, N ' 

 o MI ? 

 THE HALOGEN FAMILY 207 



Fluorine is the most active of the non-metals. It combines with 

 all the metals, but most slowly with platinum and with gold. In 

 the preparation of the gas the copper is protected from serious 

 attack by the layer of fluoride first formed. Fluorine combines 

 also with hydrogen in the cold and, unlike chlorine, without the 

 assistance of light. It combines with most of the non-metals, but 

 not with oxygen, chlorine, nitrogen, or the indifferent gases of the 

 atmosphere. 



With water (vapor or liquid) fluorine interacts, giving ozone 

 (see p. 219) and hydrogen fluoride H 2 F 2 : 

 Skeleton: F 2 + H 2 O - H 2 F 2 + 3 . 



Balanced: 3F 2 + 3H 2 -* 3H 2 F 2 + 3 . 



Hydrogen Fluoride H 2 F 2 , Preparation. When pulverized 

 calcium fluoride and concentrated sulphuric acid are placed in a 

 retort of platinum or lead and the mixture is warmed, hydrogen 

 fluoride passes over. The vapor is usually led into water, in 

 which it is very soluble (hydrofluoric acid) : 



CaF 2 + H 2 S0 4 - CaSO 4 + H 2 F 2 . 



The acid is kept in bottles of paraffin or rubber, as glass inter- 

 acts with it rapidly. 



Physical Properties. The vapor of hydrogen fluoride can 

 be condensed to a colorless liquid boiling at 19.4. Being very 

 soluble, it fumes strongly in moist air. The vapor density below 

 40 corresponds with the formula H 2 F 2 , but at higher temperatures 

 gradual dissociation to HF occurs. 



Chemical Properties. The aqueous solution has all the 

 properties of a transition acid (see p. 190). The substance has, 

 in addition, the remarkable property of acting upon silica SiC>2 

 (sand), and silicates, to give silicon tetrafluoride SiF 4 (a gas). 

 Hence it attacks glass, which is a mixture of sodium silicate 

 and calcium silicate CaSi0 3 : 



SiO 2 + 2H 2 F 2 -> SiF 4 + 2H 2 O. 

 CaSiO 3 + 3H 2 F 2 -* SiF 4 + CaF 2 + 3H 2 O. 



