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

Short answer

The correct statements are (c) 1, 2, and 4.

Step-by-step solution

Analyze Statement 1

Examine if golden yellow lead(II) iodide, \( \mathrm{PbI}_{2} \), dissolves in hot water to give a colorless solution. We know that \( \mathrm{PbI}_{2} \) is poorly soluble in cold water but dissolves in hot water to form a colorless solution due to the formation of a complex. Thus, Statement 1 is true.

Analyze Statement 2

Determine if \( \mathrm{Ba}^{2+} \) and \( \mathrm{Ca}^{2+} \) ions can be separated by adding \( \mathrm{CrO}_{4}^{2-} \) ion in an acetic acid medium. We know that \( \mathrm{BaCrO}_{4} \) is insoluble in water and can precipitate out, while \( \mathrm{CaCrO}_{4} \) is soluble in acetic acid medium. Thus, Statement 2 is true.

Analyze Statement 3

Examine if salts of calcium, copper, and nickel give a green flame color. Generally, calcium gives an orange-red flame, copper gives a blue-green flame, and nickel does not impart any distinctive flame color. Therefore, Statement 3 is not entirely correct.

Analyze Statement 4

Assess if the sulfide ion gives a violet color with alkaline sodium nitroprusside. This reaction is characteristic for the presence of sulfide ions as it forms a violet-colored complex with sodium nitroprusside in an alkaline medium. Thus, Statement 4 is true.

Determine the Correct Combination

Based on the analysis, Statements 1, 2, and 4 are true; Statement 3 is not entirely correct. Therefore, the correct combination is (c) 1, 2, and 4.

Key concepts

Solubility

Solubility is an important concept in chemistry and describes the ability of a substance to dissolve in a solvent. This property depends on the nature of the solute and the solvent as well as temperature.

• **Differential Solubility**: Some substances, like lead(II) iodide (\( \mathrm{PbI}_{2} \)), have low solubility in cold water but become more soluble in hot water. As temperature increases, more solute is able to dissolve, because the increased kinetic energy allows the solute molecules to more readily interact with the solvent molecules.
• **Complex Ion Formation**: In some cases, higher solubility at increased temperatures may result from the formation of a complex. The statement in the exercise mentioned this phenomenon with lead(II) iodide, which dissolves in hot water to form a colorless solution.

Understanding solubility is crucial in predicting whether a substance will dissolve at different temperatures and concentrations.

Flame Test

The flame test is a qualitative analysis technique used to identify the presence of certain metal ions based on the color they emit when heated in a flame. When metal ions are excited by heat, their electrons gain energy and jump to higher energy levels. As they return to their ground state, they release light.

• **Colors and Metals**: Different metal ions produce distinct colors while burning. For instance:
  • Calcium gives an orange-red flame.
  • Copper emits a blue-green flame.
  • Nickel generally does not impart a distinct color to the flame.
• **Not All Metals**: Some metals do not produce a noticeable color due to their emission spectra or the dominance of other elements' emissions when mixed.

The flame test is a simple yet informative method to help identify particular elements in a compound.

Complex Formation

Complex formation involves the combination of metal ions with other molecules or ions to form a complex ion or compound. This process is significant in various chemical reactions and analysis.

• **Definition**: A complex is formed when a central metal ion bonds with surrounding ligands, which can be ions or neutral molecules. These are typically Lewis bases donating pairs of electrons to the metal ion.
• **Stability Factors**: The formation of complexes can stabilize otherwise unstable ions or compounds. Different factors such as the size, charge, and electronic configuration of the metal ion, as well as the nature of ligands, influence the stability of complexes.
• **Analytical Uses**: Complex formation is often used in analytical chemistry to not only increase solubility, as in the dissolution of lead(II) iodide, but also to facilitate separation and identification of metals.

Understanding complex formation can aid in processes such as qualitative analysis and pharmaceutical applications.

Precipitation Reaction

A precipitation reaction occurs when two soluble salts react in solution to form one or more insoluble products, known as precipitates. These reactions are commonly utilized in chemical analysis and purification processes.

• **Mechanism**: When solutions containing different ions are mixed, if one combination of cations and anions forms a compound that is insoluble in water, a solid (precipitate) will form.
• **Example**: The reaction of \( \mathrm{Ba}^{2+} \) ions with \( \mathrm{CrO}_{4}^{2-} \) ions in the provided acetic acid medium yields barium chromate (\( \mathrm{BaCrO}_{4} \)), which precipitates out, allowing for separation from other ions like calcium.

Precipitation is a fundamental reaction type in chemistry that allows for the isolation and identification of specific compounds.