Final answer:
Adding acids to a solution results in a decrease of pH because acids increase the concentration of hydrogen ions, making the solution more acidic.
Step-by-step explanation:
When acids are added to a solution, the pH should decrease. This is because acids increase the concentration of hydrogen ions (H+), also represented as hydronium ions (H3O+), which results in a lower pH value. The pH scale is used to measure how acidic or basic a solution is, with lower values being more acidic and higher values being more basic.