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Given below are two statements :
Statement $I:$ Upon heating a borax bead dipped in cupric sulphate in a luminous flame, the colour of the bead becomes green.
Statement $II:$ The green colour observed is due to the formation of copper(I) metaborate.
In the light of the above statements, choose the most appropriate answer from the options given below :
Both Statement $I$ and Statement $II$ are true
Statement $I$ is true but Statement $II$ is false
Both Statement $I$ and Statement $II$ are false
Statement $I$ is false but Statement $I$ is true
Solution
(Borax Bead Test)
On treatment with metal salt, boric anhydride forms metaborate of the metal which gives different colours in oxidising and reducing flame.
For example, in the case of copper sulphate, following reactions occur.
$CuSO _4+ B _2 O _3 \longrightarrow Cu \left( BO _2\right)_2+ SO _3$
Two reactions may take place in reducing flame (Luminous flame)
$(i)$ The blue-green $Cu \left( BO _2\right)_2$ is reduced to colourless cuprous metaborate as :
$2 Cu ( BO _2 )_2+2 NaBO _2+ C \underset{\text { flame }}{\stackrel{\text { Luminous }}{\longrightarrow}} 2 CuBO _2+$ $ Na _2 B _4 O _7+ CO$
$(ii)$ Cupric metaborate may be reduced to metallic copper and bead appears red opaque.
$2 Cu \left( BO _2\right)_2+4 NaBO _2+2 C \underset{\text { fuminous }}{\stackrel{\text { flame }}{\longrightarrow}} 2 Cu +$ $2 Na _2 B _4 O _7+2 CO$
