Answer to Question #235675 in General Chemistry for Jersey

Question

Pure subatance and solution differences

Answer 1

Classically, substance is a matter with its own and unique physical properties and definite chemical composition. The properties of the substance cannot be changed smoothly with changing of its chemical composition, which also cannot be changed smoothly. For example water or H₂O contains 2 hydrogen atoms per 1 oxygen atom and everything with some other composition (as H₂O₂) would have completely different physical and chemical properties. Also, the substance could exist as an independent isolated phase, which does not change its composition through any kind of cooling or heating before the decomposition temperature. On the other side, a solution is also a kind of matter with smoothly changeable composition and as a result smooth changes in the physical properties. For example, sodium chloride or NaCl water solution could exist in an infinite number of different compositions with slightly different density, which is changing almost linearly from composition to composition.

It also must be noted that there are very interesting exceptions as non-stoichiometric compounds (Fe_1-xO, PdH_x etc), polymers etc.