Effect of changing hydrogen ion concentration on pH and acidity:\nA solution has pH = 5.9. If [H+] is decreased by a factor of 100, the solution becomes…

Introduction / Context:
The pH scale quantifies acidity via the negative decimal logarithm of hydrogen ion concentration. Because it is logarithmic, multiplicative changes in [H+] translate to additive changes in pH. Interpreting how a change in [H+] affects where the solution lies relative to neutrality (pH 7) is a common analytical step.

Given Data / Assumptions:

• Initial pH = 5.9.
• Hydrogen ion concentration is decreased by 100× (i.e., divided by 100).
• Aqueous solution near ambient temperature; activity effects neglected.

Concept / Approach:
If [H+] decreases by a factor of 10^2, pH increases by 2 units: pH_new = pH_old + 2. Values above 7 are classified as basic (alkaline), below 7 as acidic, and equal to 7 as neutral (at 25°C).

Step-by-Step Solution:

Verification / Alternative check:
Compute [H+]: at pH 5.9, [H+] ≈ 10^−5.9 M. After dividing by 100, [H+] ≈ 10^−7.9 M, which is less than 10^−7 M (neutral at 25°C), confirming basicity.

Why Other Options Are Wrong:

• More acidic: Would require [H+] to increase, not decrease.
• Neutral: Would require pH to be about 7.0, not 7.9.
• Same acidity: Impossible when [H+] changes by two orders of magnitude.
• Slightly acidic: pH 7.9 is alkaline, not acidic.

Common Pitfalls:
Confusing the direction of pH change with [H+] change; forgetting the logarithmic relation and using linear differences.

Final Answer:
basic