After purification, the test of purity of an organic compound is the nest important step. A pure organic compound possesses characteristic physical properties such as refractive index, specific gravity, boiling point, crystalline structure and melting point. If a purified sample shows the same properties that the pure substance is known to possess, it may be considered as pure and no further purification is required. However for practical purpose in laboratory only melting point and boiling point are used as criteria for purity. 
A pure organic solid has a definite and sharp (sudden, rapid and complete) melting point, while an impure substance has a lower and indefinite melting point. 
The melting point of a solid may be defined as the temperature at which the solid and liquid states of the compound are in equilibrium with each other at an external pressure of l atmosphere.
						

							
Analytical Methods for Testing Purity
The most accurate means of determining the purity of a substance is through the use of analytical methods. These methods, widely used in different industries, mostly involve chemical analysis, which can pinpoint the presence, identity and amount of impurities in the sample. The most simple chemical methods include gravimetry and titration. There are also the more advanced light-based or spectroscopic methods, such as UV-VIS spectroscopy, nuclear magnetic resonance and infrared spectroscopy. Chromatographic methods, such as gas chromatography and liquid chromatography, can also be used. Other methods used in testing the purity include mass spectroscopy, capillary electrophoresis, optical rotation and particle size analysis.
						

							
Physical Comparison With a Pure Standard
One of the simplest ways to check the purity of any substance is to compare the substance with a certified pure sample. Even physical comparisons can reveal a lot about the purity of a sample. Visual comparison can reveal the presence of any large impurities, such as dirt or other differently colored impurities. If the substance is nontoxic, a smell test can be used to compare it with the pure sample. Any dissimilar odors indicate the presence of at least one impurity. If the substance is edible, a taste test can be conducted. A difference between the taste of the substance and the taste of the pure sample hints at the presence of impurities.
Melting and Boiling Point Determination
The physical properties of a substance can be used to establish its purity. These properties include the melting point and boiling point. Different substances tend to have different melting and boiling points, and any pure substance will have a specific melting and boiling point. However, the presence of impurities will cause a lower melting point as well as a change in boiling point.
Colorimetric Methods
There are many colorimetric methods for determining whether a substance is pure or if there are impurities present. These usually involve the use of a chemical for detecting the presence of common impurities, which will turn the chemical a certain color. These methods are simple and are usually designed to determine the presence of impurities, not to determine the amount or the percent purity of the substance. One common use of such colorimetric methods is in forensics, where color tests are frequently used to identify illegal drugs as well as to determine their purity.