Final answer:
The pH of a solution drops and turns the neutral red dye red in color when there is an increase in hydrogen ion (H+) concentration.
Step-by-step explanation:
The pH of a solution drops and turns the neutral red dye red in color when there is an increase in hydrogen ion (H+) concentration. The neutral red dye is an acid-base indicator that changes color based on the concentration of hydronium ions (H3O+). In more acidic solutions, with higher H+ concentration, the dye appears red.