Which of the following has the highest value of pH? (a) \(\mathrm{HCl}\) (b) \(\mathrm{NaOH}\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{COOH}\)

Acids and bases are substances that can change the pH of solutions, a measure of how acidic or basic a solution is. Acidic compounds, such as hydrochloric acid (HCl) and acetic acid (CH₃COOH), release hydrogen ions (H+) when dissolved in water, resulting in a pH lower than 7. On the other hand, basic compounds, like sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)₂), release hydroxide ions (OH−) into the solution, which results in a pH greater than 7.

Understanding the properties of acids and bases is crucial in determining the pH value, an index of how strongly acidic or basic a solution is. The exercise demonstrates these concepts by comparing four compounds, two of which are acidic and two basic, to determine which one results in the highest pH value, indicating the most basic solution.

The pH scale is a numeric scale, typically ranging from 0 to 14, that indicates the acidity or basicity of a solution. It is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value. Similarly, each whole pH value above 7 is ten times more basic than the one below it. In other words, a small change in pH corresponds to a significant change in the concentration of hydrogen ions. A pH of 7 is considered neutral, which is the pH of pure water at 25°C.

When you dissolve an acidic or basic compound in water, the pH will change accordingly. Acids will decrease the pH, making the solution more acidic, while bases raise the pH, making the solution more basic. The exercise provided demonstrates how to use the pH concept to identify the strongest base among several compounds by understanding which one will have the most significant effect on raising the pH level of the solution.

Bases are classified into strong and weak based on their ability to dissociate in water. A strong base like calcium hydroxide (Ca(OH)₂) completely dissociates into its ions, resulting in a greater increase in OH− ion concentration and therefore a higher pH. Weak bases, such as ammonia (NH₃), only partially dissociate in water, leading to a lower concentration of OH− ions and consequently a smaller increase in pH.

From the exercise, the concept of strong bases is applied to determine that Ca(OH)₂, which releases two OH− ions for each molecule, makes the solution more basic than NaOH, which releases only one OH− ion per molecule. This property of calcium hydroxide is what makes it the compound with the highest pH among the options given, reflecting a more substantial ability to make the solution basic.