In anymixture, (1) the composition can be varied and (2) each component of the
mixture retains its own properties.Imagine that we have a sample of muddy river water (a heterogeneous mixture). We
might first separate the suspended dirt from the liquid by filtration. Then we could re-
move dissolved air by warming the water. Dissolved solids might be removed by cooling
the sample until some of it freezes, pouring off the liquid, and then melting the ice. Other
dissolved components might be separated by distillation or other methods. Eventually we
would obtain a sample of pure water that could not be further separated by any physical
separation methods. No matter what the original source of the impure water — the ocean,
the Mississippi River, a can of tomato juice, and so on — water samples obtained by pu-
rification all have identical composition, and, under identical conditions, they all have
identical properties. Any such sample is called a substance, or sometimes a pure substance.
A substancecannot be further broken down or purified by physical means. A sub-
stance is matter of a particular kind. Each substance has its own characteristic prop-
erties that are different from the set of properties of any other substance.Now suppose we decompose some water by passing electricity through it (Figure
1-8). (An electrolysisprocess is a chemical reaction.) We find that the water is converted
into two simpler substances, hydrogen and oxygen; more significantly, hydrogen and
Figure 1-8 Electrolysis
apparatus for small-scale chemical
decomposition of water by electrical
energy. The volume of hydrogen
produced (right) is twice that of
oxygen (left). Some dilute sulfuric
acid is added to increase the
conductivity.1-5 Mixtures, Substances, Compounds, and Elements 13The first ice that forms is quite pure.
The dissolved solids tend to stay
behind in the remaining liquid.If we use the definition given here of a
substance,the phrase pure substancemay
appear to be redundant.oxygenhydrogenwater