The change in pH is maximum when 100 ml of pH=2 solution is mixed with 100 ml of pH=12 solution.
Understanding pH Change
pH is a measure of the acidity or alkalinity of a solution. A low pH (typically 0-6) indicates acidity, a high pH (typically 8-14) indicates alkalinity (or basicity), and a pH of 7 is neutral. When solutions with different pH values are mixed, a reaction often occurs that changes the overall acidity or alkalinity of the resulting mixture. The change in pH refers to the difference between the initial pH values of the components and the final pH of the mixture.
The magnitude of the pH change depends on the initial pH values, the volumes mixed, and the types of substances involved (e.g., strong acids, weak acids, strong bases, weak bases). Mixing a strong acid with a strong base typically results in a neutralization reaction that can cause a dramatic shift in pH, especially if the initial solutions are far apart on the pH scale.
The Reference-Based Conclusion
Based on the provided reference, a specific scenario involving the mixing of solutions with significantly different pH values results in the largest observed change in pH. The reference explicitly states:
"The difference between the initial pH and final pH is largest in D. The correct option is D- 100ml of pH=2 solution is mixed with 100 ml of pH=12."
This conclusion identifies the mixing of 100 ml of a pH=2 solution with 100 ml of a pH=12 solution as the case exhibiting the maximum change in pH among the scenarios considered in the source.
Why This Mixture Results in a Large Change
Mixing a solution with a very low pH (highly acidic) and a solution with a very high pH (highly alkaline) brings together strong concentrations of hydrogen ions (H⁺) from the acidic solution and hydroxide ions (OH⁻) from the alkaline solution. These ions react to form water (H₂O) through a process called neutralization.
Initial pH values:
- pH=2 indicates a relatively high concentration of H⁺ ions ([H⁺] = 10⁻² M).
- pH=12 indicates a relatively high concentration of OH⁻ ions ([OH⁻] = 10⁻² M, considering pOH=2 and pH+pOH=14).
Mixing equal volumes of solutions with these initial pH values leads to a significant neutralization reaction. The resulting mixture will likely have a final pH much closer to neutral (pH 7) than either of the starting solutions. Moving from extreme ends of the pH scale (2 and 12) towards the middle (around 7) inherently involves a large numerical difference between the initial and final pH values, maximizing the observed change.
Consider the starting points: 2 and 12. The final pH after mixing a strong acid at pH 2 and a strong base at pH 12 in equal volumes would depend on the exact concentrations and if they are indeed strong. Assuming they represent strong acid and base solutions, the reaction would lead to a final pH near 7, or slightly acidic/basic if one was in slight excess relative to complete neutralization at pH 7. Regardless of the exact final pH near neutrality, the difference from the initial pH values of 2 and 12 is substantial. For instance, if the final pH is 7, the changes are |7-2|=5 and |7-12|=5. Compared to mixing solutions with pH values closer together (e.g., pH 4 and pH 6), where the final pH might be between 4 and 6, the magnitude of change would be much smaller.
This is why, as the reference indicates, mixing solutions at opposite ends of the pH spectrum (pH=2 and pH=12) in equal volumes results in the maximum change in pH.
