170 JUNIOR GRADE SCIENCE 



formed while the sides of the tube are bespattered with black specks. 

 When the oxide is dissolved away with hydrochloric acid, and the 

 black residue collected on a filter, washed with water and dried, it 

 is found that it has the properties of carbon. 



Action of carbon dioxide on lime-water. If carbon dioxide is 

 passed into lime-water a milkiness is seen, but if the passage of the gas 

 is continued the milkiness by and by disappears. If the clear solution 

 which results after the disappearance of the white powder or precipitate 

 is boiled, the milkiness again makes its appearance. The reason of 

 this is that the white substance of which the precipitate is formed 

 dissolves in water which has become saturated with carbon dioxide. 

 When the clear solution, which appears after the solution of the powder, 

 is boiled, the carbon dioxide is driven out of it, and the liquid again 

 becomes pure water. The precipitate reappears, because it will not 

 dissolve in water. 



The chemical change when carbon dioxide is passed into lime- 

 water. What are the facts taught by the experiments described ? 

 When an acid is added to the white powder formed when carbon dioxide 

 is passed into lime-water a brisk effervescence is noticed, and the 

 colourless, odourless gas given off is found to put out a flame. But 

 this is just what happens when the acid is dropped upon chalk, and 

 putting the facts together they suggest that the white powder is really 

 chalk, so that carbon dioxide gas combines with the lime in the lime- 

 water to form chalk. We can write : 



CARBON DIOXIDE LIME CHALK. 



From this statement it will also be seen that chalk consists of lime 

 and carbon dioxide. Further evidence of the truth of this will be 

 learnt later. 



Uses of carbon oxide. The fact that carbon dioxide is a non- 

 supporter of combustion is made use of in many forms of chemical 

 fire extinguishers, which generally contain solutions of the gas under 

 pressure, or a means of producing large quantities of it by the action 

 of acid upon a carbonate solution. As burning cannot take place 

 in this gas, the flames are therefore extinguished when the gas reaches 

 them. 



The solubility of carbon dioxide 1n water is increased by pressure, 

 and the sparkling nature of the various aerated waters, like soda-water, 

 is due to the carbon dioxide with which they have been charged at high 

 pressures, which escapes when the pressure is reduced to that of the 

 atmosphere by opening the bottle. As the pressure is very great, the 

 bottles used as fire extinguishers have to be very thick. 



