CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC 119 



hydroxide of calcium. The relation might be made more striking by writing 

 the formula thus, CaO H 2 O. 



Some oxides do not unite with water to form hydroxides, but, as far as they 

 are acted upon by acids, they give the same end product (a salt) as the hy- 

 droxides do, as may be illustrated in the following reactions : 



ZnO '+ H 2 S0 4 == ZnSO 4 + H 2 O. 

 Zn(OH) 2 + H 2 SO 4 = ZnSO 4 + 2H 2 O. 



It should be noted that one of the products that is always formed when an 

 acid acts on a metallic hydroxide, or oxide, is water. This is shown in the 

 above reactions. 



The hydroxides evidently are compounds derived from water by the re- 

 placement of part of the hydrogen in the water molecule by metal, thus 

 leaving the radical, (OH), which is known as hydroxyl, in combination with 

 metal. Hence, these compounds are called hydroxides. In a few cases hy- 

 droxides can be obtained by the direct action of the metals on water, the dis- 

 placed hydrogen escaping as a gas. This seems to be good evidence of the 

 relationship between the hydroxides and water. Most metals, however, do not 

 act on water, and their hydroxides are obtained in an indirect way. (See 

 Remarks on Tests for Metals, in the chapter on Magnesium.) 



There are some hydroxides of radicals which can unite with acids just as 

 the metallic hydroxides do, and these are also classed as basic substances. In 

 them the radical plays the part of a metal. 



Most of the metallic oxides and hydroxides are practically insoluble in 

 water, and therefore have no appreciable action on litmus paper and no taste. 

 Hence, alkaline action and taste are not a sure criterion of a basic substance. 

 But the hydroxides insoluble in water can act on acids and replace their hy- 

 drogen by metal, just as the soluble hydroxides do. 



The hydroxides differ very much in regard to specific properties, such as 

 solubility in water, color, taste, etc., but there is one feature common to all of 

 them, namely, the presence of the hydroxyl group, which is responsible for 

 the class properties upon which such compounds are classified as basic sub- 

 stances. 



It should be noted that there appears to be a tendency to easy separation 

 between the metal and the hydroxyl radical in the bases, just as there is be- 

 tween the hydrogen and the acid radical in the case of acids. The significance 

 of these facts will appear when the Ionic Theory is discussed. 



Neutralization is the term applied to the interaction between 

 acids and bases with the result that both acid and basic properties, 

 disappear i. e., are neutralized. 



All substances which are acid in character contain hydrogen as 

 one of their constituents. This hydrogen can readily be replaced by 

 metals, for instance by magnesium, when hydrogen is liberated. 

 Not all substances containing hydrogen behave in this manner ; for 

 example, magnesium does not liberate hydrogen from petroleum, 

 olive oil, sugar, etc., which all contain hydrogen. Hence the hydro- 

 gen of acids must be in a peculiar condition. That it is this hydro- 



