Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following hydroxides is insoluble in water? (a) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (b) \(\mathrm{Ba}(\mathrm{OH})_{2}\) (c) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

The insoluble hydroxide is (c) \( \mathrm{Mg(OH)}_2 \).

Step 1: Understand Solubility Rules

Solubility rules are guidelines to determine the solubility of compounds in water. While most ionic compounds containing alkali metal ions and ammonium are soluble, hydroxides show different solubility patterns. Most hydroxides are insoluble except for those of Group 1 elements and some Group 2 elements.

Step 2: Review Group 2 Hydroxide Solubility

Group 2 elements that are commonly asked about include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Calcium hydroxide \( \mathrm{Ca(OH)}_2 \) and barium hydroxide \( \mathrm{Ba(OH)}_2 \) are slightly soluble, while strontium hydroxide \( \mathrm{Sr(OH)}_2 \) is more soluble. However, magnesium hydroxide \( \mathrm{Mg(OH)}_2 \) is considered insoluble in water.

Step 3: Identify the Insoluble Hydroxide

Among the given options, \( \mathrm{Mg(OH)}_2 \) is the hydroxide known for being insoluble in water. It forms a solid precipitate when added to water.

Key Concepts

Insoluble Hydroxides

Insoluble hydroxides are compounds that do not dissolve well in water, meaning they form a solid precipitate when mixed. The solubility of hydroxides can vary greatly across different groups of the periodic table. A general rule of thumb is that most hydroxides are not soluble, except for those of alkali metals and some Group 2 elements.

For instance, magnesium hydroxide, \( \text{Mg(OH)}_2 \), is a classic example of an insoluble hydroxide. When you add it to water, instead of dissolving, it largely remains as a solid precipitate. This property is due to the strong ionic bonds between magnesium and hydroxide ions, which are not easily overcome by the polar nature of water molecules.

Recognizing insoluble hydroxides is essential when predicting the outcome of chemical reactions. It helps us understand phenomena such as precipitation and can be important in various chemical applications and water treatment processes.

Group 2 Elements

The group 2 elements in the periodic table are known as the alkaline earth metals, including magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements play a significant role in determining the solubility patterns of their respective compounds.

One key characteristic of Group 2 elements is their tendency to form slightly soluble hydroxides. However, the degree of solubility varies among the different elements:

• Magnesium hydroxide, \( \text{Mg(OH)}_2 \), is essentially insoluble in water.
• Calcium hydroxide, \( \text{Ca(OH)}_2 \), is slightly soluble, forming a mild solution.
• Strontium hydroxide, \( \text{Sr(OH)}_2 \), and barium hydroxide, \( \text{Ba(OH)}_2 \), are more soluble compared to magnesium hydroxide.

This variance in solubility is influenced by the size and charge of the ions. For students and chemists alike, understanding these patterns can help predict the behavior of these elements in water and their potential applications in different industries.

Chemical Solubility Patterns

Chemical solubility patterns refer to the general rules and tendencies for how different substances dissolve in water. These patterns are crucial for predicting how substances interact in solutions.

Solubility rules help us to predict these interactions. For instance, most alkali metal hydroxides are highly soluble, whereas transition metal hydroxides tend to be insoluble. However, the hydroxides of Group 2 elements show varying solubility.

Factors affecting solubility include:

• Temperature: Most solid solutes have increased solubility at higher temperatures.
• Ionic Nature: Compounds with higher lattice energies, like \( \text{Mg(OH)}_2 \), which have strong bonds, are less likely to dissolve.
• Molecular Structure: Polar compounds are generally more soluble in polar solvents like water.

Understanding these patterns is essential for predicting reactions involving ionic compounds and for applications across chemical industries, such as the design of pharmaceuticals and water testing.