174 



JUNIOR GRADE SCIENCE 



CottonWool 



escapes through the tube C, in which cotton- wool is packed to prevent the 

 escape of moisture. When the action is at an end draw a current of air 

 through the flask by sucking the tube C, in 

 order to displace the gas still left inside. 

 Weigh the flask again, and subtract t'ie mass 

 so obtained from that of the flask + mass of tube 

 and chalk. The resulting number is evidently 

 the mass of the gas evolved. Calculate the 

 loss of mass for 100 grams of chalk. 



If the experiment is repeated several times, 

 or done independently by different students, 

 the results obtained may be added together and 

 divided by the number of experiments, in order 

 to obtain an average result. It will )><> found 

 that 100 grams of chalk lose about 44 grams of 

 gas when acted upon by an acid. 



Changes produced by adding acid to 

 chalk. When hydrochloric acid is added 

 to a little chalk a brisk effervescence occurs 

 and a colourless, odourless gas which turns 

 lime- water milky is given off. 



If hydrochloric acid is poured upon chalk, 

 and if the solution remaining, after the 

 effervescence of the chalk with 1! 



-1, is filtered and evaporated, a now 

 substance known as calcium chloride is 

 obtained. So that, from chalk and hydro- 



Fio. 119. When the chalk 

 in If is acted upon by the acid 

 in A it loses 44 per cent, of 

 its mass. 



chloric acid, it is possible to produce carbon dioxide, calcium chloride 

 and water (which is driven off by evaporation). 



Substances which, like chalk, evolve carbon dioxide when acted 

 upon by an acid are known as carbonates, and numerous carbonates 

 exist, all possessing similar characteristics. Many of these on heating 

 also give off carbon dioxide, the residue being known to be an oxide 

 of a metal, so that they consist of carbon dioxide and a metallic 

 oxide. 



This leads to the idea that lime is also the oxide of a metal, and this 

 view is now known to be correct, the metal being named calcium. 

 Lime, therefore, is calcium oxide, and chalk calcium carbonate. 



A test for chalk and limestone. The geologist, or man of science 

 who examines and studies rocks, makes use of this experiment to discover 

 whether rocks are limestones or not. Some limestones are I/lack and 

 not easily recognised by the eye. But if, when a little acid is dropped 

 upon them, they fizz like the chalk in a test-tube does when acid is 

 poured on it, the geologist can be fairly sure, even if they are black, 

 that they are limestones all the same. 



