152 , NON-METALS AND THEIR COMBINATIONS. 



cules of the dissolved body being present with all their characteristic proper- 

 ties, and on evaporation the dissolved solid will be re-obtained unchanged. 

 Instances of this kind are solutions of sugar or table salt and water. (The 

 breaking down of molecules into ions during simple solution will be considered 

 later.) 



In chemical solutions there takes place a rearrangement of the atoms within 

 the molecules, both of the solvent and of the substance dissolved. Moreover, 

 on evaporation of the solution a substance is obtained entirely different from 

 the one which has been dissolved. Instances of this kind are the dissolving of 

 sodium in water, when sodium hydroxide is formed ; or the dissolving of zinc 

 in sulphuric acid, when zinc sulphate is formed. The term emulsion is used 

 to designate a more or less homogeneous liquid rendered opaque or rnilky by 

 the suspension in it of finely divided particles of fat, oil, or resin. The milk 

 of mammalia and the milk-like juice of certain plants are instances of true 

 emulsions. 



Many salts combine with water in crystallizing ; crystallized sodium 

 sulphate, for instance, contains more than half its weight of water. 

 This water is called ivater of crystallization, and is expelled generally 

 at a temperature of 100 C. (212F.). Some crystallized substances 

 lose water of crystallization when exposed to the air ; this property is 

 known as efflorescence. Crystals of sodium carbonate, ferrous sulphate, 

 etc., effloresce, as is shown by the formation of powder upon the crys- 

 talline surface. Substances are said to be anhydrous when they are 

 destitute of water, for instance, when crystals have lost their water 

 of crystallization or when ether or alcohol have been freed from dis- 

 solved water. The term anhydride is sometimes used for oxyacids 

 which have been deprived of all water, so that they are no longer 

 acids, but oxides. Thus, by removing water from sulphuric acid, 

 H 2 SO 4 , there is left sulphur trioxide, or sulphuric acid anhydride, 

 SO 3 . The term deliquescence is applied to the power of certain solid 

 substances to absorb moisture from the air, thereby becoming damp 

 or even liquid, as, for instance, potassium hydroxide, calcium chloride, 

 etc. Such substances are spoken of also as being hygroscopic, and 

 are used for drying gases. 



The term effervescence refers to the escape of a gas from water or 

 from any other liquid in which the gas was held under pressure or in 

 which it may be generated ; as, for instance, when an acid is added 

 to a carbonate, whereupon carbon dioxide escapes with energetic 

 bubbling. 



The explanation of effervescence and deliquescence is found in a well-known 

 principle of physics. It is well known that liquids will evaporate in a closed 

 space until the pressure of the vapor is equal to the vapor tension of the liquid, 



