Acids are substances that have a sour taste and can turn blue litmus paper red. They are found in many foods and drinks, such as citrus fruits like lemons and oranges, as well as in vinegar. Acids release hydrogen ions (H⁺) when dissolved in water, which is what gives them their sour flavor. In chemistry, acids are classified based on their strength. Strong acids, like hydrochloric acid, can completely dissociate in water, while weak acids, like acetic acid in vinegar, only partially dissociate. Acids play important roles in various processes, including digestion and industrial applications.