Question

The incorrect statement is :

• A. $Cu^{2+}$ salts give red coloured borax bead test in reducing flame
• B. $Cu^{2+}$ and $Ni^{2+}$ ions give black precipitate with $H_2S$ in presence of $HCl$ solution
• C. Ferric ion gives blood red colour with potassium thiocyanate
• D. $Cu^{2+}$ ion gives chocolate coloured precipitate with potassium ferrocyanide solution

Answer 1

Answer: (B)

$Cu^{2+}$ and $Ni^{2+}$ ions give black precipitate with $H_2S$ in presence of $HCl$ solution

Answer 2

$Cu ^{2+}$ and $Ni ^{2+}$ ions belong to Group II and Group IV, respectively,

$Cu ^{2+}+ H _{2} S \stackrel{ dil . HCl }{\longrightarrow} \underset{( B k \times k )}{ CuS } \downarrow$
$Ni ^{2+}+ H _{2} S \stackrel{ NH _{4} OH }{\longrightarrow} \underset{( Bhd )}{ NiS } \downarrow$

Group II sulphides precipitated out even under low concentration of sulphides, while Group IV sulphides require higher concentration of sulphides.

To fulfill this reaction condition, dil. HCl is chosen in Group II reagents and $NH _{4} OH$ is chosen in Group IV reagents.