54 



CHEMISTRY FOR AGRICULTURAL STUDENTS 



ihe action of the combined oxygen of water, hydrogen being 

 in this case liberated. These two methods may be tried in the 

 case of other metals. 



Cut small pieces of potassium and sodium the size of small 

 peas. Note that on exposure to air, even when cold, the 

 freshly cut surfaces become immediately tarnished, a film of 

 oxide being formed. Throw the bits into a little pure cold 

 water in a dish. Note that both metals quickly disappear, a 

 gas being evolved. In the case of the potassium, but not in 

 the case of the sodium, this gas catches fire spontaneously, 

 showing that the heat produced by the chemical combination 

 of potassium and oxygen is greater than that of sodium and 

 oxygen, and that the chemical activity of the former, in respect 

 to oxygen, is greater than the latter. Note that the flame is 

 tinged violet, due to a little of the potassium being volatilised, 

 this being the colour of the incandescent vapour of potassium 

 and its compounds. Set fire to the gas being liberated by the 

 sodium, and note that this is coloured yellow by the sodium 

 vapour. To identify the gas, invert a test-tube of water in the 

 dish (Fig. 25), and quickly insert small bits of sodium on the 



Fig. 25. 



point of a penknife, until a test-tube nearly full of the gas is 

 obtained. Shake the gas with the water remaining in the test- 

 tube, transfer by upward displacement of air to a smaller dry 

 test-tube, and ignite it. In its lightness, in its combustibility, 



