A metal gives two chlorides \(\mathrm{A}\) and \(\mathrm{B}\). A gives black precipitate with \(\mathrm{NH}_{3}\) and \(\mathrm{B}\) gives white. With KI, B gives a yellow precipitate, which is soluble in excess of KI. A and B are respectively (a) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}, \mathrm{HgCl}_{2}\) (b) \(\mathrm{HgCl}, \mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{ZnCl}_{2}, \mathrm{HgCl}_{2}\) (d) \(\mathrm{HgCl}_{2}, \mathrm{ZnCl}_{2}\)

Understanding the reaction between ammonia (h3) and various chlorides is a fundamental aspect of inorganic chemistry. When h3 is introduced to certain metal chlorides, it doesn't merely mix; it undergoes a chemical reaction that can lead to the formation of new compounds, signaling the presence of specific metals.

For example, when mercurous chloride (hg2c2) reacts with h3, it forms a distinctive black precipitate. This precipitate is a combination of elemental mercury and mercuric amidochloride (hgnh2c). This color change to black is a diagnostic reaction specific to mercurous chloride, and it helps chemists identify the presence of mercury in its +1 oxidation state. Such reactions are not just important for theoretical knowledge but also have practical applications in analytical chemistry for the identification of compounds.

The solubility of metal halides, like chlorides, in potassium iodide (KI) provides insightful information about their chemical nature. When we examine mercuric chloride (hgc2), this compound reacts with KI to produce a yellow precipitate of mercuric iodide (hgi2). However, the story doesn't end there.

If we add an excess of KI to the mixture, the precipitate dissolves, leading to the clear solution again. This happens due to the formation of a complex ion, [hgi4]^2-, which is soluble in water. This specific behavior serves as a confirmatory test for mercuric chloride. By observing a compound's solubility or lack thereof in various reagents, such as KI, scientists can deduce its chemical identity. This fundamental property is leveraged in qualitative analysis to categorize and understand different ionic compounds.

Diving deeper into the nuances of chemistry, we find that the creation of mercuric iodide (hgi2) from the reaction of mercuric chloride (hgc2) with potassium iodide (KI) showcases the way these compounds interact to form new substances. Mercuric iodide is characterized by its bright yellow precipitate, a telltale sign when conducting a qualitative analysis.

In the world of chemistry, noticing such vivid color changes is like reading a chemical text message—compounds are 'telling' us what's happening on a molecular level! When mercuric iodide is created in the presence of excess KI, it transforms into a soluble complex, rendering the solution clear once again. This is a classical demonstration of the dynamic equilibria involved in chemical reactions, illustrating reversibility and the concept of solubility products. Such reactions are not just significant for academic purposes but also have real-world applications in medicine and industry, where mercuric iodide is used for its antiseptic properties and in pigment production.