Question

Predict whether each of the following compounds is soluble in water. (a) \(\mathrm{Ni}(\mathrm{OH})_{2}\), (b) \(\mathrm{PbBr}_{2}\), (c) \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{AlPO}_{4}\) (e) \(\mathrm{AgCH}_{3} \mathrm{COO}\).

Short answer

The solubility predictions for the given compounds are: a) \(\mathrm{Ni}(\mathrm{OH})_{2}\): Insoluble b) \(\mathrm{PbBr}_{2}\): Insoluble c) \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\): Soluble d) \(\mathrm{AlPO}_{4}\): Insoluble e) \(\mathrm{AgCH}_{3} \mathrm{COO}\): Soluble

Step-by-step solution

a) Solubility of \(\mathrm{Ni}(\mathrm{OH})_{2}\)

According to rule 4, all hydroxides are insoluble except when with alkali metals and ammonia. Since \(\mathrm{Ni}(\mathrm{OH})_{2}\) is a hydroxide and Ni is not an alkali metal or ammonia, it is considered insoluble in water.

b) Solubility of \(\mathrm{PbBr}_{2}\)

According to rule 3, all bromides are soluble except for those of Ag, Hg, and Pb. Since the compound is a bromide of Pb, it is considered insoluble in water.

c) Solubility of \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\)

According to rule 1, all nitrates are soluble. Since \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) is a nitrate, it is considered soluble in water.

d) Solubility of \(\mathrm{AlPO}_{4}\)

According to rule 4, all phosphates are insoluble except when with alkali metals and ammonia. Since \(\mathrm{AlPO}_{4}\) is a phosphate and Al is not an alkali metal or ammonia, it is considered insoluble in water.

e) Solubility of \(\mathrm{AgCH}_{3} \mathrm{COO}\)

According to rule 1, all acetates are soluble. Since \(\mathrm{AgCH}_{3} \mathrm{COO}\) is an acetate, it is considered soluble in water. In conclusion, the solubility predictions for the given compounds are: a) \(\mathrm{Ni}(\mathrm{OH})_{2}\): Insoluble b) \(\mathrm{PbBr}_{2}\): Insoluble c) \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\): Soluble d) \(\mathrm{AlPO}_{4}\): Insoluble e) \(\mathrm{AgCH}_{3} \mathrm{COO}\): Soluble

Key concepts

Chemical Solubility

Understanding chemical solubility is fundamental to many aspects of chemistry. Solubility is a property referring to the ability for a given substance, the solute, to dissolve in a solvent. Water, often called the 'universal solvent', is the most common solvent used to determine solubility. A solute's structure, polarity, and intermolecular forces play significant roles in its solubility.

The solubility rules allow us to predict the solubility of ionic compounds in water. These rules are patterns observed over many experiments, which state that certain ions generally form soluble compounds and others form insoluble substances under usual conditions. These rules are not without exception, but they provide a reliable guideline for understanding which substances will dissolve in water and which will not.

For example, nitrates ((NO3)−)) and acetates ((CH3COO)−)) are generally soluble, whereas compounds like carbonates ((CO32−)) and phosphates ((PO43−)) tend to be insoluble unless they are paired with certain cations. In the exercise, it was determined that (Ba(NO3)2)) is soluble and (Ni(OH)2)) is insoluble based on these guidelines, illustrating the practical application of solubility rules.

Soluble Compounds

Soluble compounds are those that can dissolve in a solvent, creating a homogeneous solution at the molecular or ionic level. The degree of solubility can range from highly soluble, where a lot of the substance can dissolve in the solvent, to slightly soluble, where only a small amount can be dissolved.

Factors Influencing Solubility

Several factors can influence the solubility of a compound in water:

• The compound's temperature: Generally, solubility increases with temperature.
• The nature of the solute and solvent: Like dissolves like; polar solutes dissolve in polar solvents and non-polar in non-polar.
• The presence of other substances in solution can affect solubility through a common ion effect or other interactions.

In our exercise example, (Ba(NO3)2)) and (AgCH3COO)) were identified as soluble compounds. This means that in water, these compounds dissociate into their respective ions and disperse uniformly throughout the solvent, leaving no visible precipitate or undissolved residue.

Insoluble Substances

Contrary to soluble compounds, insoluble substances do not dissolve in a solvent to a significant extent. This results in the substance remaining largely intact and visible within the mixture, often settling out as a precipitate.

Common Insoluble Substances

Some common examples of usually insoluble substances include many metal hydroxides, carbonates, phosphates, and sulfides. The exercise provided cases like (Ni(OH)2)), (PbBr2)), and (AlPO4)) which are insoluble in water due to the specific combinations of their ions. In aqueous solutions, an insoluble substance will either remain in its original solid state or it will form a precipitate if the reaction between soluble substances produces an insoluble compound.

Understanding these solubility patterns is important not only in the chemistry lab but also in real-world applications such as wastewater treatment, where the removal of insoluble contaminants is essential.