What is a Compound?
Compounds and Mixtures
While the Properties of a mixture is the combined properties of the constituent elements, compounds exist as independent Entities with unique chemical properties that are diverse from those of the constituent elements. For example, Sodium Na (with its metallic properties) combines chemically with Chlorine Cl (with non metallic properties) in a ratio of 1:1 to form one molecule of Sodium chloride salt NaCl which evidently displays its own unique chemical and physical properties.
Also, though the components of a mixtures can easily be separated by physical means - using one or more of the standard separation techniques - Components of chemical compounds can only be separated by chemical reactions. For example,
Compounds and Elements
Elements, no matter the size, exists only in their pure form as depicted by the properties in the periodic Table. No matter how much a specific element combines with itself, it remains the same element. But when two different types of elements or more combine chemically they become Compounds. But for elements to form a compound, they must combined in fixed ratios.
Compounds formed from the reaction of different elements display their own unique chemical and physical reactions. The boiling point and Melting point of water is very different from Hydrogen. While Oxygen explosively supports combustion, Carbon (II) Oxide easily extinguishes a burning flame.
Compounds and Molecules
Even though Compounds are formed by the combination of two or more elements, they two can form their own Molecules. Two molecules of Hydrogen combines with one molecule of Oxygen to form One molecule of Water, and not three. Hence, Compounds contain molecules just like the constituent elements.
Chemical combinations that are not compounds
But it is not every chemical compound formed from the chemical combination of two or more elements that can be called compounds. Certain Chemical substances are known to exhibit properties that are different from real compounds due to the presence of very large amount of their Isotopes. Some other chemicals might contain large amount of impurities that are capable of altering their chemical properties.