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

Magnesium hydroxide, \( \text{Mg(OH)}_2 \), is insoluble in water.

Step 1: Understand Solubility of Hydroxides

To solve the problem, we need to determine the solubility of the given hydroxides in water. Solubility refers to the ability of a substance to dissolve in a solvent. In this context, we are focusing on the solubility of metal hydroxides in water.

Step 2: Review Solubility Rules

Hydroxides of Group 2 elements (alkaline earth metals) are generally insoluble, with the exception of barium hydroxide (Ba(OH)₂) and calcium hydroxide (Ca(OH)₂), which are slightly soluble. Stontium hydroxide (Sr(OH)₂) is more soluble compared to magnesium hydroxide (Mg(OH)₂). However, magnesium hydroxide is considered insoluble or only slightly soluble in water.

Step 3: Identify Insoluble Hydroxide

Considering the solubility rules: \[ \text{Ca(OH)}_2 \] is slightly soluble, \[ \text{Ba(OH)}_2 \] is soluble, \[ \text{Sr(OH)}_2 \] is more soluble, and \[ \text{Mg(OH)}_2 \] is typically insoluble. Therefore, magnesium hydroxide is the hydroxide that does not readily dissolve in water.

Key Concepts

Group 2 Hydroxides

Group 2 elements are known as alkaline earth metals. These include beryllium, magnesium, calcium, strontium, barium, and radium. The hydroxides of these metals are of interest when discussing their solubility. Although these elements belong to the same group, their hydroxides have varying degrees of solubility in water.
Magnesium hydroxide ( Mg(OH)_2 ) is typically considered insoluble in water. This means that it doesn't dissolve readily in water under normal conditions. On the other hand, calcium hydroxide ( Ca(OH)_2 ) and barium hydroxide ( Ba(OH)_2 ) are slightly soluble, meaning they dissolve to a small extent. Strontium hydroxide ( Sr(OH)_2 ) is more soluble than magnesium hydroxide, but still not highly soluble.

• Mg(OH)_2 - insoluble
• Ca(OH)_2 - slightly soluble
• Sr(OH)_2 - more soluble than Mg(OH)_2
• Ba(OH)_2 - soluble

Understanding these differences is key for predicting reactions and behavior of these metal hydroxides in aqueous solutions.

Insoluble Compounds

Insolubility in the context of chemical compounds like hydroxides means that the compound does not dissolve easily in water. This typically refers to a very low solubility level where only a minimal amount of the compound will dissolve in water to form a solution.
Magnesium hydroxide ( Mg(OH)_2 ) is a classic example of an insoluble compound. When added to water, it forms a suspension – a mixture where small solid particles are dispersed within the liquid, not dissolved. This is why magnesium hydroxide can be seen as a cloudy suspension rather than a clear solution.
The insolubility of such compounds is influenced by several factors:

• Ionic interactions – the stronger the forces keeping the compound's ions together, the less likely they are to separate and dissolve.
• Lattice energy – the energy holding the ions in the crystalline structure must be overcome for dissolution; higher lattice energy results in lower solubility.

Knowing about insoluble compounds helps in predicting which substances will precipitate out of a reaction, leading to practical applications in industrial and laboratory settings.

Chemical Solubility Rules

Chemical solubility rules are guidelines that help predict whether a compound will dissolve in water, forming a solution. These rules are based on experimental observations and can help simplify the task of determining solubility in different scenarios.
For hydroxides, there are specific rules related to their solubility:

• Most hydroxides are only slightly soluble in water. However, Group 1 hydroxides and barium hydroxide ( Ba(OH)_2 ) are exceptions and are considered more soluble.
• Calcium hydroxide ( Ca(OH)_2 ) is often considered slightly soluble because it can dissolve enough to form a saturated solution.
• Magnesium hydroxide ( Mg(OH)_2 ) is generally treated as insoluble.

These rules are a helpful starting point for predicting the behavior of compounds but should be complemented by empirical data and practical experimentation for more precise predictions in specific situations.