Let's analyze the given statements one by one to determine which one is true.
1. As hydrogen ion concentration increases, so does the pH value.
- This statement is false. The pH value is inversely related to the hydrogen ion concentration. When the concentration of hydrogen ions increases, the pH value decreases, indicating a more acidic solution.
2. Acids have pH values between 4.0 and 9.0.
- This statement is false. Acids have pH values below 7.0. Values between 4.0 and 9.0 would include both acidic and basic substances, but acids do not have values higher than 7.
3. As hydrogen ion concentration increases, the pH value decreases.
- This statement is true. As the concentration of hydrogen ions increases, the solution becomes more acidic, and thus the pH value decreases.
4. Acids have pH values above 7.0.
- This statement is false. Substances with pH values above 7.0 are bases, not acids. Acids have pH values below 7.0.
So, the correct and true statement regarding pH is:
- As hydrogen ion concentration increases, the pH value decreases.
Hence, the true statement is:
As hydrogen ion concentration increases, the pH value decreases.
