Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

Pick out the correct statement(s) here: (1) Golden yellow \(\mathrm{PbI}_{2}\) dissolves in hot water to give a colourless solution. (2) \(\mathrm{Ba}^{2+}\) and \(\mathrm{Ca}^{2+}\) ions can be separated by adding \(\mathrm{CrO}_{4}^{2-}\) ion in acetic acid medium (3) Salts of calcium, copper and nickel give a green flame colour. (4) The sulphide ion gives with alkaline sodium nitroprusside a violet colour. (a) 1 and 3 (b) 2 and 4 (c) 1,2 and 4 (d) 2,3 and 4

Step-by-Step Solution

Answer

Option (c) 1, 2 and 4 are correct.

Step 1: Analyze Statement 1

The statement mentions that golden yellow \(\mathrm{PbI}_{2}\) dissolves in hot water to give a colorless solution. This process is accurate because lead(II) iodide is known to dissolve at elevated temperatures, forming a colorless solution.

Step 2: Analyze Statement 2

Statement 2 suggests that \(\mathrm{Ba}^{2+}\) and \(\mathrm{Ca}^{2+}\) ions can be separated by adding \(\mathrm{CrO}_{4}^{2-}\) ion in acetic acid medium. Though \(\mathrm{BaCrO}_{4}\) can precipitate from such a mixture, \(\mathrm{Ca}^{2+}\) ions typically do not form a precipitate under these conditions, thus allowing separation.

Step 3: Analyze Statement 3

The statement suggests that salts of calcium, copper, and nickel give a green flame color. While copper compounds are known to impart a green flame color, calcium gives an orange-red flame and nickel salts generally do not impart any distinct flame color.

Step 4: Analyze Statement 4

Statement 4 claims that the sulphide ion gives a violet color with alkaline sodium nitroprusside. It is correct that the \(\mathrm{S}^{2-}\) ion, in the presence of sodium nitroprusside under alkaline conditions, produces a violet color.

Step 5: Conclusion

Based on the analysis of the statements, statements 1, 2, and 4 are correct. Thus, the correct option that includes all these statements is (c) 1, 2 and 4.

Key Concepts

Solubility

Solubility involves the ability of a substance (solute) to dissolve in a solvent (like water) and form a homogeneous solution. This depends on various factors such as temperature, pressure, and the nature of the solute and solvent. Higher temperatures often increase solubility. For example, lead(II) iodide ( PbI_2) is more soluble in hot water, hence dissolving to form a colorless solution when heated. The solubility of PbI_2 is a textbook example illustrating that solubility can change with temperature.

Flame Test

The flame test is used to identify metal ions based on the color they emit when heated in a flame. Different metals produce distinctive colors due to the excitation and subsequent relaxation of their electrons. For instance:

• Copper compounds give a characteristic green flame.
• Calcium compounds emit an orange-red flame, not green as some may mistakenly believe.
• Nickel compounds generally do not provide a distinct flame color.

This test is a quick qualitative analysis tool used in laboratories for elementary identification of specific metals.

Precipitation Reactions

Precipitation reactions occur when two solutions are combined and produce an insoluble solid, or precipitate. This is often used to separate ions in a solution. For example, in a mixture of Ba^{2+} and Ca^{2+} ions, adding chromate ( CrO_4^{2-}) in an acetic acid medium can cause BaCrO_4 to precipitate, but Ca^{2+} does not typically form a precipitate under these conditions, thus allowing the two ions to be separated. Understanding precipitation reactions is essential for conducting various chemical procedures, particularly in analytical chemistry.

Color Reactions

Color reactions are used in chemical analysis to detect specific ions or compounds. The presence of certain ions can result in vivid color changes, helping to identify them. For example:

• Adding sodium nitroprusside to sulphide ions ( S^{2-}) in an alkaline solution yields a violet color, confirming the presence of sulphide ions.

These reactions are particularly useful in qualitative analysis because they provide a visual confirmation of the presence of specific ions or functional groups in a solution.