Salt Analysis – JEE Main ChemistryPractice Questions & PYQs

This specific test serves as a diagnostic indicator for the simultaneous presence of nitrogen and sulfur in an organic substance. During the sodium fusion process, these two elements combine to form sodium thiocyanate, which subsequently reacts with $\mathrm{Fe}^{3+}$ ions-generated by the oxidation of $\mathrm{FeSO}_4$ in the presence of concentrated $\mathrm{H}_2\mathrm{SO}_4$-to produce the blood-red complex $\mathrm{Fe}(\mathrm{SCN})_3$.

Systematic salt analysis relies on the solubility product principle, where specific reagents establish controlled ionic environments to selectively precipitate cations into distinct analytical groups. Dilute $\mathrm{HCl}$ precipitates Group I cations like $\mathrm{Pb^{2+}}$ as insoluble chlorides, while the buffer combination of $\mathrm{NH_4Cl}$ and $\mathrm{NH_4OH}$ induces precipitation of Group III metals like $\mathrm{Al^{3+}}$ as their hydroxides. Group IV cations such as $\mathrm{Mn^{2+}}$ form sulfides in the presence of $\mathrm{H_2S}$ and $\mathrm{NH_4Cl} + \mathrm{NH_4OH}$, and finally, the addition of $\mathrm{Na_2CO_3}$ alongside $\mathrm{NH_4OH}$ isolates Group V cations, including $\mathrm{Sr^{2+}}$, by forming insoluble carbonates.

Qualitative inorganic analysis separates cations into systematic groups by exploiting differences in the solubility of their compounds under specific reagent conditions. The cation $\mathrm{Fe}^{3+}$ is precipitated as a hydroxide during the identification of Group III, whereas $\mathrm{Zn}^{2+}$, $\mathrm{Co}^{2+}$, and $\mathrm{Ni}^{2+}$ are categorized as Group IV because they precipitate as sulfides in the presence of hydrogen sulfide within an ammoniacal medium. Because of these distinct chemical requirements for precipitation, $\mathrm{Fe}^{3+}$ does not belong to the same classification group as the other listed transition elements.

The formation of a dark brown ring upon treatment with ferrous sulphate in an acidic medium is the definitive qualitative test for the nitrate ion ($\mathrm{NO}_3^-$), which proceeds via the creation of the nitroso-ferrous complex $[\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_5(\mathrm{NO})]\mathrm{SO}_4$. Similarly, the appearance of a deep red coloration with neutral $\mathrm{FeCl}_3$ that transforms into a brown-red precipitate upon boiling is diagnostic for the acetate ion ($\mathrm{CH}_3\mathrm{COO}^-$), as it involves the formation of a water-soluble ferric acetate complex that subsequently hydrolyzes to insoluble basic ferric acetate. Combining these results identifies the mixture as containing nitrate and acetate anions.

Confirmatory tests in qualitative analysis depend on the formation of distinct, insoluble precipitates that provide clear visual evidence of the analyte. Unlike the other compounds listed, lead nitrate, ${Pb}({NO}_3)_2$, is a colorless substance that is highly soluble in water, meaning it remains fully dissolved rather than precipitating out of the solution. Because it fails to produce a visible solid that could serve as a diagnostic indicator, it cannot be used to confirm the presence of ${Pb}^{2+}$ ions.

In qualitative analysis, the group V cations, which include $\mathrm{Ba}^{2+}$, $\mathrm{Ca}^{2+}$, and $\mathrm{Sr}^{2+}$, are identified by their reaction with ammonium carbonate, $(\mathrm{NH}_4)_2\mathrm{CO}_3$, which serves as the specific group reagent. When $\mathrm{Ba}^{2+}$ ions are present in the solution, they react with this reagent to produce $\mathrm{BaCO}_3$ as a white precipitate, a process represented by the equation $\mathrm{Ba}^{2+} + (\mathrm{NH}_4)_2\mathrm{CO}_3 \rightarrow \mathrm{BaCO}_3 \downarrow + 2\mathrm{NH}_4^+$. This distinct precipitation reaction is the standard procedure for identifying barium cations during wet testing.

Qualitative salt analysis relies on the principle of selective precipitation, where specific reaction environments are created to isolate cation groups based on their solubility products. The $Pb^{2+}$ and $Cu^{2+}$ ions represent Group II, precipitating as sulfides upon the addition of $H_2S$ in an acidic medium generated by $HCl$, whereas $Al^{3+}$ and $Fe^{3+}$ belong to Group III, precipitating as hydroxides using $NH_4OH$ in the presence of $NH_4Cl$ to regulate the hydroxide concentration. Group IV, consisting of $Co^{2+}$ and $Ni^{2+}$, requires the higher sulfide ion availability provided by $H_2S$ in the presence of $NH_4OH$, while the alkaline-earth cations $Ba^{2+}$ and $Ca^{2+}$ are classified as Group V, which are selectively precipitated as carbonates by adding $(NH_4)_2CO_3$ in the presence of $NH_4OH$.

Flame tests identify specific metal ions based on the characteristic emission spectra produced when they are heated, as electrons are promoted to higher energy levels and subsequently emit light upon returning to their ground state. Applying this to the elements, potassium is identified by a violet flame, whereas calcium corresponds to a brick red color. Similarly, strontium exhibits a crimson red emission, and barium is distinguished by an apple green flame, allowing us to accurately map the provided items to their respective colors.

