Predict the \(\mathrm{pH}(>7,<7,\) or \(\approx 7)\) of aqueous solutions containing the following salts: (a) \(\mathrm{KBr}\) (b) \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3},(\mathrm{c}) \mathrm{BaCl}_{2},(\mathrm{~d}) \mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\)

Understanding acid-base reactions is vital when predicting the pH of a solution. When salts dissolve in water, they can form ions that may affect the acidity or basicity of the solution. Acidic solutions have a pH lower than 7, basic solutions have a pH higher than 7, and neutral solutions approximate to a pH of 7.

In water, certain metal ions like (Al^{3+}) from (Al(NO_3)_3) and (Bi^{3+}) from (Bi(NO_3)_3) can hydrolyze to form hydronium ions (H_3O^+), thereby lowering the pH. The reaction between these ions and water illustrates a typical acid-base reaction where water (the base) reacts with the ions (the acids) to produce an acidic solution, as evidenced in the step-by-step solutions of (Al(NO_3)_3) and (Bi(NO_3)_3).

It is important to note that not all ions will affect pH in this manner; some ions are considered neutral in the context of acid-base chemistry, like (K^+) from (KBr) and (Ba^{2+}) from (BaCl_2). They do not react with water to form either hydronium or hydroxide ions, resulting in a neutral solution.

The solubility of salts in water is a key factor in determining the ions present in a solution. Solubility refers to how well a substance can dissolve in a solvent to create a homogenous mixture — an aqueous solution in this context. When salts dissolve in water, they break down into their constituent ions, a process known as dissociation.

• KBr: Dissolves and dissociates completely into (K^+) and (Br^-) ions.
• Al(NO_3)_3: Dissolves and dissociates into (Al^{3+}) and (NO_3^-) ions, where (Al^{3+}) undergoes further reaction with water.
• BaCl_2: Completely dissociates into (Ba^{2+}) and (Cl^-) ions in water.
• Bi(NO_3)_3: Like aluminium nitrate, it releases (Bi^{3+}) which reacts with water.

Understanding the behavior of these ions in solution enables us to explain the observed pH levels of the salts after dissolution.

The ability to predict the pH of a salt solution is an application of acid-base principles and solubility knowledge. When a salt dissolves in water, the resulting pH is influenced by the reactions of the cations ((A^+)) and anions ((B^-)) that form.

Neutral salts, like (KBr) and (BaCl_2), will not significantly affect the pH and thus the solution remains neutral, or has a pH of approximately 7. However, if a salt dissociates and forms a cation or anion that reacts with water (an acid or base reaction), the pH will be affected. For example, as seen in the exercise, (Al(NO_3)_3) creates an acidic solution due to (Al^{3+}) hydrolyzing to form hydronium ions.

By applying this knowledge, students can look at the constituent ions of a dissolved salt and determine if the ions are known strong acids or bases, weak acids or bases, or neutral. This interaction with water determines the resulting pH of the solution, and thus allows for an informed prediction even before actual measurements.