224 Domestic Science 



since it cannot be seen in the jar, but differs from air 

 in that it is able to render lime-water turbid. This gas 

 is called carbon dioxide. 



152. Before considering the explanation of this 

 formation of water and carbon dioxide, we will define 

 the exact meaning of the two names " element " and 

 " compound " as used by the chemist. 



An Element is a substance which has not yet been 

 split up into, nor built up from, simpler substances. 

 The two gases oxygen and nitrogen, the liquid mercury, 

 and the solids phosphorus, magnesium, copper, and iron, 

 are elements. In all about 80 bodies are known which 

 are believed to be elementary in character. 



A Compound is a substance produced by the chemical 

 union of two or more elements. It usually differs 

 widely in appearance and properties from any of the 

 elements of which it is composed. 



Thus water, a liquid which extinguishes flame and 

 does not itself burn, is a compound of two gases, one 

 of which, hydrogen, burns readily, while the other, 

 oxygen, causes bodies which are already burning in 

 the air to burn still more brightly when they are placed 

 in it. Again, phosphorus pentoxide is a white powder 

 which we have shown to be extremely soluble in water. 

 It is a compound of phosphorus, a yellow solid which 

 will not dissolve in water, and the colourless gas, 

 oxygen, which is only very slightly soluble in that 

 liquid. 



153. A candle is almost entirely composed of 

 compounds of the two elements carbon and hydrogen 

 (see Chapter xvn). When these bodies burn in air they 

 are oxidised, i.e. combine with oxygen, and the reactions 

 which occur may be briefly stated in the form of chemical 

 equations as follows : 



