Problem 42 - Chemistry The Central Science In Si Units

Question

Determine whether each of the following is true or false: (a) All strong bases are salts of the hydroxide ion. (b) The addition of a strong base to water produces a solution of \(\mathrm{pH}>7.0 .\) (c) Because \(\mathrm{Mg}(\mathrm{OH})_{2}\) is not very soluble, it cannot be a strong base.

Step-by-Step Solution

Answer

(a) True; (b) True; (c) False.

Step 1: Analyzing Part (a)

Evaluate if all strong bases are salts of the hydroxide ion. Strong bases typically dissociate completely in water, releasing hydroxide ions \((OH^-).\) Examples include NaOH, KOH, and LiOH, all of which are salts of the hydroxide ion. Therefore, part (a) is **true**.

Step 2: Analyzing Part (b)

Examine whether adding a strong base to water results in a solution with \(\text{pH} > 7.0.\) Strong bases fully dissociate in water, increasing the hydroxide ion concentration, which leads to a pH greater than 7. Hence, part (b) is **true**.

Step 3: Analyzing Part (c)

Check the claim regarding \(\mathrm{Mg(OH)_2}.\) Despite its low solubility, the part that does dissolve dissociates completely, releasing \((OH^-)\) ions, which is a characteristic of a strong base. However, not all of it dissolves, so it does not raise the pH significantly unless in large quantity. Thus, part (c) is **false** because the definition of a strong base depends on dissociation, not solubility.

Key Concepts

Understanding Hydroxide Ion

In chemistry, a hydroxide ion, denoted as \(OH^-\), plays a central role in defining the characteristics of strong bases. Strong bases are substances that completely dissociate in water, so each molecule of base releases a hydroxide ion into the solution. This increase in hydroxide ions is what makes a substance basic. Strong bases, like sodium hydroxide (NaOH) and potassium hydroxide (KOH), are considered salts of the hydroxide ion, because they consist of metallic cations combined with hydroxide ions.- These compounds dissolve and dissociate completely in water, allowing them to liberate hydroxide ions freely, increasing the solution's pH. Understanding the behavior of the hydroxide ion is vital because it helps predict how these substances will behave chemically in solutions, affecting both pH and reactivity.

Exploring pH Levels

The pH level of a solution is a measure of its acidity or basicity. The pH scale typically ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, while values above 7 are basic or alkaline. When a strong base is added to water, the pH of the solution increases due to the release of hydroxide ions. These ions react with hydrogen ions - which diminishes the concentration of hydrogen ions - therefore increasing the pH. For example, adding NaOH to water results in a solution with a pH greater than 7.0. The increase in pH demonstrates a decrease in acidity, with the resulting solution being more alkaline. Understanding pH is crucial as it directly influences our ability to describe and predict how different solutions will behave in various contexts, from chemical reactions to biological processes.

Solubility and Dissociation of Strong Bases

The concepts of solubility and dissociation are key when discussing strong bases. Solubility refers to how well a substance can dissolve in a solvent - usually water. A substance with high solubility dissolves readily, dispersing its molecules in the solvent. On the other hand, dissociation describes how a compound splits into its constituent ions when dissolved. Strong bases are characterized by their complete dissociation in water, which means that when they do dissolve, they dissociate completely into ions. An interesting example is magnesium hydroxide, \(\text{Mg(OH)}_2\), which is classified as a strong base because it dissociates completely, even though it is not very soluble. The limited solubility means not much of the base dissolves into the solution, which affects the overall pH unless added in sufficient amounts. The ability of a base to conduct this dissociation fully is what categorizes it as strong, rather than its solubility.