The mixture of concentrated \(\mathrm{HCl}\) and \(\mathrm{HNO}_{3}\) made in \(3: 1\) ratio contains: (a) \(\mathrm{N}_{2} \mathrm{O}_{4}\) (b) NOCl (c) \(\mathrm{NCl}_{3}\) (d) \(\mathrm{ClO}_{2}\)

At the core of understanding Aqua Regia's functions is nitrosyl chloride (NOCl), an intermediate step in many chemical reactions within the mixture. When hydrochloric acid (HCl) and nitric acid (HNO3) are combined, several reactions occur, one of which produces NOCl.

In terms of structure, NOCl is a chemical compound that consists of nitrogen (N), oxygen (O), and chlorine (Cl) atoms. It's formed when the nitrogen atom in nitric acid is partially reduced and combines with a chlorine atom, creating a highly reactive substance. The presence of NOCl in Aqua Regia is critical as it participates in further reactions that contribute to the dissolution of noble metals like gold and platinum.

In our case, knowing this specific product helps to diagnose the reactions occurring within Aqua Regia and recognize that NOCl is the correct answer to the exercise question.

One fascinating aspect of Aqua Regia is its ability to dissolve noble metals such as gold and platinum, which do not react easily with single acids. This phenomenon is known as noble metals dissolution. The reason this solution is so effective lies in the combination of its components and their individual properties.

Hydrochloric acid alone is a powerful acid, capable of dissolving many substances, but it cannot dissolve noble metals. However, when combined with nitric acid to form Aqua Regia, the mixture takes on new properties. The nitric acid acts as an oxidizing agent, producing nitrosyl chloride and free chlorine, both of which are capable of reacting with even inert metals like gold. The chlorine ions effectively oxidize the metal, forming chloroaurate ions in the case of gold, which dissolve in solution.

It's the synergistic effect of the acids and the chemical reactions they produce that enable Aqua Regia to perform the task of dissolving metals that would otherwise remain unreactive.

Delving into the realm of chemical reactions, we find it's a vast and fundamental concept in chemistry that describes the process by which substances, called reactants, are transformed into new substances, known as products. These transformations are governed by the rearrangement of atoms and the breaking and forming of chemical bonds.

A chemical equation, such as the one showing the formation of NOCl from HCl and HNO3, displays the reactants and products of a reaction and is balanced to reflect the conservation of mass. Factors like temperature, concentration, and pressure can influence the rate and extent of reactions.

Understanding these reactions in the context of the textbook exercise enhances a student's grasp of how Aqua Regia works. It's not just a mixture but a dynamic system where reactants interact through specific pathways to yield products with remarkable capabilities, like dissolving noble metals. This system is an excellent demonstration of the principles of chemistry in action, revealing how controlled chemical reactions can be harnessed to achieve desired outcomes.