Question

A salt which on heating with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gives violet vapours is (a) bromide (b) nitrate (c) iodide (d) sulphate

Short answer

The salt is iodide (c).

Step-by-step solution

Identify the Reactants and Reaction Type

When a salt is heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), it typically undergoes a chemical reaction that leads to the release of gaseous products. Our task is to identify which salt, when mixed with \(\mathrm{H}_{2} \mathrm{SO}_{4}\), produces violet vapors.

Consider the Options

Examine the given options: (a) bromide, (b) nitrate, (c) iodide, (d) sulphate. Not all anions will react with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) to form colored vapors, so we should consider known reactions that produce colored gaseous products.

Recall Known Reactions with Iodide

Among the common anions, iodide \((I^- )\) is known to react with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), producing iodine \((I_2)\) vapors. These vapors are distinctly violet.

Verify the Reaction

The reaction of potassium iodide \((KI)\), a typical iodide salt, with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) can be represented as follows: \[ 2KI + \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow I_2 (\text{violet vapours}) + \text{other products} \] The iodine vapors are violet, confirming that iodide salts produce violet vapors when heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\).

Key concepts

Violet Vapours

When discussing violet vapors in chemistry, we refer to the unique gaseous output of iodine when it is released into the air. Iodine is solid at room temperature, appearing as a dark grey or purplish-black element. However, when iodine transitions into a gaseous state, it forms distinctively violet-colored vapors. These vapors are not only visually captivating but also indicate the presence of iodine, an essential clue in chemical identification.
In the context of chemical reactions, if you see violet vapors produced, this is a strong indication of iodine being released. This phenomenon is often observed in laboratory settings when iodide compounds, such as potassium iodide, react with concentrated acids like sulfuric acid, highlighting the importance of these reactions for identifying substances based on their gaseous emissions.

Reaction with Concentrated Sulfuric Acid

Concentrated sulfuric acid ( H_2SO_4 ) is a powerful dehydrating and oxidizing agent, enabling it to deeply influence the behavior of salts. When reacting with iodide salts, sulfuric acid showcases its remarkable chemical potential by facilitating the transformation of iodide ions to elemental iodine.
When the conditions favor this chemical reaction, the iodide ions lose electrons and are transformed into iodine molecules. These iodine molecules then acquire kinetic energy in the form of heat, ultimately forming the violet vapors that we detect visually. The overall reaction highlights the multifunctional role of sulfuric acid, managing both oxidative and dehydrating capabilities that spur the release of gaseous iodine.

Chemical Reactions of Salts

Salts, which are composed of cations and anions, engage in fascinating chemical reactions when exposed to various substances. These reactions can lead to diverse outcomes, including the release of gases, formation of precipitates, or even the transformation of color in products.
A classic illustration of such a reaction involves iodide salts like potassium iodide ( KI ) engaging with concentrated sulfuric acid. Here, the acid facilitates the oxidation of iodide ions ( I^- ) to iodine ( I_2 ), observing the physical clues like violet vapors to confirm the reaction sequence. Such observations are pivotal in the study and classification of salts, underlining their significant role in broad chemical contexts.

• Salts can undergo decomposition, releasing gases.
• Some salt reactions can yield colorful products, aiding in their identification.
• Reactivity varies greatly among different salts with oxidizing acids.

Understanding chemical reactions of salts not only helps in identifying ionic compounds but also allows prediction of their behavior when subjected to strong acids like sulfuric acid. This knowledge is fundamental in both academic and applied chemistry settings.