Acidity and alkalinity are the characteristics of substances in acid-base reactions. Generally speaking, acidic substances can make the purple litmus test solution turn red, and alkaline substances can make it blue. Later, with the development of acid-base theory, people gave With a more accurate and complete definition, it gradually touched on the nature of the cause of acid and alkali. There are three scales for measuring acidity and alkalinity: pH and pOH of aqueous solution, pKa of acid and pKb of alkali, and chemical hardness of acid and alkali. Acidity and alkalinity are generally tested with pH test paper, litmus test solution, and phenolphthalein test solution. Substances with acidity and basicity are: Strong acid: hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, hydrogen bromide, hydrogen iodide, weak acid: carboxylic acid, carbonic acid, phosphoric acid, sulfurous acid, hydrogen sulfide, hydrocyanic acid, others: phenol, strong acid Weak alkali salts, strong alkalis: sodium hydroxide, potassium hydroxide, barium hydroxide, calcium hydroxide, (except for alkaline metals other than lithium) hydroxides of alkaline earth metals other than beryllium magnesium weak alkalis: hydroxides of other metals except Aluminum Beryllium Zinc Soda ash / Soda: Na2CO3, Caustic soda / Fire alkali / Caustic soda: NaOH Baking soda: NaHCO3 Dry ice: CO2 Hydrazine: N2H4 Glacial acetic acid: CH3COOH Other: Strong alkali weak acid