Answer
To decrease the PH of a neutral solution add to it an acidic substance such as lemon juice.
Explanation
The p H (potential of hydrogen) is the level of an aqueous solution which tells us how acidic or basic a substance/solution is based on it hydrogen ion concentration. The higher the concentration of hydrogen ions, the lower the p H. In distillated water that has a p H of 7(neutral), you can increase the p H of the water by adding an alkaline substance such as baking powder.
