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Pure Substance in Chemistry...Criteria for purity

 Pure substances are defined as substances that are made of only one type of atom or only one type of molecule (a group of atoms bonded together). The measure of whether a substance is pure is
known as purity. For example, pure iron would only contain iron atoms, and, as in the sugar cube mentioned above, pure sugar would only contain molecules of the substance called sucrose.
Pure substances exhibit very well-defined physical properties, or properties that are not connected with the substance's ability to combine with different substances. The temperatures where pure solids melt, known as melting points, are particularly sharp, meaning the melting occurs at a single temperature. Likewise, the temperatures where pure liquids begin to boil, or boiling points, occur at single temperatures when other factors, like air pressure, are controlled. The ability of substances to conduct electricity, or conductivity.
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.
The major criteria for purity determination are melting point and boiling point

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