pH level represents the acidity or alkalinity of a solution on a scale from 0 to 14. The lower the pH value, the higher the acidity. A pH level of 0 represents the highest acidity, indicating a very high concentration of hydrogen ions (H+) in the solution. Substances with a pH below 7 are considered acidic, while those with a pH above 7 are considered basic or alkaline. A pH level of 7 is considered neutral, indicating equal concentrations of hydrogen ions and hydroxide ions (OH-) in the solution. Examples of extremely acidic substances include battery acid, stomach acid, and concentrated hydrochloric acid.