~The commercial preparation of nitric acid.~ Fig. 38 illustrates a form of apparatus used in the preparation of nitric acid on a large scale. Sodium nitrate and sulphuric acid are heated in the iron retort A. The resulting acid vapors pass in the direction indicated by the arrows, and are condensed in the glass tubes B, which are covered with cloth kept cool by streams of water. These tubes are inclined so that the liquid resulting from the condensation of the vapors runs back into C and is drawn off into large vessels (D). [Illustration Fig. 38] ~Physical properties of nitric acid.~ Pure nitric acid is a colorless liquid, which boils at about 86 deg. and has a density of 1.56. The concentrated acid of commerce contains about 68% of the acid, the remainder being water. Such a mixture has a density of 1.4. The concentrated acid fumes somewhat in moist air, and has a sharp choking odor. ~Chemical properties.~ The most important chemical properties of nitric acid are the following. 1. _Acid properties._ As the name indicates, this substance is an acid, and has all the properties of that class of substances. It changes blue litmus red and has a sour taste in dilute solutions. It forms hydrogen ions in solution and neutralizes bases forming salts. It also acts upon the oxides of most metals, forming a salt and water. It is one of the strongest acids. 2. _Decomposition on heating._ When boiled, or exposed for some time to sunlight, it suffers a partial decomposition according to the equation 2HNO_{3} = H_{2}O + 2NO_{2} + O. The substance NO_{2}, called nitrogen peroxide, is a brownish gas, which is readily soluble in water and in nitric acid. It therefore dissolves in the undecomposed acid, and imparts a yellowish or reddish color to it. Concentrated nitric acid highly charged with this substance is called _fuming nitric acid_. 3. _Oxidizing action._ According to its formula, nitric acid contains a large percentage of oxygen, and the reaction just mentioned shows that the compound is not a very stable one, easily undergoing decomposition. These properties should make it a good oxidizing agent, and we find that this is the case. Under ordinary circumstances, when acting as an oxidizing agent, it is decomposed according to the equation 2HNO_{3} = H_{2}O + 2NO + 3O. The oxygen is taken up by the substance oxidized, and not set free, as is indicated in the equation. Thus, if carbon is oxidized by nitric acid, the oxygen combines with carbon, forming carbon dioxide (CO_{2}): C + 2O = CO_{2}. 4. _Action on metals._ We have seen that when an acid acts upon a metal hydrogen is set free. Accordingly, when nitric acid acts upon a metal, such as copper, we should expect the reaction to take place which is expressed in the equation Cu + 2HNO_{3} = Cu(NO_{3})_{2} + 2H.