Kids, sometimes it can be hard to figure out what someone means by a pure compound versus a mixture. Let's try to clear this up with an easy explanation and experiment. First, pure elements are what you see on the periodic table, and some materials exist naturally in their pure elemental form, like lead (Pb), neon (Ne), iron (Fe), etc. Some other elements are "diatomic", like nitrogen (N₂ ), oxygen (O₂ ), and hydrogen (H₂ ) in their natural state. Then we have to deal with the compounds. These are pure materials made up of two or more elements on the periodic table and represented by a distinct molecule, like water (H₂O) and ammonia (NH₃ ) and sodium chloride (NaCl) salt.

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

Finally there are the mixtures. These are combinations of compounds and account for most of what you encounter during the day. Take salt water, for example. Mix some regular table salt in water and you have a mixture. It is not a new compound because you can't write a formula for salt water, rather it is a combination (a "homogeneous" one). Mix together salt with some white table sugar and you also have a mixture (a "heterogeneous" one). White sugar is a pure compound called sucrose. Other examples of mixtures are: milk (water, milk fat, proteins, lactose, etc.), blood (white blood cells, red blood cells - with hemoglobin molecules, water, platelets, electrolytes - salts, etc.), and dirt (silica or silicon dioxide or sand, decayed plants, moisture or water, etc.). It is much easier to make a mixture than it is to make a pure compound! Try to find some more examples of mixtures and compounds during a regular day. 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
September 1998