Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. When Lassaigne's solution is treated with dil.HNO, and \(\mathrm{AgNO}_{3}\), a white precipitate soluble in ammonium hydroxide shows the presence of (a) chlorine (b) bromine (c) sulphur (d) nitrogen

Short answer

Answer: (a) Chlorine

Step-by-step solution

Write down the given information

We are given that Lassaigne's solution is treated with dilute HNO and \(\mathrm{AgNO}_{3}\), and this results in a white precipitate that is soluble in ammonium hydroxide.

Analyze the reactions of the given elements

We need to find out which element forms a white precipitate with these reagents that is soluble in ammonium hydroxide. Let's analyze the reactions for each of the given elements: (a) Chlorine: When treated with \(\mathrm{AgNO}_{3}\), chlorine forms \(\mathrm{AgCl}\), which is a white precipitate. Moreover, \(\mathrm{AgCl}\) is soluble in ammonium hydroxide. So, chlorine is a possible answer. (b) Bromine: In the presence of \(\mathrm{AgNO}_{3}\), bromine forms \(\mathrm{AgBr}\), which is a pale yellow precipitate. Since the precipitate should be white, bromine can be ruled out. (c) Sulphur: Though sulphur reacts with \(\mathrm{AgNO}_{3}\) to give a white precipitate of \(\mathrm{Ag}_{2}\mathrm{S}\), it is insoluble in ammonium hydroxide. So, sulphur can be ruled out. (d) Nitrogen: Nitrogen doesn't form a white precipitate with \(\mathrm{AgNO}_{3}\). So, nitrogen can be ruled out.

Choose the correct answer

From the above analysis, we can conclude that chlorine is the only element that forms a white precipitate that is soluble in ammonium hydroxide when treated with dilute HNO and \(\mathrm{AgNO}_{3}\). Therefore, the correct answer is (a) chlorine.

Key concepts

Lassaigne's Test

Lassaigne's Test is a classic qualitative analysis used to detect non-metals in organic compounds. It is particularly useful for identifying the presence of elements like

• Chlorine
• Bromine
• Sulfur

The test involves heating the organic compound with sodium metal to form a fusion product. This "sodium fusion extract" contains sodium halides, sulfides, or cyanides if the respective elements were in the original compound.
This test allows you to convert the non-metals into ionic forms that are easier to detect and analyze. It's a crucial step in determining the composition of complex compounds.

Procedure

To perform the test, the sodium fusion extract is treated with chemicals like silver nitrate or lead acetate, leading to precipitate formation. This provides a visual cue, helping chemists confirm the presence of specific elements within the compound.
Identifying the colors and solubilities of these precipitates distinguishes between different non-metals.

Ammonium Hydroxide Solubility

Ammonium hydroxide solubility is a key property used in chemical analysis to differentiate between substances. Particularly with silver halides, solubility in ammonium hydroxide provides clues about identity.
Silver chloride (\(\mathrm{AgCl}\)) is a white precipitate that readily dissolves in ammonium hydroxide, while other silver halides like silver bromide (\(\mathrm{AgBr}\)) do not dissolve as easily.

Understanding the Process

When ammonium hydroxide is added to precipitates like silver chloride, it forms soluble complex ions rendering the solid invisible. This reaction provides a straightforward way to test for chlorine presence.
It's an efficient approach in separating and identifying compounds based on selective solubility.

• Solubility tests are often used in conjunction with other analyses for more accurate results.
• The method is quite reliable, assisting chemists in narrowing down possible compounds in a mixture.

AgNO3 Precipitate Test

The \(\mathrm{AgNO}_{3}\) (silver nitrate) precipitate test is a prominent method in qualitative inorganic analysis, especially in detecting halides. When metals like chlorine, bromine, and iodine form halides, they react distinctively with silver nitrate.
This reaction leads to the formation of characteristic precipitates, helping chemists to easily identify unknown substances.

The Reaction

When silver nitrate solution is added to a solution containing halide ions, an insoluble silver halide precipitates out:

• Chlorine forms \(\mathrm{AgCl}\), a white precipitate.
• Bromine forms \(\mathrm{AgBr}\), a pale yellow precipitate.
• Iodine forms \(\mathrm{AgI}\), a yellow precipitate.

This color difference helps distinguish between halides. The solubility attributes following these reactions further aids in narrowing down the elements present.
These reactions are highly reliable, making them a staple in chemistry labs for analyzing unknown samples.