Purity+of+a+substance

How do you determine whether there are impurities in a substance?
4. The substance forms more than 1 spot on chromatogram.
 * 1) It has a lower boiling point.
 * 2) It has a higher melting point.
 * 3) It boils over a range of temperatures.

Why is knowing the purity of a substance so important?
In drugs, certain substances have to be pure or they will not take effect, thereby causing doctors to be unable to give effective medication to patients.

In foodstuffs, certain substances cannot be put into the foodstuff or the foodstuff might become harmful to the body. This might cause illnesses or death.

Therefore, it is important that the purity of a substance is tested carefully and accurately.