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