How are mixtures defined?
By what is mixed within them.
If it is a homogeneous mixture, it could be a solution. If it is a heterogeneous mixture, you can physically tell the constituents apart.
Think salt water versus granite rock.
A mixture has to have more than one compound present, or different elements not part of a single compound. So it is a substance with two substances or more making it up.
Basically, a mixture is something that is not 100% pure and has more than one compound inside of it. For example the air is thought of as 'pure' but it is actually a mixture as it contains oxygen, nitrogen, carbon dioxide and other gases.
Typical mixture include water and sand, most often seen in science lessons. These two objects are not chemically bonded together, they are just simply mixed together. Other common types of mixtures include precipitates. In all honesty is is best not to over thing it. Also, a mixture counters the definition of a pure element. A mixture is not pure as a pure element is 1 type of atom or compound, where a mixture is usually a mixture of atoms or compounds.