\([\mathrm{X}]+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow[\mathrm{Y}]\) a colourless gas with irritating smell \([\mathrm{Y}]+\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow\) Green solution \([\mathrm{X}]\) and \([\mathrm{Y}]\) are (a) \(\mathrm{Cl}^{-}, \mathrm{HCl}\) (b) \(\mathrm{SO}_{3}^{2-}, \mathrm{SO}_{2}\) (c) \(\mathrm{CO}_{3}^{2-}, \mathrm{CO}_{2}\) (d) \(\mathrm{S}^{2-}, \mathrm{H}_{2} \mathrm{~S}\)

In the context of the NEET chemistry curriculum, acid-base reactions are a fundamental concept that requires a strong understanding. An acid-base reaction, in simple terms, is a chemical process in which an acid and a base interact, typically resulting in the formation of water and a salt. However, reactions can take a more complex route, especially when involving gases.

For instance, the reaction between compound X (SO_{3}^{2-}) and sulfuric acid (H_{2}SO_{4}) can be seen as an acid-base reaction. Here, the sulfuric acid donates a proton (H^{+}), and the sulfite ion (SO_{3}^{2-}) accepts it, leading to the formation of sulfur dioxide gas (SO_{2}), which is identified by its colorless appearance and pungent smell. This reaction demonstrates the characteristic behavior of acids and bases in a chemical interaction, resulting in the evolution of a gas, in this case SO_{2}.

Understanding these interactions is crucial for students because it enables the prediction of products in chemical reactions. It also provides a foundation to understand the conservation of matter—identifying how atoms are rearranged rather than lost or gained.

Redox reactions, short for 'reduction-oxidation reactions', are another vital type of chemical reaction involving the transfer of electrons between substances. These reactions are ubiquitous in chemistry. Oxidation refers to the loss of electrons, while reduction implies the gain of electrons.

In our example, when sulfur dioxide (SO_{2}) reacts with potassium dichromate (K_{2}Cr_{2}O_{7}) and sulfuric acid, the green color of the solution suggests the reduction of chromium (Cr) ions from a +6 oxidation state to a +3. This indicates that SO_{2} is serving as a reducing agent because it donates electrons to the chromium ions during the reaction. On the other hand, the chromium ions are being reduced, and the sulfur in sulfur dioxide is oxidized, demonstrating the complementary processes of reduction and oxidation within the same chemical equation.

Grasping the concept of redox reactions is essential as they play a key role in various chemical processes, including cellular respiration, photosynthesis, batteries, and corrosion. For NEET aspirants, developing a clear understanding of how to identify the agent undergoing oxidation or reduction is fundamental for solving related problems.

Gas evolution reactions are chemical transformations that result in the production of a gas as one of the products. These reactions are prevalent in both organic and inorganic chemistry, and they often have noticeable characteristics such as effervescence or specific odors.

In our exercise, the reaction between sulfite (SO_{3}^{2-}) and sulfuric acid is a classic example of a gas evolution reaction, resulting in the production of sulfur dioxide (SO_{2}). This gas is easily identified by its colorless nature and distinctive, irritating odor. Gas evolution reactions are not restricted to acid-base interactions; they can also be the result of decomposition reactions, single displacement, or even redox processes.

For NEET chemistry students, recognizing gas evolution reactions is crucial because it not only involves understanding the chemical process but also interpreting the physical signals, such as color change, smell, or effervescence. This ability is integral to practical chemistry, where observations lead to conclusions about the identity of reactants or products in a given reaction.