What happens when (write chemical equations):
(a) Orthoboric acid is heated gradually.
(b) `B_(2)O_(3)` and ferric oxide are heated in oxidising flame.
(c) A mixture of boron and sodium hydroxide is fussed at high temperature.
(d) A mixture of `B_(2)O_(3)` and Mg powder is fused and teh product is boiled with dilute HCl.

Explanation

(a) Upon heatin, orthoboric acid undergoes the following changes:
`H_(3)BO_(3)overset(370k)rarrHBO_(2)+H_(2)O`
`4HBO_(2)overset(410k)rarrH_(2)B_(4)O_(7)+H_(2)O`
`H_(2)B_(4)O_(7)overset("Red hot")rarr2B_(2)O_(3)+H_(2)O`
(b) Ferric metaborate is formed and the head acquires a yellow colour.
`Fe_(2)O_(3)+3B_(2)O_(3)rarr2Fe(BO_(2))_(3)`
(c) On fusing boron with sodium hydroxide, sodium borate is formed.
`2B+6NaOHoverset(("Fuse"))rarr2Na_(3)BO_(3)+3H_(2)`
(d) On fusing `B_(2)O_(3)` and Mg powder, the product is a mixture of `MgO` and boron. the mass is boled with dilute HCl when MgO dissovles to form soluble `MgCl_(2)` and boron gets precipitated.
`B_(2)O_(3)+3Mgoverset("Fuse")rarr3MgO+2B`