Classify each of the following as an element,compound,or mixture:
Gold,Air,Marble,Milk,Sugar.

(N/A) $1$. Element: Gold $(Au)$ is a pure substance consisting of only one type of atom.
$2$. Compound: Marble $(CaCO_3)$ and Sugar $(C_{12}H_{22}O_{11})$ are compounds because they consist of two or more elements chemically combined in a fixed ratio.
$3$. Mixture: Air and Milk are mixtures. Air is a homogeneous mixture of various gases,while milk is a heterogeneous mixture (colloid) of water,fats,proteins,and minerals.