Which statement about pH is correct?

Which statement about pH is correct?

• A. pH is a linear scale of H+ concentration.
• B. pH measures the temperature of the solution.
• C. pH measures the concentration of hydroxide ions only.
• D. pH is a logarithmic scale of H+ concentration; neutral pH is 7; higher numbers are basic; lower numbers are acidic.

Answer: (D)

The main idea is that pH is a logarithmic measure of hydrogen ion activity, not a simple linear count of H+ ions. Because pH is defined as -log10[H+], a one-unit change in pH corresponds to a tenfold change in hydrogen ion concentration. That’s why the scale compresses the vast range of possible [H+] values into a manageable number range. At room temperature, a pH of 7 is considered neutral; values below 7 indicate acidity and values above 7 indicate basicity. This explanation captures why higher numbers mean less H+, and why the scale is logarithmic rather than linear. It’s not about temperature, and it doesn’t measure hydroxide ion concentration directly (that’s a separate pOH concept, with pH and pOH summing to about 14 at 25°C). Also, pH reflects hydrogen ion activity, not merely concentration, due to interactions in solution.

The main idea is that pH is a logarithmic measure of hydrogen ion activity, not a simple linear count of H+ ions. Because pH is defined as -log10[H+], a one-unit change in pH corresponds to a tenfold change in hydrogen ion concentration. That’s why the scale compresses the vast range of possible [H+] values into a manageable number range. At room temperature, a pH of 7 is considered neutral; values below 7 indicate acidity and values above 7 indicate basicity. This explanation captures why higher numbers mean less H+, and why the scale is logarithmic rather than linear. It’s not about temperature, and it doesn’t measure hydroxide ion concentration directly (that’s a separate pOH concept, with pH and pOH summing to about 14 at 25°C). Also, pH reflects hydrogen ion activity, not merely concentration, due to interactions in solution.