EXAMINATION OF COMMON SUBSTANCES 



97 



Lime is also most valuable in agriculture. Its action in getting rid 

 of " sourness " in soil and making food available for plants will be 

 explained later. 



38. EXAMINATION OF ALUM. 



L Examine as described in Chapter XVII. 



ii. Liquid formed when alum is heated. Notice that a vapour is given 

 off which condenses to a clear liquid at the top of the test-tube. In order 

 to identify this liquid collect a quantity of it. 

 Half fill a large test-tube with powdered alum ; 

 fit the test-tube with a long right-angled delivery 

 tube, place in a horizontal position in a clamp 

 attached to a retort stand, with the end of the 

 delivery tube dipping into a dry test-tube 

 (Fig. 74). Heat the alum gently and note the 

 condensation of the vapour and collection of 

 liquid. When the alum has become solid, cease 

 heating and examine the liquid. It is clear like 

 water, and has little if any taste. If it really 

 is water it ought to have the same density and 

 boiling and freezing points as water. 



The whole class production of liquid should 

 be collected and these determinations made. 



Density. Use the small U-tube method 

 (p. 48). 



Boiling point. Adopt the method described 

 for finding the boiling point of water (p. 78). 



Freezing point. Place some of the liquid in 

 a test-tube with a thermometer and surround 

 it with a mixture of powdered ice and salt. 

 Stir well and notice the temperature at which 

 the mercury remains stationary. -when alum is heated. 



Alum. Alum is a transparent crystalline solid, with a characteristic 

 astringent taste. It is soluble in water, the solution turning litmus 

 red. When heated it is noticed that alum melts (i.e. changes from the 

 solid to the liquid state) and then froths up ; a vapour is formed and 

 drops of liquid are noticed on the sides of the test-tube. When a 

 quantity of this liquid is collected and examined it is found closely to 

 resemble water in appearance and properties, and investigation shows 

 it to have the same density and boiling and freezing points. (Some- 

 times a little alum makes its way over during the heating process and 

 causes the liquid to have a slight taste and action on litmus.) It is 

 concluded that the liquid is water. This is certainly a fact worth} 7 

 of notice. The water in the alum must exist there in some peculiar 

 manner, since the ordinary solid form of water ice only exists at a 



J.O.S. G 



