JEE MAIN - Chemistry (2023 - 31st January Evening Shift - No. 11)
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 light of the above statements, choose the most appropriate answer from the options given below :
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 light of the above statements, choose the most appropriate answer from the options given below :
Both Statement I and Statement II are false
Both Statement I and Statement II are true
Statement I is false but Statement II is true
Statement I is true but Statement II is false
Explanation
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.
$$ \mathrm{CuSO}_4+\mathrm{B}_2 \mathrm{O}_3 \mathrel{\mathop{\kern0pt\longrightarrow} \limits_{\left( {Oxidation} \right)}^{Non - lu\min ous\,flame}} \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+\mathrm{SO}_3 $$
Two reactions may take place in reducing flame (Luminous flame)
(i) The blue-green $\mathrm{Cu}\left(\mathrm{BO}_2\right)_2$ is reduced to colourless cuprous metaborate as :
$$ \begin{aligned} & 2 \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+2 \mathrm{NaBO}_2+\mathrm{C} \stackrel{\text { Luminous }}{\longrightarrow} \\\\ & 2 \mathrm{CuBO}_2+\mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7+\mathrm{CO} \end{aligned} $$
(ii) Cupric metaborate may be reduced to metallic copper and bead appears red opaque.
$$ \begin{aligned} & 2 \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+4 \mathrm{NaBO}_2+2 \mathrm{C} \underset{\text { flame }}{\stackrel{\text { Luminous }}{\longrightarrow}} \\\\ & 2 \mathrm{Cu}+2 \mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7+2 \mathrm{CO} \end{aligned} $$
$$ \mathrm{CuSO}_4+\mathrm{B}_2 \mathrm{O}_3 \mathrel{\mathop{\kern0pt\longrightarrow} \limits_{\left( {Oxidation} \right)}^{Non - lu\min ous\,flame}} \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+\mathrm{SO}_3 $$
Two reactions may take place in reducing flame (Luminous flame)
(i) The blue-green $\mathrm{Cu}\left(\mathrm{BO}_2\right)_2$ is reduced to colourless cuprous metaborate as :
$$ \begin{aligned} & 2 \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+2 \mathrm{NaBO}_2+\mathrm{C} \stackrel{\text { Luminous }}{\longrightarrow} \\\\ & 2 \mathrm{CuBO}_2+\mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7+\mathrm{CO} \end{aligned} $$
(ii) Cupric metaborate may be reduced to metallic copper and bead appears red opaque.
$$ \begin{aligned} & 2 \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+4 \mathrm{NaBO}_2+2 \mathrm{C} \underset{\text { flame }}{\stackrel{\text { Luminous }}{\longrightarrow}} \\\\ & 2 \mathrm{Cu}+2 \mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7+2 \mathrm{CO} \end{aligned} $$
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