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) \(\mathrm{Sr}(\mathrm{OH})_{2}\)

Short answer

Mg(OH)_2 is insoluble in water.

Step-by-step solution

Understanding Solubility

To determine which hydroxide is insoluble in water, we first need to understand the concept of solubility. Most hydroxides are only sparingly soluble in water, except for those of the alkali metals (like lithium, sodium, and potassium). Hydroxides of alkaline earth metals typically have varying solubilities.

Analyzing the Solubility of Alkaline Earth Metal Hydroxides

The hydroxides mentioned are all from alkaline earth metals: calcium (Ca), barium (Ba), magnesium (Mg), and strontium (Sr). These metals form hydroxides with varying solubility levels in water.

Examining Solubility Trends

From known solubility rules, Mg(OH)_2 is known to be sparingly soluble or practically insoluble in water, forming a white precipitate. On the other hand, Ca(OH)_2 is sparingly soluble but dissolves to some extent, Ba(OH)_2 is more soluble, and Sr(OH)_2 is also relatively soluble.

Conclusion Based on Solubility

Based on the solubility rules and trends for hydroxides of alkaline earth metals, Mg(OH)_2 is the hydroxide that is least soluble in water and generally considered insoluble compared to the others listed.

Key concepts

Alkaline Earth Metals

Alkaline earth metals are the elements found in Group 2 of the periodic table. These metals include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They are known for their shiny appearance and are typically very reactive, especially with water and oxygen.

When these metals react with water, they often form hydroxides, which can vary in solubility. The reactivity and the properties of their hydroxide compounds are influenced by the atomic structure of the metals. Alkaline earth metals have two valence electrons, which they readily lose in chemical reactions, forming cations with a +2 charge.

Compared to their group neighbors, the alkali metals, these metals are harder and denser. They also exhibit a higher melting point, making them quite distinct in their physical properties.

Understanding these basic characteristics helps explain why hydroxides of alkaline earth metals differ in solubility. Knowing the general tendencies of these metals provides a foundation for predicting their behavior in chemical reactions.

Insoluble Compounds

An insoluble compound is one that dissolves very little in a solvent, such as water. Solubility can depend on temperature, pressure, and the nature of both the solute and the solvent.

For alkaline earth metal hydroxides, solubility is particularly inconsistent. Magnesium hydroxide, for instance, is considered an insoluble compound. It does not dissolve well in water, forming a white precipitate instead.

In contrast, other hydroxides like those of calcium or barium are more soluble, with calcium hydroxide being sparingly soluble and barium hydroxide quite soluble. The solubility of compounds is important in various applications, including chemical production and environmental science.

It's crucial to remember that a compound typically described as insoluble might still dissolve slightly, but not enough to be considered soluble under normal conditions.

Solubility Trends

Solubility trends refer to the predictable patterns in which compounds dissolve in solvents like water. For alkaline earth metal hydroxides, solubility generally increases as you move down the group on the periodic table.

This trend can be attributed to the increasing atomic size and decreasing lattice energy of the compounds formed by these metals as you go down the group. This means that magnesium hydroxide (Mg(OH)₂) is much less soluble than barium hydroxide (Ba(OH)₂).

Each metal hydroxide from this group exhibits a unique solubility level based on its position in the group. While general rules help in predicting solubility, specific conditions such as temperature also play a significant role.

Understanding these solubility trends assists in making predictions about reaction outcomes and can be vital for processes in chemistry, biology, and industry.