A refinery will pay less for impure gold than pure gold.
It is only very slightly more dense than pure water.
Pure ethanol is less dense.
Generally an impure material is cheaper than a pure material; the word advantage is not so adequate here - can use pure or impure, this depends on the specific applications.
impure because they are made of more than 1 substance
A compound can be pure if it consists of only one type of molecule. If a compound contains more than one type of molecule or impurities, then it is considered impure.
There's no correlation between the composition of a substance and its density. Pure ethanol is less dense than potato-leek soup, but pure mercury is more dense. Potato-leek soup is less dense than pure mercury, but concrete is more dense.
An aluminum nail that is less dense than the element aluminum would not be made of pure aluminum, but would have to be an alloy of aluminum and some other substance. If an aluminum nail is made of pure aluminum, it would have the same density as the "actual" element.
This depends on the specific application of a material.
no
As a whole, trees are less dense than water (both pure and salt). This difference makes them Fan
For a simple experiment to distinguish between pure and impure naphthalene, you can try sublimation. Place a small amount of naphthalene in a test tube and heat it gently. If the naphthalene sublimes (converts from solid to gas without melting), leaving no residue, it is likely pure. However, if there is residue left behind, it suggests the naphthalene is impure.