Which statement correctly describes the relationship between pH and hydrogen ion concentration in aqueous solution?

• A. As pH decreases, the hydrogen ion concentration $\left[H^{+}\right]$ decreases.
• B. pH is directly proportional to the hydrogen ion concentration $\left[H^{+}\right]$.
• C. pH is independent of the hydrogen ion concentration $\left[H^{+}\right]$.
• D. As pH decreases, the hydrogen ion concentration $\left[H^{+}\right]$ increases.

Answer: (D)

Verified step by step guidance

1. Recall the definition of pH, which is given by the formula: \(\text{pH} = -\log_{10} [\mathrm{H}^+]\), where \([\mathrm{H}^+]\) is the hydrogen ion concentration in moles per liter.
2. Understand that the logarithmic relationship means pH and \([\mathrm{H}^+]\) are inversely related: as \([\mathrm{H}^+]\) increases, the value of \(-\log_{10} [\mathrm{H}^+]\) decreases, so pH decreases.
3. Analyze the options: if pH decreases, then \([\mathrm{H}^+]\) must increase, because a lower pH corresponds to a higher acidity (more hydrogen ions).
4. Recognize that pH is not directly proportional to \([\mathrm{H}^+]\) because of the logarithmic scale; a direct proportionality would mean both increase or decrease together linearly, which is not the case.
5. Conclude that the correct description is: 'As pH decreases, the hydrogen ion concentration \([\mathrm{H}^+]\) increases.'