Animated Solution for Chemistry - d and f-Block Elements: A red solid is insoluble in water. However, it becomes soluble if some KI is added to water. Heating the red solid in a test tube results in liberation of some violet coloured fumes and droplets of a metal appear on the cooler parts of the test tube. The red solid is

Imagine you are standing in a chemistry laboratory, holding a test tube containing a mysterious, bright red powder. You try adding some distilled water and shaking it, but the solid stubbornly sits at the bottom. It is completely insoluble.

This is a classic scenario in qualitative inorganic analysis. We are given a physical observation—a red, water-insoluble solid—and we need to deduce its chemical identity using a series of specific tests. The problem provides us with two powerful clues: its reaction with Potassium Iodide and its behavior upon heating. Let's break down these clues one by one to unmask our mystery compound.

The Solubility Trick

The first major hint is that our stubborn red solid magically dissolves when we add an aqueous solution of Potassium Iodide ($KI$).

In the realm of transition metal chemistry, when an insoluble precipitate dissolves upon the addition of a reagent containing a common ion (like adding iodide to a metal iodide), it is almost always due to complex formation. The solid reacts with the excess ligand to form a new, water-soluble complex ion.

If we look at our options, Mercuric Iodide ($Hg{I}_2$) is a very famous bright red precipitate. Let's see how it reacts with $KI$. When excess $KI$ is added to $Hg{I}_2$, the iodide ions act as ligands and coordinate with the central mercury ion. This forms Potassium Tetraiodomercurate(II), a highly soluble complex:

This reaction perfectly explains the sudden disappearance of the red solid, strongly suggesting that our compound is indeed $Hg{I}_2$. But in chemistry, we always want a second confirmation.

Trial by Fire

The second test is a thermal decomposition. The problem states that heating the original red solid directly in a test tube liberates violet fumes, and droplets of a metal appear on the cooler upper parts of the tube.

This is a dead giveaway! In inorganic chemistry, violet fumes are the universal signature of Iodine gas ($I_2$). Furthermore, the only metal that exists as a liquid at room temperature and would condense as silvery droplets is Mercury ($Hg$).

Let's write down the thermal decomposition reaction for Mercuric Iodide:

When heated strongly, the bonds in $Hg{I}_2$ break. The iodine is released as a beautiful violet vapor, which sublimes upwards. The mercury metal vaporizes due to the heat and then condenses back into liquid droplets as soon as it hits the cooler glass walls of the upper test tube.

Both clues flawlessly align with the chemical properties of Mercuric Iodide. The formation of the soluble $K_2[Hg{I}_4]$ complex explains the solubility in $KI$, and the thermal decomposition perfectly accounts for the violet iodine fumes and the mercury droplets.

As a bonus, it is crucial to remember that the soluble complex we formed, $K_2[Hg{I}_4]$, is not just a random byproduct. When this solution is made alkaline (usually by adding $KOH$), it becomes Nessler's Reagent. This is a highly specific and sensitive reagent used in analytical chemistry to detect the presence of ammonia ($N{H}_3$), yielding a characteristic brown precipitate. Always try to connect these qualitative tests to their broader analytical applications!