Question

The composition of golden spangles is:
A. $PbCr{O_4}$
B. $Pb{I_2}$
C. $A{s_2}{S_3}$
D. $BaCr{O_4}$

Answer 1

Hint: Golden spangles is the common name given to a yellow precipitate. This precipitate is formed when two colourless solutions of lead nitrate and potassium iodide are made to react at low temperatures.

Complete step by step answer:
The golden spangles test is a characteristic test used in salt analysis and for various other purposes. Let us see how this is made possible:
The reactants are two colourless solutions. The first one is an aqueous solution of lead nitrate, while the second one is a colourless solution of potassium iodide. Let us see what happens when these two reagents are mixed together:
The ions from each compound split into the solution. Therefore, we have:
$Pb{(N{O_3})_2} \rightleftharpoons P{b^{2 + }} + 2NO_3^ - $
$KI \rightleftharpoons {K^ + } + {I^ - }$
Now, when these are mixed together, the ions are free to react with each other. A double displacement reaction occurs, and the ions, more specifically, the anions, are exchanged between the two compounds. The reaction is as follows:
$Pb{(N{O_3})_2} + 2KI \to Pb{I_2} + 2KN{O_3}$
Thus, we can see that the double displacement reaction has caused the nitrate ion from the lead nitrate and iodide ion from the potassium iodide to get mutually exchanged, forming lead iodide and potassium nitrate.
The lead iodide so formed is insoluble and is formed as a bright yellow precipitate. When shaken slightly, the precipitate particles fall down in the solution like raindrops falling. Thus, this experiment is also known as the golden rain experiment. The golden spangles formed are thus, particles of lead iodide ($Pb{I_2}$).

So, the correct answer is Option B .

Note: The reaction will only show this test at low temperatures. When heated, the constituent ions re-dissolve and the solution loses its shiny colour and becomes colourless. The reaction is usually carried out in an Erlenmeyer flask, and is a characteristic test of lead iodide. Sometimes, a cloudiness is observed, which can be removed by adding a few drops of acid.