Question

On strongly heating, a blue salt leaves a black residue, which of the following cations can be present in the salt.
A.$F{e^{2 + }}$
B.$F{e^{3 + }}$
C.$C{u^{2 + }}$
D.\[Z{n^{2 + }}\]

Answer 1

Hint :To solve this question we need to know the colours of some metallic salts and ions.
Cations are positive charge carrying ions that are the metals or electropositive atoms that donate a pair of electrons to electronegative atoms, and also they are attracted to the cathode in electrolysis.
The colour of the compounds of transition metals may be attributed to the presence of incomplete $(n - 1)d$ subshell.

Complete Step By Step Answer:
$F{e^{2 + }}$ ion is of green colour, $F{e^{3 + }}$ ion is of yellow/brown colour, $C{u^{2 + }}$ion is observed to have blue colour in aqueous solution and $Z{n^{2 + }}$ has completely filled d-orbitals and has no unpaired electrons so it is colourless.$CuS{O_4}.5{H_2}O$
$CuS{O_4}.5{H_2}O$ copper (II) sulphate pentahydrate when heated loses the water of crystallisation and becomes anhydrous sulphate.
  $CuS{O_4}.5{H_2}O\xrightarrow{\blacktriangle }CuS{O_4} + 5{H_2}O \uparrow $
Cupric sulphate which is white in colour, further decomposes into cupric oxide (which is black in colour) and sulphur trioxide.
$CuS{O_4}\xrightarrow{\blacktriangle }CuO + S{O_3} \uparrow$ $CuS{O_4}\xrightarrow{\blacktriangle }CuO + S{O_3} \uparrow $
On adding water to anhydrous copper sulphate, the blue colour of copper sulphate pentahydrate can be regained, this reaction is an exothermic reaction,
Therefore the correct answer to the question is option C. $C{u^{2 + }}$ on strongly heating, a blue salt leaves a black residue.

Note :
Copper does not occur abundantly in nature about$$ $1 \times {10^{ - 4}}{\raise0.5ex\hbox{$\scriptstyle 0$}
\kern-0.1em/\kern-0.15em
\lower0.25ex\hbox{$\scriptstyle 0$}}$ of the earth’s crust. It occurs as native as well as in a combined state.
Copper is used for making electric wires in the electrical industry, this is because of the very high electrical conductivity of copper, it is also used in several alloys which are tougher than metal itself. The common alloys are brass, bronze, gun metal, coinage metal, etc.