chemistry technology

If sugar and water, two pure substances, are mixed together, a solution result, uniform throughout
in  its  properties,  in  which  the  sugar  can  neither  be  seen  with  a  microscope  nor  filtered  out.  It  is  not distinguishable from a pure substance in appearance.
The experimental distinction between a pure substance and solution is quite simple when the solute
/the dissolved substance/ is not volatile so that it is left behind when the solvent is evaporated. However, when both are volatile the matter is not quite so simple and it is necessary to find out whether any change
in composition and hence in properties occurs during a change in state.
Suppose we wish to determine whether air is a pure substance or a solution. One method would be
to  liquefy  a  certain  amount  and  then  observe  what  happens  to  it  as  it  slowly  evaporates.  As  the evaporation proceeds one may observe that
a- The light blue color gradually becomes deeper
b- The temperature of the liquid slowly rises
c- The densities of both liquid and gas change.
Any  one  of  these  as  well  as  other  possible  observations  show  that  air  must  contain  two  or  more components  whose  relative  amounts  change  during  the  evaporation,  causing  the  observed  changes  in properties due  to differences  between  the  components in  color, volatility, density,  chemical  behavior. Still other properties might have been used.
The term solution is not restricted to liquid solutions. All gases are completely miscible with each other, forming but one phase, so that every mixture of gases is a solution. Alloys of silver and gold, no matter  what  the  relative  amounts  of  the  two  metals,  contain  but  one  kind  of  crystal,/the  properties  of which change continuously with the composition/, thus being a solid solution.
If liquid air is distilled in a scientifically constructed still, it is possible to separate it into two nearly pure constituents. One of these constituents, nitrogen, is found to be slightly lighter than air; it can be condensed to a colorless liquid boiling at -1940C; it is very inert chemically, reacting with but few other substances. The other constituent, oxygen, is slightly heavier than air; it gives, when condensed at low temperatures, a blue liquid boiling at -182.50C, and it reacts readily with many substances.
As another illustration, suppose we have a solid metal, which appears to be perfectly homogeneous under  the  microscope.  We  could  determine  whether  it  is  a  solution  or  a  pure  substance  by  melting  it, dipping  into the  melt  a  suitable  thermometer  and  letting  it  cool  slowly,  taking temperature  readings  at regular intervals, and plotting temperature against time.