How many times more acidic is pH 7 than pH 9?

To determine how many times more acidic pH 7 is than pH 9, it is important to understand the pH scale and its logarithmic nature. The pH scale ranges from 0 to 14, with lower values being more acidic and higher values being more basic.

Each whole number change on the pH scale corresponds to a tenfold change in hydrogen ion concentration. A pH of 7 is considered neutral, while a pH of 9 indicates a basic solution. This means that moving from pH 9 to pH 7 represents a decrease of 2 units on the pH scale.

Since each unit represents a tenfold increase in acidity, a decrease of two units means:

10 (for pH 8) × 10 (for pH 7) = 100.

Thus, pH 7 is 100 times more acidic than pH 9, confirming that the correct answer is indeed that pH 7 is 100 times more acidic. Understanding this relationship through the logarithmic scale of pH is essential for effectively analyzing acidity and alkalinity in various substances.