When an acidic solution is diluted, what happens to its pH?

Analysis of the Question

The question asks how the pH of an acidic solution changes when water is added to it (dilution). To answer this, we need to understand the relationship between pH, hydrogen ion concentration, and dilution.

Concept Explanation

1. Definition of pH: The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: $$\text{pH} = -\log_{10}[\text{H}^+]$$
2. The Relationship: Because of the negative sign in the formula, there is an inverse relationship between pH and the concentration of hydrogen ions ($[\text{H}^+]$). • High $[\text{H}^+]$ = Low pH (more acidic) • Low $[\text{H}^+]$ = High pH (less acidic)
3. Effect of Dilution: When you dilute an acidic solution by adding water, you are increasing the total volume of the solution. This causes the concentration of hydrogen ions ($[\text{H}^+]$) to decrease. As the concentration of hydrogen ions decreases, the solution becomes less acidic, causing the pH value to move closer to 7 (the neutral point).

Step-by-Step Solution

• Step 1: Identify the initial state. An acidic solution has a high concentration of $\text{H}^+$ ions, resulting in a low pH (typically below 7).
• Step 2: Apply dilution. Adding water increases the volume, which spreads the existing $\text{H}^+$ ions over a larger space, thereby lowering their concentration.
• Step 3: Observe the effect on pH. Since $[\text{H}^+]$ decreases, the value of $-\log_{10}[\text{H}^+]$ must increase.
• Step 4: Conclusion. As the acid becomes more dilute, the pH value rises (e.g., a solution with pH 2 might move toward pH 3 or 4 upon dilution).

Conclusion

Exam Tip

Remember the "pH Scale Rule": The pH scale is like a ladder. For acids, the "stronger" (more concentrated) the acid, the lower it is on the ladder (closer to 0). When you dilute an acid, you are moving it "up" the ladder toward 7, which means the pH value is increasing.