The formation of a white precipitate upon the addition of barium chloride suggests the potential presence of anions like sulfite, which further reacts with dilute hydrochloric acid to liberate gas 'X'. This gas is sulfur dioxide, $\mathrm{SO}_2$, which is chemically confirmed by its distinct, pungent odor of burning sulfur. The underlying chemical transformation is represented by the equation $\mathrm{BaSO}_3 + 2\mathrm{HCl} \rightarrow \mathrm{BaCl}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{SO}_2 \uparrow$, which identifies the sulfite ion, $\mathrm{SO}_3^{2-}$, as the anion in the inorganic salt.

The flame test relies on the characteristic light emitted by metal ions when they are excited by heat, allowing for qualitative identification based on distinct colors. Among the cations listed, $\mathrm{Cu}^{2+}$ is unique because it produces a green flame with a distinctive blue centre. In comparison, $\mathrm{Sr}^{2+}$ results in a crimson red color, $\mathrm{Ca}^{2+}$ exhibits a brick red flame, and $\mathrm{Ba}^{2+}$ shows a green flame, but none of these share the specific blue-centered appearance characteristic of copper.

When preparing to test for halogens, the sodium fusion extract often contains interfering substances like sodium cyanide or sodium sulphide if the original organic compound possessed nitrogen or sulphur. If left untreated, these ions react with silver nitrate to produce precipitates that would compromise the reliability of the halogen identification. By boiling the extract with concentrated $\mathrm{HNO}_3$, these specific impurities are decomposed, allowing for a clean and unambiguous detection of halogens.

In the qualitative analysis of metal ions, the presence of $\mathrm{Ni}^{2+}$ is confirmed by its reaction with dimethylglyoxime in an alkaline solution. This specific chemical interaction results in the formation of the complex $[\mathrm{Ni}(\mathrm{dmg})_2]$, which appears as a characteristic bright red precipitate.

The addition of concentrated $HCl$ creates an acidic environment that suppresses sulfide ion concentration through the common ion effect, ensuring that only metal sulfides with extremely low solubility products can precipitate. $Al^{3+}$ and $Fe^{3+}$ do not form precipitates because their sulfides hydrolyze in water, whereas $Zn^{2+}$ and $Ni^{2+}$ remain in solution because they require a basic medium to induce precipitation. Since $Ca^{2+}$ and $Ba^{2+}$ form sulfides that are inherently soluble, they also stay in the aqueous phase. Ultimately, $Cu^{2+}$ is the only ion present that forms a $CuS$ precipitate, resulting in a single cation being removed.

The identification of $Ni^{2+}$ ions is a standard analytical procedure that relies on the reaction between nickel and dimethylglyoxime in an alkaline medium, facilitated by $NH_4OH$, to yield a characteristic red precipitate. This specific chemical interaction confirms the validity of the first statement regarding the detection of nickel. Dimethylglyoxime is defined as a neutral bidentate ligand because it utilizes two nitrogen atoms to coordinate with the metal ion simultaneously, forming a stable chelate structure. This classification confirms that the second statement is also accurate, reflecting the fundamental bonding behavior of the ligand.

When a copper(II) salt reacts with potassium iodide, the iodide ion functions as a reducing agent that converts copper(II) into copper(I). This redox process leads to the precipitation of copper(I) iodide, which is represented as the dimer $\mathrm{Cu}_2 \mathrm{I}_2$ alongside the liberation of iodine, following the reaction: $2 \mathrm{Cu}^{+2} + 4 \mathrm{I}^{-} \rightarrow \mathrm{Cu}_2 \mathrm{I}_2 (s) + \mathrm{I}_2$

Separation via acid-base extraction utilizes the difference in $pK_a$ values to selectively convert acidic or basic components into water-soluble ionic salts. $NaHCO_3$ is a sufficiently strong base to deprotonate carboxylic acids like $m$-chlorobenzoic acid, extracting them into the aqueous phase as fraction A, while $NaOH$ is a stronger base required to deprotonate phenols like $m$-chlorophenol into water-soluble phenoxides, isolating them in the aqueous phase as fraction B. The basic compound $m$-chloroaniline remains in the organic layer throughout these processes because it does not react with basic reagents, meaning it constitutes the final organic layer, fraction C.

The Lucas test employs Lucas reagent, consisting of anhydrous $\mathrm{ZnCl}_2$ and concentrated $\mathrm{HCl}$, to classify alcohols based on their reactivity. Nitrogen quantification is achieved via the Dumas method, which uses $\mathrm{CuO}$ to liberate nitrogen gas in an atmosphere of $\mathrm{CO}_2$, and the Kjeldahl method, which relies on digestion with concentrated $\mathrm{H}_2\mathrm{SO}_4$ to convert organic nitrogen into ammonium sulfate. Lastly, the Hinsberg test uses benzenesulfonyl chloride ($\mathrm{C}_6\mathrm{H}_5\mathrm{SO}_2\mathrm{Cl}$) in aqueous $\mathrm{KOH}$ to identify and distinguish amines through the formation and solubility properties of sulfonamide products.

The cation is $Zn^{2+}$ because it is the only Group IV ion that forms a sulfide, $ZnS$, which is soluble in dilute $HCl$. Upon reacting with $NaOH$, the $Zn^{2+}$ solution produces a white precipitate of $Zn(OH)_2$, while treatment with potassium ferrocyanide yields a characteristic bluish-white precipitate of $Zn_2[Fe(CN)_6]$. Other Group IV sulfides like $CoS$ and $NiS$ remain insoluble in dilute $HCl$, and $Mn^{2+}$ does not form the required ferrocyanide product.