In borax bead test, which compound is formed?
A. Orthoborate
B. Metaborate
C. Double oxide
D. Tetraborate

Hint:The borax bead test is used in quantitative inorganic analysis for the detections of cations in the sample. In borax bead testing the borax undergoes decomposition reaction by releasing water molecules.

Complete step by step answer:
Borax bead test is a preliminary test for detection of cation radicals in inorganic qualitative determination. It is one of the most traditional tests used for the detection.
Borax is a chemical compound with chemical formula \[N{a_2}{B_4}{O_7}.10{H_2}O\] and chemical name sodium pyroborate. It is a hydrous crystalline salt. On heating the borax loses water of crystallization and swells up to form fluffy mass. The products obtained are sodium tetraborate and ten molecules of water.
\[\;N{a_2}{B_4}{O_7}.10{H_2}O\xrightarrow{\Delta }N{a_2}{B_4}{O_7} + 10{H_2}O\]
The white coloured porous mass thus obtained on further heating melts to give a clear liquid which undergoes solidification to a transparent glossy bead. The products of the reaction are boric anhydride and sodium metaborate.
\[N{a_2}{B_4}{O_7}\xrightarrow{\Delta }{B_2}{O_3} + 2NaB{O_2}\]
Hence option B is the correct answer.
The bead thus formed is known as borax bead which is used to detect coloured radicals. The test is called the borax bead test.
For this a small loop of a platinum wire is made and heated in a flame until red hot. It is then dipped into powdered borax where borax gets stuck in the loop. The loop is again heated in the hottest part of the flame where swelling occurs followed by forming a colourless, transparent glass-like bead.
When the beat is heated with any inorganic transition metal oxide like \[CuO\] or \[CoO\] a blue coloured \[Cu{\left( {B{O_2}} \right)_2}\] or \[Co{\left( {B{O_2}} \right)_2}\] bead is formed. Several cations like iron, chromium, cobalt, copper, nickel and manganese generate characteristic colour beads and make the detection easier.

Note:
The process of losing water molecules is termed as loss of water of crystallization. The water molecules are responsible for the crystal lattice structure of the salt.