386 SMITH'S INTERMEDIATE CHEMISTRY 



tar, and also in the manufacture of bleaching powder. Other uses 

 are mentioned under the hydroxide. 



Quicklime deteriorates when exposed to the air. It combines 

 both with the moisture and carbon dioxide in the atmosphere and 

 becomes air-slaked. 



Calcium Hydroxide Ca(OH) 2 . The hydroxide is a white, 

 amorphous powder. It is slightly soluble in water (about 1 : 600 

 at 18), giving lime-water. The solution has a strong alkaline 

 reaction, however, showing that so much as is dissolved is very 

 largely ionized. Milk of lime, a saturated solution with a large 

 excess of calcium hydroxide suspended in it, is employed in many 

 operations (see e.g. pp. 166, 371). As the dissolved part under- 

 goes chemical change, more goes into solution. Being cheap, 

 it is used whenever an alkali is needed, provided a dilute alkali 

 will serve the purpose. It interacts with acids giving salts of 

 calcium, and shows the other properties of a base (pp. 167, 192). 



Slaked lime is used in making mortar (see below) and alkalies 

 (p. 166) and in purifying sugar (p. 402). It is employed to re- 

 move the hair from hides, before tanning, an action which recalls 

 the solubility of wool (sheep's hair) in an alkali (p. 1). It finds 

 application, also, in softening water (p. 389) and as whitewash. 



Mortar. Mortar is made by mixing slaked lime with three or 

 four times its bulk of sand, and making the whole into a paste 

 with water. When the water evaporates, a porous, rather crumbly 

 material remains. This, however, at once begins to harden, 

 owing to the action of the carbon dioxide in the air upon the lime: 



Ca(OH) 2 + C0 2 -* CaC0 3 + H 2 | . 



The crystalline calcite (CaC0 3 ) adheres to, and is interlaced with 

 the sand, and gives a rigid, though porous, structure attached 

 firmly to the brick or stone. The pores facilitate the penetration 

 of the air into the deeper parts and thus provide, both for the 

 fresh supplies of carbon dioxide required for the continuance of 



