Celebrate With Chemistry

Chemistry is the study of matter and the changes that take place with that matter.

Thursday, August 03, 2006

Methods Of Purification -Pure Substances and Mixtures

Pure Substances

a pure substance contains only one type of substance,
meaning it only contains one type of atom or molecule.

pure water is a pure substance.
and sea water is a mixture as it is water containing dissolved salt as well as other dissolved substances,
which is referred to as the impurity.


a pure solid has a fixed and exact melting point.
the presence of an impurity lowers the melting point and that causes the substance to melt over a range of temperature.

the addition of salt to ice.

pure ice melts at exactly 0 degrees celcius.
when salt is added, it lowers the melting point
to values between -5 to -25 degrees celcius.


the presence of any impurity raises the boiling point.

the addition of salt to water

water boils at 100 degrees celcius.
however, when salt is added, it raises the boiling point.


mixtures are formed by a physical change only.
because of this, they are easy to separate into pure substances.

this process is called purification and this can be achieved by simple physical purification methods like
dissolving, filtering, evaporating, distillation and chromatography.



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