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

(b) 2 and 4

Step 1: Analyzing Statement 1

Check if lead(II) iodide (\(\mathrm{PbI}_2\)) dissolves in hot water. \(\mathrm{PbI}_2\) is known to be sparingly soluble in cold water but becomes more soluble in hot water, forming a yellow solution, not a colorless one.

Step 2: Evaluating Statement 2

Determine if \(\mathrm{Ba}^{2+}\) and \(\mathrm{Ca}^{2+}\) ions can be effectively separated using \(\mathrm{CrO}_4^{2-}\) in acetic acid. These ions can be separated by using \(\mathrm{CrO}_4^{2-}\) because \(\mathrm{BaCrO}_4\) is insoluble, leading to its precipitation, while \(\mathrm{CaCrO}_4\) is soluble.

Step 3: Evaluating Statement 3

Check whether salts of calcium, copper, and nickel produce a green flame color. Calcium gives a characteristic orange-red flame, copper produces a green flame, and nickel typically does not give a distinctive green flame unless certain conditions are met.

Step 4: Evaluating Statement 4

Investigate whether the sulphide ion with alkaline sodium nitroprusside produces a violet color. This reaction is known to produce a noticeable violet color when \(\mathrm{S}^{2-}\) ions are present.

Key Concepts

Solubility

Solubility is a crucial concept in chemistry that describes how a substance (the solute) dissolves in a solvent to form a solution. This concept helps predict the behavior of substances in various conditions. For instance, lead(II) iodide (\(\mathrm{PbI}_2\)) is sparingly soluble in cold water, meaning only a small amount dissolves. As water temperature increases, so does the solubility of \(\mathrm{PbI}_2\). This is because the increased kinetic energy at higher temperatures allows for greater interaction between solute and solvent particles. However, it’s important to note that while \(\mathrm{PbI}_2\) forms a yellow solution when dissolved, it is not colorless as initially thought in the exercise.

Ion Separation

Ion separation is the process of distinguishing and isolating ions from a mixture based on their chemical properties. Elements such as barium (\(\mathrm{Ba}^{2+}\)) and calcium (\(\mathrm{Ca}^{2+}\)) can be separated using specific reagents that cause one to precipitate while the other remains dissolved. In this case, \(\mathrm{CrO}_4^{2-}\) ions in an acetic acid medium can separate these two because \(\mathrm{BaCrO}_4\) is insoluble and precipitates out, whereas \(\mathrm{CaCrO}_4\) remains soluble. This principle is fundamental in inorganic chemistry, especially in processes like water purification and the analysis of minerals.

Flame Test

The flame test is a quick scientific technique used to detect the presence of certain metal ions based on the color they emit when burned. Each element emits a unique color when it is heat-excited due to the different energies of photons released. Copper, for instance, emits a green flame, indicative of its presence. However, calcium gives off an orange-red flame, and nickel generally does not emit a green flame. Misconceptions about flame colors often arise, emphasizing the importance of correct experimental conditions when conducting a flame test.

Chemical Reactions

Chemical reactions involve the transformation of substances through breaking and forming bonds, resulting in the creation of new products. An example is the reaction between sulphide ions (\(\mathrm{S}^{2-}\)) and alkaline sodium nitroprusside, yielding a violet color. This color change is a visible indicator of the reaction occurring and is often used in qualitative analysis to detect the presence of specific ions. Understanding the principles of chemical reactions is foundational in predicting how different materials interact and change under varying conditions.