* Pure substances have a fixed and definite composition. They are made up of only one type of molecule or element.
* Impurities are any substances that are present in the compound that are not part of its defined chemical structure. These impurities can be other elements, compounds, or even different forms of the same compound (like different isotopes).
Examples:
* Pure water (H₂O): This is a pure substance because it contains only hydrogen and oxygen atoms in a fixed ratio.
* Tap water: This is not a pure substance because it contains dissolved minerals, salts, and other contaminants.
* Salt (NaCl): This is a pure substance when it only contains sodium chloride molecules.
* Salt with added iodine: This is not a pure substance because it contains iodine in addition to sodium chloride.
Important Note:
Even a very small amount of impurity can make a compound not a pure substance. However, the definition of "pure" can be flexible depending on the context. For example, in some applications, a compound may be considered "pure" if it contains less than a certain percentage of impurities.
