Question

Which of the following cation forms white precipitate with dil NaOH, which is soluble in excess of NaOH
A.\[C{d^{2 + }}\]
B.$F{e^{2 + }}$
C.$M{g^{2 + }}$
D.$P{b^{2 + }}$

Answer 1

Hint: When two soluble salt solutions are mixed, a precipitation reaction occurs, resulting in the formation of an insoluble salt. The precipitate is the insoluble salt that precipitates out of solution, hence the reaction's term. The presence of different ions in solution can be determined using precipitation reactions.

Complete answer: The base has a sour flavour and is slick to the touch. An alkali is a base that dissolves in water. Salts are formed when certain compounds react chemically with acids. On red litmus paper, bases are known to turn blue.
Cadmium:
\[{\text{C}}{{\text{d}}^{2 + }}({\text{aq}}) + 2{\text{O}}{{\text{H}}^ - }({\text{aq}}) \rightleftharpoons {\text{Cd}}{({\text{OH}})_2}(\;{\text{s}})\]
\[Cd{\left( {OH} \right)_2}\]is precipitated by sodium hydroxide, but the precipitate does not dissolve in excess hydroxide. It's a pink colour.
Iron:
Sodium hydroxide also produces \[Fe{\left( {OH} \right)_2}\]and \[Fe{\left( {OH} \right)_3}\]from the corresponding oxidation states of iron in aqueous solution. It does not dissolve in excess in NaOH and is brown in colour.
\[{\text{F}}{{\text{e}}^{2 + }}({\text{aq}}) + 2{\text{O}}{{\text{H}}^ - }({\text{aq}}) \to {\text{Fe}}{({\text{OH}})_2}(\;{\text{s}})\]
Lead:
\[{\text{P}}{{\text{b}}^{2 + }}({\text{aq}}) + Na{\text{OH}}({\text{aq}}) \to Pb{({\text{OH}})_2}(\;{\text{s}})\xrightarrow{{excess.O{H^ - }}}N{a_2}Pb{O_2}\]
Sodium plumbite is formed which is soluble in excess NaOH.
The following is an example of an oxidation-reduction (redox) reaction:
NaOH is an oxidising agent, while Pb is a reducing agent.
Magnesium:
\[M{g^{2 + }}({\text{aq}}) + Na{\text{OH}}({\text{aq}}) \to Mg{({\text{OH}})_2}(\;{\text{s}})\]
It does not dissolve in excess in NaOH
Hence option D is correct.

Note:
When alkali (base) reacts with metal, salt and hydrogen gas are produced. As sodium hydroxide reacts with zinc wire, it produces hydrogen gas and sodium zincate. As sodium hydroxide reacts with aluminium wire, sodium aluminate and hydrogen gas are formed.