Question

Predict which of the following compounds are insoluble in water: (a) strontium carbonate, \(\mathrm{Sr} \mathrm{CO}_{3}\) (b) calcium hydroxide, \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) nickel(II) sulfide, NiS (d) mercury(II) bromide, \(\mathrm{HgBr}_{2}\)

Short answer

Insoluble: SrCO₃, NiS. Soluble: Ca(OH)₂ (slightly), HgBr₂.

Step-by-step solution

Understand Solubility Rules

To determine if a compound is insoluble in water, we can apply general solubility rules. Common rules include: carbonates (such as SrCO₃) are generally insoluble except for those of alkali metals; hydroxides (such as Ca(OH)₂) are generally insoluble except for those of alkali metals and barium; sulfides (like NiS) are generally insoluble except for alkali metals and ammonium; most bromides (including HgBr₂) are soluble except those with lead(II), silver, and mercury(I).

Apply Solubility Rules to SrCO₃

Strontium carbonate (SrCO₃) is a carbonate, and according to the solubility rules, most carbonates are insoluble except those of alkali metals and ammonium. Since strontium is an alkaline earth metal, SrCO₃ is insoluble in water.

Apply Solubility Rules to Ca(OH)₂

Calcium hydroxide (Ca(OH)₂) is a hydroxide. Hydroxides are generally insoluble except for those of alkali metals and barium. Calcium is an alkaline earth metal, not an alkali metal or barium; however, Ca(OH)₂ is slightly soluble, meaning it has limited solubility.

Apply Solubility Rules to NiS

Nickel(II) sulfide (NiS) is a sulfide. According to solubility rules, most sulfides are insoluble except for those with alkali metals and ammonium. Nickel is neither an alkali metal nor ammonium, so NiS is insoluble in water.

Apply Solubility Rules to HgBr₂

Mercury(II) bromide (HgBr₂) falls under the category of bromides, which are generally soluble except with lead(II), silver, and mercury(I). Since mercury(II) is not mercury(I), HgBr₂ is expected to be soluble in water.

Key concepts

Insoluble compounds

Insolubility in water is mainly determined by general solubility rules, which are often encountered by students in chemistry studies. These rules help identify whether a compound can dissolve in water or not based on the type of ions they contain. Knowing these rules helps in predicting the behavior of compounds when they are mixed with water.

Some compounds are considered insoluble, meaning they do not dissolve in water, or they dissolve very slightly. When a compound is termed insoluble, it typically means that the amount that dissolves is negligible, and thus, it forms a precipitate rather than a solution. This is crucial in many applications such as precipitation reactions, where forming an insoluble compound helps drive a reaction to completion.

The common ions that lead to insoluble compounds include:

• Carbonates (CO₃²⁻)
• Hydroxides (OH⁻)
• Sulfides (S²⁻)

These are often insoluble unless paired with alkali metals (such as lithium, sodium, and potassium) or ammonium (NH₄⁺). Understanding these types of interactions helps understand everyday chemistries and industrial applications.

Solubility of carbonates

Carbonates are compounds that contain the carbonate ion, ext{CO₃²⁻}. Generally, carbonates are considered insoluble in water. However, there are prominent exceptions to this rule.

For instance, carbonates of alkali metals and ammonium are indeed soluble in water. This is because these compounds tend to dissociate readily in water, releasing ions that are highly solvable. Examples include the carbonates of sodium ( ext{Na₂CO₃}) and potassium ( ext{K₂CO₃}). These substances dissolve in water, allowing them to participate in reactions in aqueous solutions effectively.

Strontium carbonate ( ext{SrCO₃}) serves as a key example that follows the general rule: it is insoluble. Strontium belongs to the alkaline earth metals such as magnesium and calcium, whose carbonates are known for their limited solubility. When mixed with water, SrCO₃ typically results in a suspension, not a solution, thus classifying it as insoluble.

Solubility of hydroxides

Hydroxides contain the hydroxide ion ( ext{OH⁻}) and, by general rules, are typically insoluble in water. But, like carbonates, particular exceptions exist.

Hydroxides of alkali metals such as lithium hydroxide ( ext{LiOH}), sodium hydroxide ( ext{NaOH}), and potassium hydroxide ( ext{KOH}) are all soluble. Barium hydroxide is another exception and shows considerable solubility. However, calcium hydroxide ( ext{Ca(OH)₂}) is an interesting case to consider. Although it is classified as insoluble, it demonstrates slight solubility in water by allowing a minimal amount to dissolve. This property makes it useful in certain applications like limewater tests, yet it will mostly remain as solid when added to water.

Hydroxides form fascinating subjects in chemistry due to their ability to affect pH levels. The dissociation of soluble hydroxides results in an increase in ext{OH⁻} concentration leading to a solution becoming basic or alkaline.

Solubility of sulfides

Sulfides feature the sulfide ion ( ext{S²⁻}) and are categorized as generally insoluble in water.

Like with carbonates and hydroxides, exceptions include sulfides with alkali metals and ammonium, which are soluble. These exceptions are mostly due to the high solubility of their corresponding ions facilitating the dissolving process in water.

Nickel(II) sulfide ( ext{NiS}), for example, is an illustration of an insoluble sulfide. When NiS is placed in water, it does not dissolve significantly, which is consistent with the general solubility rules for sulfides except those involving elements from Groups 1 and 2 on the periodic table. Practical applications revolve around using such properties to conduct precipitation reactions in labs and industries to separate compounds.

Understanding sulfide solubility is essential in fields like geochemistry and environmental science, where sulfide presence affects mineral formation and soil composition.