Final answer:
The second solution must have had a higher concentration of OH- ions to result in a higher pH when mixed with the first acidic solution.
Step-by-step explanation:
If two clear solutions are mixed where the first has a pH of 4 and the resulting mixture has a pH of 5, it indicates that the second solution must have a higher concentration of OH- ions than the first solution. The pH scale is used to determine the acidity or basicity of a solution. A pH lower than 7 is acidic, and a higher pH indicates a basic solution. Since the first solution is an acid (pH of 4), and the mixture resulted in a higher pH (less acidic), we can infer that the second solution has a pH above 4. The resulting pH of 5 suggests that the second solution was less acidic, or basic, which means it had a higher concentration of hydroxide ions (OH-) compared to the first one. Options such as fewer suspended solids, a higher temperature, or more dissolved NaCl particles would not cause a higher pH on their own, as the pH is specifically a measure of the concentration of hydronium (H3O+) and hydroxide ions.