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A certain salt (X) gives the following t...

A certain salt `(X)` gives the following tests :
(a) Its aqueous solution is alkaline to litmus.
(b) On strong heating, it swells up to give a glassy material `(Y)`.
(c) When conc. `H_(2) SO_(4)` is added to a hot concentrated solution of `(X)`, white crystal of a weak acid `(Z)` separates out.
Identify `(X), (Y) and (Z)` and write down the chemical equations for reaction at steps `a, b` and `c`.

Text Solution

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As given, `(X)` on strong heating, sweels to give a glassy bead `(X)` is borax, `Na_(2) B_(4) O_(7). 10 H_(2) O`
(a) `Na_(2) B_(4) O_(7) + 7H_(2) O hArr underset ("Strong base") (2NaOH) + underset("Weak acid") (4H_(3) BO_(3)`.
In aqueous solution, borax hydrolyse to give a strong base, `NaOH`, and a weak acid, `H_(3) BO_(3)`. Hence, the solution will be alkaline to litmus.
(b) `Na_(2) B_(4) O_(7) . 10 H_(2) O overset (Delta)rarr Na_(2) B_(4) O_(7) + 10 H_(2) O`
`Na_(2)B_(4)O_(7) overset("Strong heating")rarr underset("Sodium metaborate") (NaBO_(2)) + underset("Boric anhydride")(B_(2) O_(3))`
` B_(2) O_(3)` forms glassy bead.
( c) `underset("Hot and conc")(Na_(2) B_(4)O_(7)) + underset("Conc") (H_(2) SO_(4))+ 5H_(2) O rarr Na_(2) SO_(4) + 4H_(3) BO_(3) darr`
Sodium sulphite, `Na_(2) SO_(4)`, is soluble is water and white crystals of boric acid, `H_(3) BO_(3)` (weak acid), separates out.
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