CALCIUM, MAGNESIUM, ETC. 137 



form, as apatite rock, see Fig. 58. Calcium phosphate is 

 extensively used for the preparation of commercial fer- 

 tilizers as explained in Section 109. 



174. Mortar. When quicklime is slaked and mixed 

 with sand, it forms at first a mechanical mixture. When 

 it is placed upon the walls of buildings, a chemical change, 

 known as the hardening or setting process, takes place. 

 When this change occurs, the moisture is expelled and 

 the carbon dioxid of the air changes the calcium hydroxid 

 to calcium carbonate. In the slaking of lime and setting 

 of mortar, the following reactions take place : 



(1) CaO + H 2 = Ca(OH) 2 . 



(2) Ca(OH) 2 + CO 2 = CaCO 3 . 



When magnesium carbonate and aluminum silicate are 

 present, forming a part of the composition of the original 

 lime rock, hydraulic cement is produced which has the 

 property of setting under water. 



Experiment 28. Testing quality of lime. Place about 40 

 grams of lime, CaO, in an evaporating dish and moisten with water 

 warmed to about 35 C. Note the reaction. Good lime readily 

 undergoes the slaking process. Place some of the slaked lime in 

 a bottle, add about 100 cc. of distilled water, shake vigorously and 

 leave the lime in contact with the water for four hours or longer, 

 then filter some of the solution of lime water, and test it by 

 forcing respired air through it as explained in Experiment 24. 



Place about one-half gram of the slaked lime in a test-tube, add 

 10 cc. of water and then a few drops of HC1. When action ceases, 

 add more HC1, a little at a time, and heat. The material which 

 fails to dissolve usually consists of insoluble silica and clay, Lime 

 of a high degree of purity contains less than 10 per cent, of acid-in- 

 soluble impurities. 



Questions, (i) Was any heat evolved when the lime was 



