Question

When a solid substance is a mixture of \(\mathrm{KBr}\) and \(\mathrm{KNO}_{3} .\) Which of the following does not give a distinguishing test? (a) Adding \(\mathrm{AgNO}_{3}\) solution in presence of dilute \(\mathrm{HNO}_{3}\) to a solution of the mixture in water (b) Warming the mixture with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) Adding dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\) followed by drops of \(\mathrm{KMnO}_{4}\), to an aqueous solution of the substance, then adding \(\mathrm{CCl}_{4}\) and shaking (d) Adding chlorine water to an aqueous solution of the substance, followed by adding \(\mathrm{CCl}_{4}\) and shaking.

Short answer

Option (a), (b), (c), and (d) all give a distinguishing test.

Step-by-step solution

Analyzing Option (a)

When AgNO₃ is added to a solution that contains KBr and KNO₃, the bromide ions (Br⁻) will react with silver ions (Ag⁺) to form a white precipitate of AgBr, whereas no reaction occurs with NO₃⁻ ions. This means that AgNO₃ can distinguish between KBr (which forms a precipitate) and KNO₃ (which does not). Thus, this option does provide a distinguishing test.

Analyzing Option (b)

Warming the mixture with concentrated H₂SO₄ can help differentiate between KBr and KNO₃. KBr reacts with H₂SO₄ at high temperatures to release fumes of bromine gas (Br₂), while KNO₃ primarily breaks down into gases like NO₂. Thus, this option is also a distinguishing test.

Analyzing Option (c)

Adding dilute H₂SO₄ followed by KMnO₄ involves an oxidation reaction in which Br⁻ ions can react with KMnO₄ in an acidic medium. The solution changes color due to the reaction. KNO₃ does not partake in this reaction, which again distinguishes between the two substances. This option provides a distinguishing test.

Analyzing Option (d)

Adding chlorine water and then CCl₄ is used to cause a phase transfer of bromine. Chlorine water converts Br⁻ to Br₂ which gives an orange color in CCl₄. However, chlorine water does not have a significant visible reaction with NO₃⁻ from KNO₃, making this a valid distinguishing test between KBr and KNO₃.

Key concepts

KBr vs KNO3 distinction

Differentiating between potassium bromide (KBr) and potassium nitrate (KNO₃) is essential in inorganic chemistry, especially when dealing with mixtures. These two salts appear quite similar, being white crystalline solids, but certain tests can help in telling them apart.

One effective method involves the use of silver nitrate (AgNO₃). When added to a solution containing these salts, it reacts with bromide ions (Br⁻) from KBr, forming a white precipitate of silver bromide (AgBr). In contrast, the nitrate ions (NO₃⁻) from KNO₃ do not react with silver ions, hence no precipitate is formed.

Another approach to distinguish them involves the warm reaction with concentrated sulfuric acid (H₂SO₄). KBr reacts under these conditions to produce brown bromine fumes, while KNO₃ decomposes differently, forming gases such as nitrogen dioxide (NO₂), which is brown and pungent.

These chemical properties are utilized in practical scenarios to separate and identify these compounds in mixtures efficiently. Having this knowledge is crucial in qualitative analysis and understanding the reactivity patterns of different anions.

Chemical reaction tests

Chemical reactions offer distinct tests for distinguishing between substances like KBr and KNO₃. Key reactions involve observing changes such as precipitate formation, gas evolution, and color changes when reagents are introduced to the sample.

Reactions with silver nitrate prove especially useful. The formation of a white precipitate indicates the presence of bromide ions. Alternatively, warming the sample with concentrated sulfuric acid presents differently. The fumigating bromine gas is evidence specific to KBr, while the decomposition of KNO₃ does not produce such direct indications.

Additionally, reactions using dilute sulfuric acid and potassium permanganate (KMnO₄) provide visual clues. In an acidic environment, bromide ions will undergo oxidation, changing the solution color, while nitrate ions do not react similarly. This leads to evident differences based on color changes and confirms the presence of bromide.

Each reaction is a piece of the analytical puzzle, forming a clear picture of the components in the mixture and underscoring the importance of reaction-based tests in salt analysis.

Qualitative analysis of salts

Qualitative analysis in inorganic chemistry involves identifying the components of a salt mixture based on their chemical properties. For mixtures containing KBr and KNO₃, this process involves strategic selection of reactions that produce observable changes unique to each compound.

The use of silver nitrate is a classic test. With KBr, the reaction leads to a precipitate, a straightforward observation that is not seen with KNO₃, thereby indicating the presence of bromide. Another method involves phase transfer with chlorine water, followed by carbon tetrachloride (CCl₄). Upon adding chlorine, bromide ions convert to bromine, which then dissolves in CCl₄, imparting an orange color, while nitrate ions remain unaffected.

These tests show the satirical nature of qualitative analysis: using distinct reactions to discern and identify substances based on differences in standard reactivity. The tests are a staple in understanding ions' roles in reactions and often serve as stepping stones to more advanced analytical techniques.