The colour of the bead in the borax bead test | vedclass

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Question

(C) The correct option is $(C)$ metal metaborates.
Explanation:
When borax $(Na_2B_4O_7 \cdot 10H_2O)$ is heated,it loses its water of crystallizati

Vedclass · Accepted Answer

metal metaborates

Vedclass · Answer

(C) The correct option is $(C)$ metal metaborates.
Explanation:
When borax $(Na_2B_4O_7 \cdot 10H_2O)$ is heated,it loses its water of crystallization and swells up. On further heating,it melts to form a transparent glassy bead consisting of sodium metaborate $(NaBO_2)$ and boric anhydride $(B_2O_3)$.
$Na_2B_4O_7 \cdot 10H_2O \xrightarrow{\Delta} Na_2B_4O_7 + 10H_2O$
$Na_2B_4O_7 \xrightarrow{\Delta} 2NaBO_2 + B_2O_3$
When this bead is heated with a transition metal salt in an oxidizing flame,the metal salt decomposes into the corresponding metal oxide,which then reacts with boric anhydride to form coloured metal metaborates,which are responsible for the characteristic colours observed in the test.
$MO + B_2O_3 \rightarrow M(BO_2)_2$ (coloured bead)

The colour of the bead in the borax bead test is mainly due to the formation of:

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