Acidity and basicity refer to the properties of substances that determine their behavior in chemical reactions. An acidic substance has a pH less than 7 and can donate protons (H⁺ ions) in a solution, while a basic substance has a pH greater than 7 and can accept protons. Common examples of acids include hydrochloric acid and citric acid, while bases include sodium hydroxide and ammonia. The pH scale, which ranges from 0 to 14, is used to measure acidity and basicity. A pH of 7 is considered neutral, indicating a balance between acids and bases. Understanding these concepts is essential in various fields, including chemistry, biology, and environmental science.