Problem 124 - Introductory Chemistry Atoms First

Question

Based solely on concentrations, when is an aqueous solution judged to be acidic? Give two answers to this question.

Step-by-Step Solution

Answer

An aqueous solution is judged to be acidic based on two main aspects: hydrogen ion (H+) concentration and the pH scale. 1. The solution is considered acidic when the concentration of hydrogen ions (H+) is greater than the concentration of hydroxide ions (OH-), that is, when \([H^+] > [OH^-]\). 2. Using the pH scale, an aqueous solution is judged to be acidic when its pH is less than 7.

Step 1: Explanation of an Acidic Solution

An aqueous solution is considered acidic when it has a higher concentration of hydrogen ions (H+) than hydroxide ions (OH-). In other words, the solution has an excess of H+ ions compared to OH- ions, resulting in acidic properties.

Step 2: Answer 1: Based on H+ concentration

An aqueous solution is judged to be acidic when the concentration of hydrogen ions (H+) is greater than the concentration of hydroxide ions (OH-). That is, when \([H^+] > [OH^-]\).

Step 3: Answer 2: Based on pH scale

The pH scale is a measure of the acidity or alkalinity of a solution, ranging from 0 to 14. A solution with a pH value less than 7 is considered acidic. So, an aqueous solution is judged to be acidic when its pH is less than 7.

Key Concepts

Understanding the pH Scale

The pH scale is a critical tool used to determine the acidity or alkalinity of an aqueous solution. It ranges from 0 to 14, where each number represents the concentration of hydrogen ions (\(\text{H}^+\)) in the solution. A pH value is a logarithmic measure, meaning each whole number change represents a tenfold difference in acidity or alkalinity.
When a solution has a pH of less than 7, it is deemed acidic. This happens when there are more hydrogen ions present compared to hydroxide ions. A neutral solution, like pure water, has a pH of exactly 7. Solutions with a pH greater than 7 are basic or alkaline and have more hydroxide ions than hydrogen ions.

• pH below 7: acidic
• pH of 7: neutral
• pH above 7: basic

This scale helps chemists and scientists assess and compare the acidity levels of different solutions, providing a standard framework for understanding chemical properties.

Hydrogen Ion Concentration

Hydrogen ion concentration in a solution is a key determinant of its acidity. The more hydrogen ions (\(\text{H}^+\)) present, the more acidic the solution becomes. This occurs because hydrogen ions lower the pH of the solution. They are positively charged ions that interact with other components of the solution.
In aqueous solutions, water molecules can dissociate into hydrogen ions and hydroxide ions (OH-). When an external acidic substance is added, it increases the number of hydrogen ions. For a solution to remain acidic, the hydrogen ion concentration must be greater than the hydroxide ion concentration, expressed symbolically as \([\text{H}^+] > [\text{OH}^-]\).
Understanding this concept is important for comprehending how solutions behave chemically and how acids differ from bases based on the ion concentrations. Knowing hydrogen ion concentration helps in predicting how a solution will interact with other chemicals or biological systems.

Hydroxide Ion Concentration

Hydroxide ions (\(\text{OH}^-\)) are negatively charged ions that play a significant role in determining a solution's basicity. In contrast to acids, basic solutions contain a higher concentration of hydroxide ions. This results in an increase in pH value.
In any aqueous solution, some water molecules become hydroxide ions through dissociation. In a basic or alkaline solution, the concentration of these ions exceeds the concentration of hydrogen ions, leading to a higher pH. This relationship between hydroxide ions and hydrogen ions is crucial because it explains the dual nature of solutions as either acidic or basic based on their ion content.
For a solution to be considered basic, the hydroxide ion concentration must be greater than the hydrogen ion concentration, often stated as \([\text{OH}^-] > [\text{H}^+]\). Recognizing this allows scientists to manage chemical processes by predicting changes in ion concentrations and developing solutions to buffer against unwanted pH shifts.