If a neutral solution of water, with pH = 7.00, is cooled to 10 °C, the pH rises to 7.27. Which of the following three statements is correct for the cooled water: (i) [H+] > [OH-], (ii) [H+] = [OH-], or (iii) [H+] < [OH-]?

Verified step by step guidance

1. Understand that pH is a measure of the hydrogen ion concentration, [H+], in a solution. A pH of 7.00 at 25 °C indicates a neutral solution where [H+] = [OH-].
2. Recognize that the pH scale is temperature-dependent. As temperature decreases, the ion product of water (Kw) changes, affecting the pH of neutrality.
3. Note that at 10 °C, the pH of neutrality is higher than 7.00, specifically 7.27, indicating that the concentrations of [H+] and [OH-] are still equal, but both are lower than at 25 °C.
4. Since the pH of the cooled water is 7.27, which is the new neutral pH at 10 °C, it implies that [H+] = [OH-] even though the pH is higher than 7.00.
5. Conclude that for the cooled water at 10 °C with pH 7.27, the correct statement is (ii) [H+] = [OH-].

Key Concepts

pH and Ion Concentration

pH is a measure of the hydrogen ion concentration in a solution, defined as pH = -log[H+]. In pure water at 25 °C, the concentrations of hydrogen ions [H+] and hydroxide ions [OH-] are equal, resulting in a neutral pH of 7. As temperature changes, the ion product of water also changes, affecting the pH.

Temperature Effects on Water Ionization

The ionization of water is temperature-dependent. At lower temperatures, the equilibrium shifts, resulting in a decrease in the ion product of water (Kw). This means that at 10 °C, the concentration of [H+] and [OH-] will not be equal, leading to a change in pH and indicating that the solution is no longer neutral.

Understanding Neutrality in Solutions

A neutral solution is characterized by equal concentrations of hydrogen ions and hydroxide ions. When the pH of water increases from 7.00 to 7.27 upon cooling, it indicates that the concentration of [H+] has decreased relative to [OH-], thus the solution is no longer neutral, and [H+] < [OH-] in the cooled water.