Problem 19 - Chemistry: The Central Science

Question

Using solubility guidelines, predict whether each of the following compounds is soluble or insoluble in water: (a) \(\mathrm{NiCl}_{2}\) (b) \(\mathrm{Ag}_{2} \mathrm{~S},(\mathrm{c}) \mathrm{Cs}_{3} \mathrm{PO}_{4}\) (d) \(\mathrm{SrCO}_{3}\) (e) \(\mathrm{PbSO}_{4}\).

Step-by-Step Solution

Answer

Using solubility guidelines, we can predict the solubility of the following compounds in water: (a) NiCl₂ is soluble. (b) Ag₂S is insoluble. (c) Cs₃PO₄ is soluble. (d) SrCO₃ is insoluble. (e) PbSO₄ is insoluble.

Step 1: (a) NiCl₂ solubility

According to solubility rule 3, most chlorides are soluble, except for silver, lead, and mercury(I) chlorides. NiCl₂ is a chloride compound and does not fall into these exceptions, so it is soluble in water.

Step 2: (b) Ag₂S solubility

According to solubility rule 6, most sulfides are insoluble, except for those of alkali metals and ammonium. Ag₂S is a sulfide compound and does not fall into these exceptions, so it is insoluble in water.

Step 3: (c) Cs₃PO₄ solubility

According to solubility rule 6, most phosphates are insoluble, except for those of alkali metals and ammonium. Cs₃PO₄ is a phosphate compound with an alkali metal (Cs), so it is soluble in water.

Step 4: (d) SrCO₃ solubility

According to solubility rule 6, most carbonates are insoluble, except for those of alkali metals and ammonium. SrCO₃ is a carbonate compound and does not fall into these exceptions, so it is insoluble in water.

Step 5: (e) PbSO₄ solubility

According to solubility rule 4, most sulfates are soluble, except for those of calcium, strontium, barium, mercury(I), lead, and silver. PbSO₄ is a sulfate compound and is one of these exceptions, so it is insoluble in water.

Key Concepts

Solubility in Water

Solubility is a chemical property that refers to the ability of a substance to dissolve in a solvent. In most chemistry discussions, the solvent is commonly water. When a compound is soluble in water, it means that it can dissolve smoothly, forming a homogenous solution. The degree of solubility depends on interactions between water molecules and the particles of the compound.

It's essential to understand that solubility can be vastly different depending on the nature of the solute and solvent. While many salts such as NaCl are soluble in water, others like sand are not. Solubility is affected by factors such as temperature, pressure, and chemical structure. Commonly, solubility rules are used to predict how likely it is for a substance to dissolve in water. By understanding these rules, students can easily determine whether specific compounds are water-soluble.

Water-Solubility Exceptions

Though many compounds follow general solubility rules, there are always exceptions. These exceptions are critical to note because they can impact the practical outcome of experiments.

• Some chlorides, such as those of silver, lead, and mercury(I), are insoluble even though most chlorides are soluble.
• Similarly, while most sulfates are soluble, calcium, strontium, barium, mercury(I), lead, and silver sulfates do not dissolve in water.
• Carbonates, phosphates, and sulfides also have specific exceptions, whereby they are mostly insoluble except if they contain alkali metals or ammonium.

Recognizing these exceptions helps to identify compounds that might behave in unexpected ways in reactions or solutions. Rather than assuming all compounds behave the same way, it is important to test each for water-solubility.

Solubility of Chlorides

Chlorides are generally soluble in water, which makes them noticeably reliable for making solutions for reactions. However, the solubility rule for chlorides comes with noteworthy exceptions.

Chlorides of silver (\(\text{AgCl}\)), lead (\(\text{PbCl}_2\)), and mercury(I) (\(\text{Hg}_2\text{Cl}_2\)) are not soluble. This indicates that, despite being chlorides, these do not dissolve to form homogeneous solutions in water. Because of this, they often form a precipitate when mixed in solutions. Understanding these exceptions is important for laboratory work, as it allows chemists to predict the formation of solid by-products in chemical reactions.

Solubility of Sulfides

Most sulfides are insoluble in water, meaning they do not easily dissolve to form a solution. This general lack of solubility has a couple of notable exceptions.

• Sulfides of alkali metals, such as sodium sulfide (\(\text{Na}_2\text{S}\)), are soluble. These can dissolve in water to form clear solutions.
• Ammonium sulfide (\(\text{(NH}_4\text{)}_2\text{S}\)) is another sulfide that is soluble in water.

Knowing the solubility of sulfides is crucial, especially in industrial and chemical processes where the formation or prevention of solid precipitates is necessary. It also plays an important role in the fields of geology and medicine, where sulfide solubility impacts both extraction methods and pharmaceutical formulations.