Question

Which of the following compounds are water soluble? (a) \(\mathrm{ZnSO}_{4}\) (b) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right](\mathrm{OH})_{2}\) (c) \(\mathrm{ZnS}\) (d) \(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\)

Short answer

\(\mathrm{ZnSO}_{4}\), \(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\), and \([\mathrm{Zn}(\mathrm{NH}_{3})_{4}](\mathrm{OH})_{2}\) are water soluble.

Step-by-step solution

Understanding Water Solubility

Water solubility is determined by the ability of a compound to dissolve in water. Typically, salts containing alkali metals (like sodium, Na) and nitrates are soluble. Compounds containing sulfate ions are generally soluble except for some exceptions.

Analyze \\(\mathrm{ZnSO}_{4}\\)

Zinc sulfate (\(\mathrm{ZnSO}_{4}\)) involves the sulfate ion, which is generally soluble in water. Therefore, \(\mathrm{ZnSO}_{4}\) is expected to be water soluble.

Analyze \\([\mathrm{Zn}(\mathrm{NH}_{3})_{4}](\mathrm{OH})_{2}\\)

This is a complex ion with hydroxide. Complex ions like \([\mathrm{Zn}(\mathrm{NH}_{3})_{4}]^{2+}\) are water-soluble because the surrounding ammonia can keep the zinc ion solvated, although hydroxides are usually insoluble, the overall complex can still dissolve in water.

Analyze \\(\mathrm{ZnS}\\)

Zinc sulfide (\(\mathrm{ZnS}\)) is typically insoluble in water owing to its tendency to form a precipitate, as sulfides of metals are often insoluble except for those of alkali metals and some alkaline earth metals.

Analyze \\(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\\)

Sodium zincate (\(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\)) contains sodium, an alkali metal. Compounds containing sodium are generally soluble in water.

Key concepts

Zinc Compounds

Zinc is a transition metal, known for its silvery-blue appearance. In chemistry, compounds containing zinc present various interesting properties. Some common zinc compounds include zinc sulfate (\(\mathrm{ZnSO}_{4}\)), zinc sulfide (\(\mathrm{ZnS}\)), and zincate like sodium zincate (\(\mathrm{Na}_{2}\mathrm{ZnO}_{2}\)). Zinc can form both simple and complex compounds:

• Zinc sulfate (\(\mathrm{ZnSO}_{4}\)): Typically used in agriculture and medicine, owing to its high solubility in water.
• Zinc sulfide (\(\mathrm{ZnS}\)): Known for its insolubility in water, it is commonly used in luminous paints.
• Complex Zinc compounds: Include compounds like \([\mathrm{Zn}(\mathrm{NH}_{3})_{4}](\mathrm{OH})_{2})\), which exhibit unique solubility properties due to the presence of ammonia.

Zinc's ability to form both soluble and insoluble compounds depends significantly on the ligands and ions it is bound to. Understanding these properties is essential in inorganic chemistry.

Solubility Rules

Solubility rules are essential guidelines that help us predict the solubility of different compounds in water. These rules are based on empirical observations and are useful for predicting whether substances will dissolve in water to produce solutions:

• Salts of Alkali Metals: Almost all salts of alkali metals like sodium (\(\mathrm{Na}^+\)) are soluble in water.
• Nitrates, Acetates, and Most Sulfates: Compounds containing nitrate (\(\mathrm{NO}_3^-\)) or acetate (\(\mathrm{C}_2\mathrm{H}_3\mathrm{O}_2^-\)) ions, and most sulfate ions (\(\mathrm{SO}_4^{2-}\)) are typically soluble.
• Exceptions: Sulfates of barium, calcium, and lead are exceptions, often forming insoluble compounds.
• Hydroxides and Sulfides: Generally insoluble, except for those of alkali metals and some alkaline earth metals.

By understanding and applying these rules, we can assess whether a given compound might dissolve in water and thus determine its applications and potential reactions.

Inorganic Chemistry

Inorganic chemistry is the branch of chemistry that deals with inorganic compounds. These compounds typically lack carbon-hydrogen bonds, which are the hallmark of organic compounds. Instead, inorganic chemistry focuses on elements and compounds such as metals, minerals, and organometallics. Zinc compounds are a part of this wide field:

• Metals and Their Compounds: Inorganic chemistry studies metals like zinc and their interactions with other elements and compounds.


• Catalysis and Material Science: Often involves inorganic compounds, utilizing metals and their complexes to speed up chemical reactions.


• Environmental and Biological Roles: Inorganics are crucial in environmental processes and biological systems, offering vital nutrients and participating in catalytic activities.

The study of zinc and its compounds is integral to understanding broader interactions and applications in the natural and industrial world. This understanding is crucial for applications ranging from medicine to agriculture.

Complex Ions

Complex ions are fascinating arrangements in chemistry formed when a central metal ion is surrounded by molecules or ions known as ligands. These ligands can donate electron pairs to the metal ion to form a stable complex:

• Formation of Complex Ions: Typically involves transition metals like zinc, which can bind with various ligands.


• Ligands: Can be neutral molecules like \(\mathrm{NH}_3\), or charged species like \(\mathrm{Cl}^-\); their arrangement around the metal ion influences the complex's properties.


• Stability and Solubility: Some complexes, such as \([\mathrm{Zn}(\mathrm{NH}_3)_{4}]^{2+}\), are soluble in water due to their stable, polar nature, despite hydroxide being typically insoluble.

Overall, complex ions play a significant role in modifying the solubility of metals in water, enabling diverse chemical processes and applications in both lab and industry.