That the gas obtained is different from air and oxygen may be shown by holding a bottle of it mouth downward and bringing a lighted splint into it. The hydrogen is ignited and burns with an almost colorless flame. [Illustration Fig. 9] ~Preparation from acids~ (_usual laboratory method_). While hydrogen can be prepared from water, either by the action of the electric current or by the action of certain metals, these methods are not economical and are therefore but little used. In the laboratory hydrogen is generally prepared from compounds known as acids, all of which contain hydrogen. When acids are brought in contact with certain metals, the metals dissolve and set free the hydrogen of the acid. Although this reaction is a quite general one, it has been found most convenient in preparing hydrogen by this method to use either zinc or iron as the metal and either hydrochloric or sulphuric acid as the acid. Hydrochloric acid is a compound consisting of 2.77% hydrogen and 97.23% chlorine, while sulphuric acid consists of 2.05% hydrogen, 32.70% sulphur, and 65.25% oxygen. The changes which take place in the preparation of hydrogen from zinc and sulphuric acid (diluted with water) may be represented as follows: _ _ _ _ | hydrogen |(sulphuric | zinc |(zinc zinc + | sulphur | acid) = | sulphur | sulphate) + hydrogen |_oxygen _| |_oxygen _| In other words, the zinc has taken the place of the hydrogen in sulphuric acid. The resulting compound contains zinc, sulphur, and oxygen, and is known as zinc sulphate. This remains dissolved in the water present in the acid. It may be obtained in the form of a white solid by evaporating the liquid left after the metal has passed into solution. When zinc and hydrochloric acid are used the following changes take place: _ _ _ _ | hydrogen |(hydrochloric | zinc |(zinc zinc + |_chlorine_| acid) = |_chlorine_| chloride) + hydrogen When iron is used the changes which take place are exactly similar to those just given for zinc. [Illustration Fig. 10.] ~Directions for preparing hydrogen from acids.~ The preparation of hydrogen from acids is carried out in the laboratory as follows: The metal is placed in a flask or wide-mouthed bottle A (Fig. 10) and the acid is added slowly through the funnel tube B. The metal dissolves in the acid, while the hydrogen which is liberated escapes through the exit tube C and is collected over water. It is evident that the hydrogen which passes over first is mixed with the air from the bottle A. Hence care must be taken not to bring a flame near the exit tube, since, as has been stated previously, such a mixture explodes with great violence when brought in contact with a flame.