An acid is a substance that turns the pH of the solution lower on the pH scale. The pH scale is a scale that runs from 0 to 14, with 7 representing the neutral point in the middle. Distilled water has a pH of 7. So the more acidic a substance is, the lower the number on the pH scale. Bases are substances that have a higher nuber on the pH scale. Anything higher than 7 is considered a base. The same is true about adding basic substances to water, you increase the pH towards the right side of the scale, indicating a base. The pH scale is a scale indicating the number of hydrogen ions in solution, so the lower the number, the more hydrogen ions there are present in the solution. The pH scale may also be an indicator of the hydroxide ion presence in the solution (OH-). The higher the number, the more hydroxide ions present in the solution. The more substance you dissolve in solution, the more acidic or basic the solution is going to be.