Which pure substance can be classified as an element

A pure substance cannot be separated into 2 or more substances by physical means. Pure substances have one set of physical and chemical properties. The particles of a pure substance are alike no matter where they are found. There are two types of pure substances: elements and compounds.

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.