64 CHEMISTRY FOR AGRICULTURAL STUDENTS 



obtained the formulae, arrange the oxides in groups as 

 ino7ioxides^ dioxides^ and trioxides. Calculate the formula that 

 most nearly expresses the composition of red lead as deter- 

 mined by the foregoing experiment. 



To distinguish the oxides of a metal, it is usual to specify the colour, or 

 to use the terminations -oiis and -ic to represent the lower and higher 

 oxides respectively. Unstable higher oxides are usually distinguished as 

 peroxides. The highest oxides of metals are sometimes acidic. There 

 are, for instance, a chromium trioxide CrOs, and manganese heptoxide, 

 MnaO;, which exist combined with potash in " bichromate of potash" and 

 *' permanganate of potash." The oxides of some metals, though basic, act 

 as acidic oxides to more basic oxides. Thus alumina dissolves in caustic 

 soda solution, forming a salt. This reaction serves to distinguish alumina 

 from the other bases met with in agriculture. 



It has already been observed that the basic oxides, ,if 

 dissolved by water, form alkaline liquids, but they differ in 

 their solubility. Of the oxides studied, potassium and sodium 

 oxide will be found to be soluble in water in almost all pro- 

 portions ; lime, strontia, and baryta soluble to a limited extent ; 

 magnesia, litharge, ferrous oxide and oxide of zinc very slightly 

 soluble; the remaining oxides practically insoluble. Verify 

 these statements experimentally. The act of solution appears 

 to be accompanied by chemical combination, for heat is 

 produced. Slake some quicklime with cold water. Note 

 the change in appearance that takes place (immediately if the 

 lime be pure, slowly if impure), the heating of the mixture, 

 and the final disappearance of the water, all evidence of 

 chemical combination having occurred. The compound 

 produced (slaked lime) may be termed hydrate of lime. 



Determine the proportions in which lime combines with water. 

 Heat a weighed quantity — say i gram — of pure quicklime to 

 low redness in a weighed capsule. Cover the capsule, allow to 

 cool in a dessicator (Fig. 27) over more quicklime, itself a good 

 drying agent, and, when cold, weigh, Add excess of water, 

 cover with a beaker till slaking is complete, and place in the 



