The most common acids are sulfuric, hydrochloric, and nitric. Among the more common bases are caustic soda, caustic potash, and lime. A convenient way to recognize whether a substance has acid or basic properties is by means of certain color-changes. Litmus is a coloring matter which is ordinarily blue. If a solution which is colored blue with litmus be treated with a drop or two of an acid, the color is changed to red. If the red solution be treated with a few drops of a solution of a base, the blue color is restored. Many substances change in color according to whether the solutions in which they are present are acid or alkaline. An infusion of red cabbage, for example, changes color when treated with an acid, and recovers its color when again treated with an alkali. [Sidenote: Formation of common salt] What happens in the chemical sense in this neutralizing process is nicely illustrated by the formation of common salt from hydrochloric acid and caustic soda, also called sodium hydroxid. When these two substances are dissolved in water, and the solutions mixed, the chemical action is as follows: HCL + NaOH = H₂O + NaCl Hydrochloric acid + Caustic soda = Water + Common salt (Muriatic acid) (Sodium hydroxid) (Sodium chloride) [Sidenote: Common examples of neutralization] The strong hydrochloric acid with its pungent odor and sour taste, and the caustic alkali with its equally characteristic properties have both disappeared, and in their place we find nothing more wonderful than common salt dissolved in water. Other forms of neutralization that are very common are vinegar (acetic acid C₂H₄O₂) and soda, or sour milk (lactic acid C₃H₆O₃) and soda. When bread is "sour," we mean that there was not enough soda to neutralize the acid. PRINCIPLES OF NEUTRALIZING ALKALIS If we should try many experiments of neutralizing alkalis with acids, we would discover these general rules: 1 All acids contain hydrogen. 2 All alkalis contain oxygen and hydrogen in equal proportions. 3 When these substances react, the hydrogen of the acid joins the hydrogen of the base or alkali, forming water, H₂O. 4 The metal of the base always replaces the hydrogen of the acid. 2KOH + H₂SO₄ = K₂SO₄ + 2H₂O Potassium hydroxid + Sulfuric acid = Potassium Sulfate + Water (alkali or base) (acid) (Salt) (In the above equation the potassium (K) of the potassium hydroxid replaces the Hydrogen (H) in the sulfuric acid.) 5 The other elements of the original compounds unite to form a new substance, which is neither acid nor alkali, but which is termed a salt. The names of a few common acids, bases and salts, and their chemical formulas, are given here, as many of them will be important in the pursuance of this work. ACIDS