chemistry technology

Different  materials  may  be  distinguished  by  their  properties,  the  most  obvious  of  which  is  the physical state, or state of aggregation, on the basis of which all materials are classified as solids, liquids, and gases.

The characteristic feature of gas is that its molecules are not held together, but move about freely. Because of this freedom of molecular motion a gas does not possess either definite shape or definite size,
it shapes itself to its container. A liquid, on the other hand, has a definite volume, but does not have a definite shape. Only a solid is characterized both by a definite shape and definite size.
By the word substance a chemist means an essentially pure substance/ Actually, all substances are more  or  less  impure/  When  referring  to  very  impure  substances,  solutions,  and  mixtures,  the  word material should be used instead.
All  substances  can  be  divided  into  two  classes:  elementary  substances  and  compounds.  An elementary substance is a substance, which consists of atoms of only one kind, a compound is a substance which consists of atoms of two or more different kinds. These atoms of two or more different kinds must
be  present  in  a  definite  numerical  ratio  since  substances  are  defined  as  having  a  definite  invariant composition. Thus an elementary substance is composed of two or more elements./To avoid confusion, it
is necessary for us to state exactly what a particular kind or atom in the above definition of an element means. By this expression we mean an atom whose nucleus has a given electrical charge. All nuclei have positive electrical charges which are equal to or integral multiples of the charges of the electron/ with an opposite sign./The integer which expresses this relation is called the atomic number.
The  word mixture  is  used  to  refer  to a  homogeneous  material/  exhibiting a  uniform structure/, which is not a pure substance, or to a heterogeneous aggregate of two or more substances. The ingredients
of a mixture are called its component. Sometimes a mixture consisting mainly of one component, with much smaller amounts of others, is called an impure substance. The components present in the smaller amounts are called impurities.