130 JUNIOR GRADE SCIENCE 



When first collected it may have a slight smell like bleaching powder, 

 due to the presence, as an impurity, of a little chlorine, but this odour 

 disappears on standing over water for a time. It can be got rid of more 

 quickly by passing the gas through a solution of caustic potash during 

 the preparation. 



The pure gas has no colour, no odour and no taste. It has no 

 action on litmus, and is for this reason said to be neutral. Ordinary 

 combustible substances burn more brightly in oxygen than in the air. 



Oxygen has no effect on substances like sulphur and carbon when 

 they are at the same temperature as the room, but if these elements 

 are heated to the point of ignition the oxygen combines with them very 

 readily, causing them to burn vigorously. 



Some substances which will not burn under ordinary conditions 

 can be made to burn in oxygen, and the case of iron affords a good 

 example of this. Consider what the result would be if there were only 

 oxygen in the air. As soon as iron got red-hot it would start burning. 

 Iron could not in these circumstances be used for grates, furnaces and 

 similar things. 



Oxygen is not very soluble in water one hundred parts of water 

 dissolve three parts of this gas. That the amount of oxygen dissolved 

 by water is very small is seen by the fact that oxygen prepared for 

 experiment is usually collected over water. But though the amount 

 is small it is of great importance in the economy of nature, for it is due 

 to this dissolved oxygen that water animals are able to breathe. Oxygen 

 can, however, be readily dissolved by some liquids, such as a solution 

 of pyrogallol in caustic potash. 



Though oxygen exists in a gaseous condition under ordinary con- 

 ditions of temperature and pressure, yet it can, by lowering the tem- 

 perature and very much increasing the pressure, be made to assume 

 the liquid, and even the solid, state. 



Oxygen is indispensable to life. It is the constituent of the atmo- 

 sphere which is used up in the processes of combustion, decay and 

 fermentation. 



Formation of oxides. In all the cases of burning studied experi- 

 mentally new substances have been formed, differing greatly from the 

 original substance and from the oxygen in which it was burned, 



To such new substances the name compound is given. To the 

 compounds formed by the burning of substances in oxygen the general 

 name oxide is given. 



Classification of oxides. At an earlier stage it was mentioned that 

 the action of substances on litmus was of assistance in classifying them. 

 In the case of the oxides tabulated above it is noticed that their action 



