Acids and bases are two important categories of substances in chemistry. Acids are substances that can donate protons (H⁺ ions) in a solution, often having a sour taste and turning blue litmus paper red. Common examples of acids include citric acid found in citrus fruits and hydrochloric acid in the stomach. Bases, on the other hand, are substances that can accept protons or donate hydroxide ions (OH⁻) in a solution. They usually have a bitter taste and can turn red litmus paper blue. Examples of bases include sodium hydroxide used in cleaning products and baking soda, which is often used in cooking.