Problem 64 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is insoluble in water? (a) \(\mathrm{CaF}_{2}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{HgCl}_{2}\) (d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

CaF₂ is insoluble in water.

Step 1: Understand Solubility

To determine which compound is insoluble in water, recall that solubility rules can help. Compounds containing alkali metals (eg., NaCl) and nitrates (eg., Ca(NO₃)₂) are generally soluble. Halides are soluble except when paired with certain metals like silver and lead.

Step 2: Examine Each Option

Review each compound: (a) CaF₂: Fluorides are generally soluble, but exceptions exist with alkaline earth metals like calcium, making it insoluble. (b) NaCl: Contains an alkali metal, making it soluble. (c) HgCl₂: Mercury(II) salts are often less soluble, but HgCl₂ is moderately soluble. (d) Ca(NO₃)₂: Nitrates are always soluble.

Step 3: Identify the Insoluble Compound

Based on the solubility principles, CaF₂ is the compound that does not dissolve in water because it is an exception to the general rule that most fluorides are soluble.

Key Concepts

Insoluble Compounds

Insoluble compounds are those that do not dissolve in a solvent, like water. Whether a compound is soluble or not depends primarily on its ionic nature and the interactions between its ions and the solvent molecules. For ionic compounds, solubility is influenced by the strength of ionic bonds and lattice energy, referring to the energy necessary to separate the ions. If this energy is substantial, the compound tends to remain undissolved.

In the context of the exercise, we're analyzing the solubility of compounds like calcium fluoride (\(\text{CaF}_2\)). Although fluorides are generally soluble, \(\text{CaF}_2\) is an exception because calcium is an alkaline earth metal, and the bonding in \(\text{CaF}_2\) is strong enough to render it insoluble in water. Keep in mind that solubility is a spectrum, and some compounds are slightly soluble rather than completely insoluble.

Solubility of Ionic Compounds

Understanding the solubility of ionic compounds is crucial for predicting whether a compound will dissolve in water. Ionic compounds consist of positively charged cations and negatively charged anions. Solubility rules help determine which combinations of ions will dissolve.

Several factors influence solubility, such as the strength of the attraction between the ions compared to the attraction with water molecules. The general solubility rules for ionic compounds include:

• Compounds with nitrate ions \((\text{NO}_3^-)\) are soluble.
• Alkali metal salts (like sodium chloride, \(\text{NaCl}\)) are typically soluble.
• Halides are usually soluble except when bonded with certain metals, making exceptions like \(\text{HgCl}_2\) moderately soluble.

By using these rules, one can predict the likelihood of a compound dissolving in water. This is essential in various applications from industrial processes to everyday cooking.

Alkali Metals and Solubility

Alkali metals such as lithium, sodium, and potassium, form a key part of solubility rules because compounds that include alkali metals are almost always soluble in water. This predictability simplifies the understanding of many reactions and solutions.

The reason alkali metals promote solubility lies in their high reactivity and low ionization energies. This enables them to dissociate easily when in contact with water, forming solutions more readily than other elements.

• Sodium chloride \((\text{NaCl})\), containing the alkali metal sodium, is a classic example of a soluble ionic compound.
• Their ability to form stable aqueous solutions makes alkali metal compounds essential in various functions including medicine and industry.

Hence, when you encounter a compound that includes an alkali metal, you can almost always confidently assume it is soluble in water.