xiv SIMPLE CHEMICAL OPERATIONS 209 



sulphur, and carbon will serve as good instances. But the line 

 between the two classes is not a hard and fast one, for one or 

 two substances possess some of the properties which distinguish 

 a metal, and yet for other reasons, which the student will under- 

 stand better later, are not classed with the metals, but with the 

 non-metals. Arsenic may be cited as an instance of a substance 

 which possesses properties common to both classes. 



Chemical Operations. A number of chemical operations are 

 so frequently employed, that it will be well at once to familiarise 

 ourselves with them, and we may do this by the following ex- 

 periments. 



EXPT. 198. Place a piece of sugar in water, and note that it 

 soon disappears, and has given a sweet taste to the whole of 

 the water, so that the particles of the sugar must be spread 

 through the entire mass of the water. 



The sugar is said to have dissolved and formed a solution of 

 sugar. Similarly, a very great number of substances can dis- 

 solve, or are soluble, in water, but not all to the same extent, 

 and when no more of the substance can be dissolved the solution 

 is said to be saturated. 



EXPT. 199. Weigh out 50 grams of each of the following 

 substances : finely powdered nitre, sugar, salt, and to each 

 add water, in small quantities, with vigorous shaking after 

 each addition. Determine thus the quantity of water neces- 

 sary to form a saturated solution of each. 



Solution is a Physical Change. EXPT. 200. Weigh out a 

 quantity of salt in an evaporating basin and dissolve it in 

 water. Heat gently over a Bunsen burner so that the water 

 boils and evaporates away completely. Note that a white 

 solid remains in the basin, and again weigh. Satisfy yourself 

 that the weight is equal to the weight of the basin and salt 

 before solution, and that the solid left is still salt. 



Hence during solution 1 we have in general no chemical 

 change, but merely a change of physical state, although we 

 shall find later that in certain cases solution may be accompanied 

 by chemical changes. 



By very gently warming water or other liquid, or even by 

 allowing it to remain exposed to the air for some time, the liquid 



1 The act of solution is now commonly spoken of as dissolution, the term solu- 

 tion being restricted to the product formed by the dissolution. 



