xvi WATER AND THE ELEMENTS COMPOSING IT 245 



Temporary and Permanent Hardness. Hard waters differ 

 among themselves. Some can be softened by mere boiling, and 

 when this is so the hardness is said to be temporary. If the 

 hardness is not removed after the water has been boiled, and 

 the water requires the addition of a chemical to soften it, such 

 hardness is termed permanent. 



EXPT. 250. Filter and then evaporate to dryness some 

 samples of a water which you have found to be hard. Observe 

 a white solid is left. Add to this white solid a little hydro- 

 chloric acid. Observe that it effervesces. 



EXPT. 251. Filter and then evaporate samples of soft water, 

 if any are obtainable. You will most probably find that very 

 little residue remains and that no effervescence results on the 

 addition of acid. 



Hence the hardness of water is due to some white solid 

 dissolved in the water, and this solid effervesces on the addition 

 of acid. 



We shall return to this in a later chapter. 



CHIEF POINTS OF CHAPTER XVI. 



Water is a clear liquid with a bine-green colour. It boils at 100 C. , 

 when it is converted into steam. It freezes at C., becoming ice. 

 Its density at 4 C. is 1. It possesses great solvent power. 



Hydrogen is contained in Water. This has been found by causing 

 heated iron to drive it out of the water, when the iron combines with 

 the oxygen which is left, to form oxide of iron. Sodium turns the 

 hydrogen out of water at ordinary temperatures. The hydrogen 

 can be collected and examined. 



Preparation of Hydrogen. Hydrogen is best prepared by acting 

 upon a dilute acid with a metal. Sulphuric acid and zinc were 

 found to be suitable. 



Sulphuric Acid and Zinc form Hydrogen and Zinc Sulphate. 



Properties of Hydrogen. It is a colourless, odourless gas, con- 

 siderably lighter than air. It does not support combustion, but 

 itself burns in air. It forms an explosive mixture when mixed with 

 air. 



Production of Oxide of Hydrogen. By collecting the product of 

 combustion when hydrogen is burnt, and examining it, it is found to 

 be a clear liquid with density 1, boiling point 100 C., freezing 

 poiut C. The product, or oxide of hydrogen, is thus seen to be 

 really water. Hydrogen in burning produces water ichich is, there- 

 fore, an oxide of hydrogen. 



Composition of Water by Volume. This can be determined by 

 means of an Eudiometer, into which known volumes of pure dry 



