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.

The correct answer is option 3: A mixture can vary in composition, but apure substance has a set composition.

Explanation:

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.