168  COLLEGE   CHEMISTRY

Chemical  Properties. —  Hydrogen  iodide  is  the  least  stable  of  the
hydrogen  halides.  When  heated  it  begins  visibly  to  decompose  into
its  constituents  at  180°.  On  account  of  the  ease  with  which  it  parts
with  the  hydrogen  which  it  contains,  it  can  be  burned  in  oxygen
gas,  4HI  +  O2  — »  2H20  +  2I2.  When  the  gas  is  mixed  with  chlorine,
a  violent  chemical  change,  accompanied  by  a  flash  of  light,  occurs,
the  iodine  is  set  free,  and  hydrogen  chloride  is  produced,  C12  +  2HI— »
2HC1  +  I2.  Bromine  vapor  will  similarly  displace  the  iodine  from
hydrogen  iodide.
Chemical  Properties  of  Hydriodic  Acid  HI,  Aq.  —  In  most
respects  the  aqueous  solution  behaves  exactly  like  hydrochloric  and
hydrobromic  acids.  With  oxidizing  agents,  for  example,  such  as
manganese  dioxide,  it  gives  free  iodine,  just  as  the  others  give  free
chlorine  and  bromine,  respectively.  Here,  however,  the  oxidation  is
so  much  more  easily  carried  out,  that  it  is  slowly  effected  by  atmos-
pheric oxygen,  so  that  hydriodic  acid  left  exposed  to  the  air  gradually
becomes  brown  (02  +  4HI  ~>  2H20  +  2I2).
Although  the  dry  gas  is  not  an  acid,  the  solution  has  all  the  ordi-
nary properties  of  this  class  of  substances  (cf.  pp.  63,  111,  123,  161).
The  hydrogen  may  be  displaced  by  metals  like  zinc  and  magnesium.
The  acid  interacts  wiih  oxides  and  hydroxides,  forming  iodides  and
water.
FLUORINE  F2.
