\(\mathrm{NaNO}_{2}\) gives cloured fumes on treatment with dil. Acids. The colour of fumes is (a) Brown (b) Reddish (c) Reddish-brown (d) None of these

Sodium nitrite ((NaNO_{2}) is an inorganic compound widely used in the chemical industry, particularly known for its participation in reactions with dilute acids. In academia, we often look at its interaction with acids as a helpful demonstration of chemical reaction properties. When (NaNO_{2}) reacts with a dilute acid, it forms nitrous acid ((HNO_{2})). This is a spontaneous reaction where the dilute acid protonates the (NaNO_{2}), leading to the interchange of ions and creation of nitrous acid. However, nitrous acid is unstable under normal conditions and quickly undergoes further transformations. Understanding these reactions is crucial for students not only in terms of recognizing chemical properties but also in grasping the nuances of acid-base chemistry and the behavior of nitrogen oxides.

• Sodium nitrite is a white crystalline powder, soluble in water.
• It has important uses, including in the preservation of food.
• The reaction with dilute acids demonstrates the concept of acid-base neutralization with a twist — the formation of a gaseous byproduct.

Nitrogen dioxide ((NO_{2})) plays a significant role both in environmental chemistry and as an indicator in chemical reactions. It is one of several oxides of nitrogen and is a reddish-brown gas that has a sharp, biting odor and is a prominent air pollutant. Recognizing (NO_{2}) is relatively easy due to its characteristic color, which can be useful in identifying reactions that produce it, such as the decomposition of nitrous acid formed from sodium nitrite and dilute acid.

• (NO_{2}) often results from the combustion of fossil fuels, contributing to air pollution and acid rain.
• In laboratory settings, (NO_{2}) can serve as a visual cue in experiments to deduce the presence of certain nitrogen compounds.
• Its reddish-brown coloration is due to the absorption of light, which excites the molecules to a higher energy state.

The understanding of (NO_{2})'s properties and reactions is important for those studying environmental science and industrial processes as well as for general comprehension of atmospheric chemistry.

When we discuss the instability of nitrous acid ((HNO_{2})), we refer to its tendency to decompose, especially at higher temperatures, but even at room temperature it is not completely stable. This decomposition is not just a simple breakdown; it leads to the formation of nitrogen dioxide ((NO_{2})), water ((H_{2}O)), and in some cases, nitric oxide ((NO)) and other nitrogen oxides. The decomposition is an example of a multi-step process that may involve intermediate species.

• The decomposition of (HNO_{2}) showcases principles of chemical kinetics and reaction mechanisms.
• Unraveling the steps involved in the decomposition gives insight into how certain reaction intermediates can influence the rate and outcome of a reaction.
• The detection of (NO_{2}) can be used as an indirect method to study the behavior of nitrous acid, despite its fleeting presence.

By examining the decomposition of nitrous acid, students can better comprehend the complexities of chemical stability and reactivity, including the factors that influence how and why certain substances decompose.