98 



ELEMENTS AND COMPOUNDS 



The air is a physical mixture of nitrogen, oxygen, steam, 

 carbon dioxide, and small amounts of other gases. 



Of course we have mixtures of compounds, as well as of 

 elements. Natural water is always a mixture (cf. 80). 

 All soils, and most rocks, are mixtures (cf. 285 and 294). 

 The forms in which the solid elements and compounds 

 are found in nature are called minerals. 



103. Preparation of Hydrogen. The most common 

 way to prepare hydrogen is to bring certain metals and 

 certain acids together. All adds are compounds con- 

 taining hydrogen (cf. 214) ; but all do not give it off with 

 the same ease. The best acids to use are dilute sulphuric 

 acid and dilute hydrochloric acid; the best metals are 

 zinc and iron. 



The apparatus needed (Fig. 79) is a bottle provided with a stopper 

 having two holes, a "thistle tube," and a delivery tube reaching to a 



O 



B 



__H_ydroge 



Air 



FIG. 79. 

 (A) Making Hydrogen and Collecting It over Water. 



(B) Collecting It over Air. 



water-pan. The thistle tube is the opening through which fresh 

 supplies of acid are put into the bottle; it also allows the hydrogen to 

 escape if the delivery tube becomes stopped up. Hydrogen is not very 



