You find out that it does not dissolve in water, has a very high melting point and conducts electricity when molten. the substance is probably

The substance is a metal, mainly iron or copper.

Metals are classified as those elements which have a strong structure, hard, malleable, ductile, sonorous and have high heat and electricity conductivity.

Metals are not soluble in water in atomic state because they are in crystal forms which are very hard to be broken by water.

They are conductor for both heat and electricity. Metals have free electrons in their outermost shell which can be free in the lattice which helps to conduct electricity even in molten state.

And metals do usually have very high melting points. So they aren't easy to melt at all. Main conductors that are used are iron and copper.

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.