Question

How do we determine the purity of a substance?

Answer 1

if it is made of up one kind of atom or molcule

Answer 2

Purity can be defined as the absence of any impurities -- or types of matter other than the substance itself.                                                                               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. 
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.
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.