What are different techniques of determining whether a substance is pure or not?

Here are the criteria of purity:

• Melting Point Determination: Pure crystalline solids melt at a sharp, specific temperature. Impurities lower the melting point and cause it to melt over a range of temperatures.

• Boiling Point Determination: Pure liquids boil at a sharp, specific temperature at a given pressure. Impurities raise the boiling point (for non-volatile solutes) and cause it to boil over a range of temperatures.

• Density Measurement: Pure substances have a characteristic density at a specific temperature and pressure. Impurities will alter the density.

• Chromatography: Techniques such as Thin Layer Chromatography (TLC) or Gas Chromatography (GC) can separate components of a mixture. A pure substance will show only one spot or peak.