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Acid. A chemical compound that yields one or more hydrogen ions (or protons, H,) in solution. Acids occur as solids, liquids or gases, possess a sour taste, and turn blue litmus paper (q.v.) red. They react with certain metals to form salts (q.v.) and liberate hydrogen (q.v.), with oxides (q.v.) to form salts and water, and they neutralize bases (q.v.) to form salts and water. Depending on their origin, they are differentiated into mineral acids (q.v.) and organic acids (q.v), and may further be divided into strong acids with a high degree of dissociation (q.v.) or reactivity, and weak acids with a low degree of dissociation or reactivity. When in solution, they produce a pH (q.v.) value below the neutral point of 7, i.e., in the acidic region of the scale.