Problem 13 - Chemistry in 30 days

Question

A white salt is insoluble in cold water but soluble in boiling water. Its solution when treated with potassium chromate solution gives yellow precipitate. The salt may be (a) \(\mathrm{BaCl}_{2}\) (b) \(\mathrm{SrCl}_{2}\) (?) \(\mathrm{PbCl}_{2}\) (d) \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

The salt is likely \( \mathrm{PbCl}_{2} \).

Step 1: Analyze Solubility Based on Temperature

First, identify the salt from the given options that is insoluble in cold water but soluble in boiling water. Lead(II) chloride, \( \mathrm{PbCl}_{2} \), has the property of being poorly soluble in cold water but becomes soluble when the water is heated to boiling.

Step 2: Understand the Reaction With Potassium Chromate

Next, understand that when a solution containing \( \mathrm{Pb}^{2+} \) ions is treated with potassium chromate, \( \mathrm{K}_2\mathrm{CrO}_4 \), a yellow precipitate of lead(II) chromate, \( \mathrm{PbCrO}_4 \), is formed.

Step 3: Match the Properties to Identify the Salt

Combine the solubility information with the reaction observation. The salt that matches both being insoluble in cold water and forming a yellow precipitate with potassium chromate solution is \( \mathrm{PbCl}_{2} \).

Key Concepts

Solubility in Water

Lead(II) chloride, known as \( \mathrm{PbCl}_{2} \), presents an interesting behavior in terms of solubility. In the context of solubility, substances can either dissolve in water or remain undissolved, and temperature can significantly affect this trait. \( \mathrm{PbCl}_{2} \) is characterized as being poorly soluble in cold water, which means it doesn't dissolve easily at lower temperatures. This property is distinct since many salts dissolve well in both cold and hot water. However, when the water is heated to boiling, \( \mathrm{PbCl}_{2} \) becomes soluble. This transition from insoluble in cold to soluble in boiling water is essential for experiments requiring lead(II) chloride in its dissolved state. This phenomenon can be attributed to the fact that with increased temperature, the kinetic energy of water molecules rises, enhancing the solubility of certain salts like \( \mathrm{PbCl}_{2} \). This unique behavior makes lead(II) chloride particularly suited for experiments that require precise control over the solubility conditions.

Reaction with Potassium Chromate

When solutions contain particular ions, they can undergo reactions that result in visible changes. When a solution containing \( \mathrm{Pb}^{2+} \) ions, such as one derived from dissolved Lead(II) chloride, is mixed with a potassium chromate solution, something fascinating occurs. Potassium chromate, or \( \mathrm{K}_2\mathrm{CrO}_4 \), introduces \( \mathrm{CrO}_4^{2-} \) ions into the mix.

• The \( \mathrm{Pb}^{2+} \) ions from lead(II) chloride meet with the \( \mathrm{CrO}_4^{2-} \) ions from potassium chromate.
• They combine through a simple chemical precipitation reaction.
• This process forms lead(II) chromate, known chemically as \( \mathrm{PbCrO}_4 \).

The formation of \( \mathrm{PbCrO}_4 \) is quite clear due to the resultant yellow precipitate, a solid that forms and settles out of the solution. This reaction is a classic double displacement reaction where components exchange partners to form new products.

Yellow Precipitate

The term "yellow precipitate" in chemistry describes the visibly noticeable solid that appears when certain reactions occur, such as when lead(II) chloride interacts with potassium chromate. During this specific chemical encounter, the resulting substance, lead(II) chromate, exhibits a bright yellow color, making it easy to identify.

• Precipitates form when the product of a reaction is insoluble in the given solvent, which in this case is water.
• This leads to the solid particles separating from the liquid.
• Precipitates are crucial indicators in reactions, signaling the formation of a new substance.

For students and chemists alike, recognizing the yellow precipitate as \( \mathrm{PbCrO}_4 \) provides insight into the reaction’s progress and confirms the presence of lead ions in the original solution. The vivid color and formation of solid particles are key observational indicators that a chemical transformation has occurred.