Final answer:
The second solution must be basic with a pH higher than 7 to raise the pH of the first acidic solution from 4 to 5 upon mixing, reflecting the logarithmic nature of the pH scale where each whole pH value below 7 is 10 times more acidic than the next.
Step-by-step explanation:
The pH of a solution is a measure of its acidity or basicity. When you mix two solutions with different pH levels, the resulting pH will depend on the relative volumes and concentrations of hydronium ions of the solutions. In this case, the first solution has a pH of 4, which indicates it is an acidic solution. To raise this pH to 5 upon mixing with a second solution, the second solution must be basic because the resulting pH is higher than that of the first solution and is moving towards neutrality (pH 7).
Because the pH scale is logarithmic, each whole pH value below 7 is 10 times more acidic than the next higher value. Thus, a solution with a pH of 4 has 10 times more hydronium ions than a solution with a pH of 5. To increase the pH from 4 to 5, the second solution must have a significantly lower concentration of hydronium ions, meaning its pH must be above 7.
However, it is clear that the second solution must be a basic solution, having a pH higher than 7, in order to raise the pH of the first solution when mixed.