212 GROUPV
the actual temperature depending on purity. Black phosphorus does
not ignite until even higher temperatures.ARSENIC, ANTIMONY AND BISMUTHNone of the common allotropic forms of these metals is affected by
air unless they are heated, when all burn to the (III) oxide.- Reaction with acids
Hydrochloric and dilute sulphuric acids have no appreciable action
at room temperature on the pure Group V elements.
Concentrated sulphuric acid and nitric acid—powerful oxidising
agents—attack all the elements except nitrogen, particularly when
the acids are warm. The products obtained reflect changes in
stability of the oxidation states V and III of the Group V elements.
Both white and red phosphorus dissolve in, for example, con-
centrated nitric acid to form phosphoric(V) acid, the reaction
between hot acid and white phosphorus being particularly violent.
Arsenic dissolves in concentrated nitric acid forming arsenic(V)
acid, H 3 AsO 4 , but in dilute nitric acid and concentrated sulphuric
acid the main product is the arsenic(III) acid, H 3 AsO 3. The
more metallic element, antimony, dissolves to form the (III) oxide
Sb 4 O 6 with moderately concentrated nitric acid, but the (V) oxide
Sb 2 O 5 (structure unknown) with the more concentrated acid.
Bismuth, however, forms the salt bismuth(Ill) nitrate Bi(NO 3 ) 3.
5H 2 O.- Reaction with alkalis
The change from non-metallic to metallic properties of the Group V
elements as the atomic mass of the element increases is shown in
their reactions with alkalis.
The head element nitrogen does not react. White phosphorus,
however, reacts when warmed with a concentrated solution of a
strong alkali to form phosphine, a reaction which can be regarded
as a disproportionation reaction of phosphorus:
P 4 4- 3KOH + 3H 2 O -» 3KH 2 PO 2 + PH 3 T
potassium phosphine
phosphinate
(hypophosphite)