What is a main difference between a mixture and a pure substance?
a mixture is only a liquid, but a pure substance can be in any state.
a mixture looks the same throughout, but a pure substance does not.
a mixture can vary in composition, but a pure substance has a set composition.
a mixture can be made up of a single compound, but a pure substance cannot.

A mixture can vary in composition, but a pure substance has a set composition.

Explanation:

    In general, pure substances are hardly found isolated in nature and are found in the form of mixtures, ie, associated with other substances. This means that we and almost everything around us are examples of mixtures of the most varied pure substances.

    Pure substances are materials that have constant chemical composition and physical and chemical properties as they do not change at constant pressure and temperature.

   Mixture is the union of two or more different substances (regardless of whether they are simple or compound). It has different and variable (non-fixed) physical characteristics (melting point, boiling point, density, toughness, etc.) compared to its constituent substances.

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the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.