300 ELEMENTARY GENERAL SCIENCE CHAT. 



nitric acid on copper. If, however, an oxide of the metal be 

 used instead of the metal itself, such secondary actions are 

 avoided, as the following experiments show. 



EXPT. 302. Burn some magnesium, and collect the white 

 solid formed. Observe that it is a white powder insoluble in 

 water, which glows when strongly heated, but does not 

 undergo any chemical change. 



To some of this oxide add some diluted sulphuric acid, but 

 not quite enough to dissolve all the oxide. Observe that the 

 oxide dissolves ivithout the evolution of gas. Filter, partially 

 evaporate, and allow it to crystallise. 



The white crystals so obtained consist of magnesium sulphate. 

 They possess a bitter saline taste, and may be easily proved to 

 contain water of crystallisation (p. 239). They are commonly 

 known as Epsom Salts, and are used as an aperient. In this case, 

 then, no hydrogen was evolved, since the oxygen of the oxide 

 was capable of combining with it to form water ; thus : 



MgO + H 2 S0 4 = MgS0 4 + H 2 0. 



Magnesium Magnesium 



Oxide. Sulphate. 



Similar results may be obtained if nitric or hydrochloric acid 

 be used in place of sulphuric acid, with the formation of magne- 

 sium nitrate, Mg(NO 3 ) 2 , and magnesium chloride, MgCl 2 : 



MgO + 2HN0 3 = Mg(N0 3 ) 2 + H 2 O. 

 MgO + 2HCl = MgCl 2 +H 2 O. 



Calcium Salts. Other metallic oxides can in a similar manner 

 react with acids, with the production of salts and water. 



EXPT. 303. Examine lime (oxide of calcium). Note its 

 effect on litmus, its slight solubility, action of heat, &c. (page 

 286), and, as above, endeavour to prepare the calcium sulphate, 

 calcium chloride, and calcium nitrate. 



You will find that crystals do not result, but that calcium 

 chloride and calcium nitrate are both white solids which readily 

 absorb moisture from the air. Owing to this the former is, as 

 we have seen, largely used for drying gases, &c. The sulphate, 

 however, does not dissolve in water (at least, it dissolves 

 only to a very small extent, viz., about 2 parts per 1,000), and 

 hence the acid may at first be thought to have no action, but 



