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

Understanding the concepts of pH and pOH is essential when studying the chemistry of aqueous solutions. The term 'pH' is an abbreviation for 'potential of hydrogen' and refers to the acidity or basicity of a solution. It is a scale used to specify how acidic or basic (alkaline) a water-based solution is.

pH is calculated as the negative logarithm of the hydrogen ion concentration, expressed mathematically as: \[ pH = -\log_{10} [H^{+}] \] The pH scale typically ranges from 0 to 14, with 7 being neutral. A pH less than 7 indicates acidity, while a pH greater than 7 indicates basicity.

Complementing pH, pOH represents the 'potential of hydroxide' and is related to the alkalinity (basic nature) of a solution. It's determined by taking the negative logarithm of the hydroxide ion concentration: \[ pOH = -\log_{10} [OH^{-}] \] If the sum of pH and pOH equals 14, the solution is said to be neutral. This balance is a result of the water dissociation constant (\( K_W = [H^{+}][OH^{-}] \) at 25°C). When dealing with aqueous basic solutions, if the pH is above 7, or equivalently the pOH is below 7, the solution is basic. Understanding this balance is key to grasping the fundamental nature of aqueous solutions.

The concentration of hydrogen ions, denoted as \( [H^{+}] \), is a critical parameter in determining the acidity of a solution. As mentioned, the pH of a solution is directly related to the hydrogen ion concentration through a simple logarithmic relationship.

In pure water at 25°C, the concentration of hydrogen ions is \( 1 \times 10^{-7} \) molar, which is neutral on the pH scale (\( pH = 7 \)). But in an aqueous basic solution, the hydrogen ion concentration is lower than \( 1 \times 10^{-7} \) molar, making the pH higher than 7. In simple terms, the more diluted the hydrogen ions are, or the lower their concentration, the more basic the solution becomes.

It's important for students to remember that small changes in hydrogen ion concentration can lead to significant shifts in pH due to the logarithmic scale. For instance, a solution with a hydrogen ion concentration of \( 1 \times 10^{-6} \) molar has a pH of 6, which is ten times more acidic than pure water.

Conversely, the concentration of hydroxide ions (\( [OH^{-}] \)) in a solution is a measure of its basicity. Basic, or alkaline, solutions have a higher hydroxide ion concentration than neutral water. In neutral water at 25°C, the hydroxide ion concentration is the same as the hydrogen ion concentration, \( 1 \times 10^{-7} \) molar.

For a solution to be considered basic, its \( [OH^{-}] \) must exceed this neutral concentration, thus lowering the pOH below 7. The higher the concentration of hydroxide ions, the more basic the solution, and correspondingly, the higher its pH will be.

For instance, if a solution has an \( [OH^{-}] \) of \( 1 \times 10^{-4} \) molar, its pOH would be 4, and consequently, the pH would be 10, making it a strongly basic solution. It's crucial for students to grasp this concept as it lays the foundation for understanding the behavior of solutions and their reactions in various chemical contexts.