Classification of Compounds: Acids and Bases

Chemical compounds are categorized into four major families: acids, bases, oxides, and salts.

• Acids: Characterized by a chemical formula beginning with hydrogen (except organic acids which end in -COOH).
• Examples: Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric acid (HNO3).
• Physical and chemical properties: sour taste, corrosive to metals (releasing flammable hydrogen gas: Zn + 2HCl -> ZnCl2 + H2↑), and turning litmus paper red.

• Bases and Alkalis: An alkali is a subset of bases that dissolve in water to produce hydroxide ions (OH-).
• Famous rule: All alkalis are bases, but not all bases are alkalis!
• (For example, CuO and Fe(OH)3 are bases that do not dissolve in water, hence they are not alkalis).
• Properties: bitter taste, soapy feel, and turning litmus paper blue.