xiv SIMPLE CHEMICAL OPERATIONS 215 



means, be broken up into anything simpler. They can be sub- 

 divided into metals and non-metals. 



Chemical Operations.- Solution is the process by which some sub- 

 stances, when placed in water or other liquids, disappear and their 

 particles spread through the entire mass of the water or other 

 liquid. 



When no more of the substance will dissolve the liquid is said to 

 be saturated. Solution is generally a physical change and is 

 unaccompanied by any changes of weight. 



Distillation is the process by means of which we are able to 

 separate the dissolved substance from the solution containing it. 

 The liquid itself is recovered by condensing the vapour. 



Filtration consists in utilising the porosity of unglazed paper or 

 other material for separating insoluble substances from solutions 

 with which they are mixed ; or, it is a means of purifying liquids 

 and isolating solids. 



Crystallisation denotes the separation of a dissolved solid in masses 

 having regular simper, from a solution as the liquid evaporates or cools. 

 The regular solids formed are called crystals. 



Precipitation means the formation and separation of an insoluble 

 substance from a solution. 



Compounds and Mixtures. 



Substances 



In a mixture the components exist side by side and can be sepa- 

 rated by simple mechanical methods. The ingredients may be 

 present in any proportion and the properties of the mixture are in- 

 termediate between those of the constituents. 



In a compound the components cannot be separated by the simple 

 means available in the case of mixtures. The properties of the 

 compound are quite different from those of the constituents and 

 those constituents are always present in certain definite proportions 

 which for each compound are invariable. 



QUESTIONS ON CHAPTER XIV. 



1. How may muddy water be (a) made clear, (/>) purified from 

 dissolved material ? 



2. Explain the differences between a mixture and a compound. 



3. What do you mean by a "physical " and what by a " chemical " 

 change ? If a substance changes under the influence of heat how 

 would you endeavour to find out whether the change is chemical or 

 merely physical ? 



4. A mixture of salt and powdered glass is given to you. How 

 could you (a) obtain separately the two constituents, (6) find the 



