A clear liquid in an open container is allowed to evaporate. After three days, a solid is left in the container. Was the clear liquid an element, a compound or a mixture? How do you know?


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First let's define a few terms.  An element is a pure chemical in its simplest form that contains a single type of atom.  An example is carbon.  Pure carbon will be composed entirely of carbon atoms with no other atoms mixed it.  A chemical compound is a single substance that is composed or more than one element.  Water is an example.  A sample of water is a single substance where every molecule is H2O (one atom of oxygen and two atoms of hydrogen combined).  A mixture is a medium that is composed of more than one substance.  The mixture can be either entirely uniform (homogeneous) or non-uniform (heterogeneous). 

In your example, the clear liquid eventually ends up as a solid over time.  Since you can see that two different substances are involved with different phases of matter (a solid and a liquid), we know that the original clear liquid is a mixture.  It is most likely a solid like salt or sugar dissolved in water.  That contains two different substances combined, hence it is a mixture.

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