Problem 54 - A Complete Resource Book in Chemistry for JEE Main

Question

The composition of golden spangles is (a) \(\mathrm{PbI}_{2}\) (b) \(\mathrm{BaCrO}_{4}\) (c) \(\mathrm{As}_{2} \mathrm{~S}_{3}\) (d) \(\mathrm{PbCrO}_{4}\)

Step-by-Step Solution

Answer

(a) PbI_2

Step 1: Understand the context

Golden spangles are known for their shiny, flaky appearance, which is commonly seen in certain chemical compounds when they precipitate out of a solution.

Step 2: List known compounds with golden appearance

Lead Iodide ( 71I_2 7) is known to appear as bright yellow crystals or spangles under certain conditions, which gives it a golden appearance.

Step 3: Analyze each option's properties

- 7PbI_2 7 is known for exhibiting golden spangles when it crystallizes. - 7BaCrO_4 7 is a yellow powder rather than spangles. - 7As_2S_3 7 is known for its yellow to orange-red color but not specifically forming spangles. - 7PbCrO_4 7 is a vibrant yellow powder and also does not form golden spangles.

Step 4: Match the compound with golden spangles

Based on the properties of forming golden spangles, 7PbI_2 7 is traditionally known for its golden appearance due to its crystalline structure when precipitated.

Key Concepts

Lead Iodide

Lead Iodide, known chemically as \( \text{PbI}_2 \), is a fascinating compound that results in the distinct appearance of golden spangles when it crystallizes. This compound forms bright yellow crystals that gleam like gold, giving it its notable characteristic. Lead Iodide is produced through a chemical reaction, often involving lead nitrate and potassium iodide solutions.

During the reaction, these solutions are mixed, creating a precipitation of Lead Iodide as a solid. This occurs because lead ions and iodide ions meet, bond, and settle out of the solution. Below this magical appearance lies fundamental chemistry.

• Due to its high density and toxicity, lead iodide must be handled with care.
• It is also sensitive to light, which can transform its color, further adding to its intriguing visual characteristics.

Crystalline structure

Crystalline structures are highly ordered and repeat in patterns at the atomic level. Lead Iodide forms crystals that are not just visually stunning but also offer a peek into the orderly nature of such substances.

In crystalline Lead Iodide, atoms are neatly arranged, forming what looks like a lattice with regular intervals. This organization results in distinct physical properties, such as the shiny appearance of the spangles.

• The regularity in the structure affects how light interacts with the material, often causing the beautiful sheen.
• Crystalline formations are also more stable due to their specific and repeating arrangement.

This stability and beauty make crystalline Lead Iodide particularly notable in science.

Precipitation

Precipitation is a core process in chemistry where a solid forms within a solution and settles out, often forming crystals. This occurs when the concentration of dissolved ions exceeds their solubility, prompting atoms or molecules to cluster and form a solid.

For example, in the creation of golden spangles with Lead Iodide, precipitation takes place when lead nitrate and potassium iodide solutions are mixed.

• The resulting solid, Lead Iodide, falls out of the solution, forming those iconic spangles.
• This process is not just essential for forming such compounds but is also crucial in many natural and industrial processes, such as water purification.

This transformation from dissolved ions to tangible, often beautiful solids, underscores the chemistry behind familiar and useful reactions.