Final answer:
To neutralize a solution with a pH of 4, one should add a solution with a pH of 10. Since a pH lower than 7 is acidic, and higher is basic, a pH of 10 solution is sufficiently basic to neutralize the acidic solution to a closer to neutral pH.
Step-by-step explanation:
The most effective way to neutralize a solution with a pH of 4 would be to add a solution with a pH of 10. A pH value below 7 indicates an acidic solution, whereas a pH above 7 indicates a basic solution. Neutralization involves adding a base to an acid to reach a pH close to the neutral value of 7. When mixing solutions, the result will be somewhere between the two original pH values, depending on the amount of each solution used. Bringing the pH up to 7 from 4 requires adding a basic substance, thus a solution with a pH of 10. This will raise the pH toward neutrality more effectively than a neutral solution (pH of 7), an even more acidic solution (pH of 3), or a less basic solution (pH of 8).