The pH scale is a measure of how acidic or basic a solution is. It ranges from 0 to 14, with 7 being neutral. A pH of less than 7 indicates acidity, while a pH greater than 7 indicates basicity. The pH scale is logarithmic, meaning that a change of 1 pH unit corresponds to a ten-fold change in hydrogen ion concentration. For example, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4. The pH of a solution is determined by the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in the solution. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. The pH scale is used in various fields, including chemistry, biology, and environmental science, to measure the acidity or basicity of solutions.