78 CHEMISTRY FOR AGRICULTURAL STUDENTS 



therefore insoluble in, acids, the reaction provides a test for 

 sulphuric acid and the sulphates. To (i) distilled water, (2) tap 

 water, and (3) solution of sodium sulphate, add a few drops of 

 baryta water and then nitric acid. A permanent white precipi- 

 tate will be obtained where sulphate was present. Write an 

 equation representing the reaction between sodium sulphate 

 and baryta. 



XIX. COMMON SALT: HYDROCHLORIC ACID 



Of the salts examined, so far none correspond in taste and 

 appearance with common salt. Recrystallise some common 

 salt by evaporating an aqueous solution till crystallisation 

 begins ; then allow to stand. Note that each 

 crystal has the form of a cube (Fig. 34). When 

 carefully crystallised, the cubes build them- 

 selves together into hollow inverted pyramids. 

 Insert a grain of salt on a platinum wire into 



Fig. 34. a non-luminous flame ; note the yellow colora- 



tion of the flame. What metal does this show 

 is present? Place some salt in a test-tube and add strong 

 sulphuric acid. Is a volatile acid thus liberated? Test the 

 vapour with blue litmus paper. 



Prepare some of the acid from common salt in the apparatus 

 figured. In the flask A place 50 grams of salt, and in B 

 just enough water to cover the bottom of the flask and the 

 orifice of the glass tube, and thus to wash the gas as it passes 

 through. Pour a little strong sulphuric acid through the 

 funnel into A, adding more as required from time to time to 

 produce a constant flow of gas. Heat gently when necessary. 

 When the water in B has become saturated, collect the gas by 



