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.

thermophiles, notice the prefix thermo.

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