Acid-base chemistry studies the properties and reactions of acids and bases. An acid is a substance that donates protons (H⁺ ions) in a solution, while a base accepts protons. This interaction can affect the pH level of a solution, which measures its acidity or alkalinity. The Brønsted-Lowry theory is one way to understand these reactions, defining acids and bases based on proton transfer. Another important concept is the pH scale, which ranges from 0 to 14, indicating how acidic or basic a solution is. A pH of 7 is neutral, while values below or above indicate acidity or basicity, respectively.