Is there a strict difference between gas and dust? In Earthly environment most things become gaseous if heated enough. The temperature of interstellar medium seems to range mostly between 10 and 10 000 Kelvin. Is gas/dust an analog for hot/cold, or does the phase diagram of the element in question matter too? Can metals and molecules be gas in astronomical terms?
Yes, metals and other elements and molecules can exist in gaseous form under the right conditions of temperature and pressure. A "gas" is simply one of the fundamental states of matter, as in solid, liquid, or gas (and a few other states outside the scope of this question). But as a gas, these substances exist entirely as either individual atoms, individual elemental molecules, or individual compound molecules of multiple atoms (e.g. carbon dioxide).
Dust, on the other hand, is comprised of tiny particulate matter that has undergone the stronger intermolecular bonds to create substances like ice, silicates, and carbon compounds that float around in varying densities between the stars and between the galaxies. Since these particles are still extremely small (typically a fraction of a micron across), they can appear to be a gas, but these tiny, irregularly-shaped objects still exist individually in a solid or liquid state.
I may just add to the excellent answer by Robert that interstellar dust particles, very much like cigarette smoke in air, hangs in the interstellar gas and interacts with it both kinematically (is dragged along with it depending on the particle size) and energetically (exchanges heat, which can result in significant cooling of the gas). Dust particles also interact with the (stellar) radiation and can be evaporated due to high-energy radiation, but can also grow by condensing from the surrounding gas.
All larger solid astronomical objects (planets, asteroids etc, but not stellar remnants) have formed from dust, which in turn has formed from the heavier elements in the inter stellar gas.
For many astronomical purposes, dust is annoying, as it blocks the light, in particular the shorter wave lengths (reddening and darking the light of stars), hiding stars, in particular in the mid-plane of the Milky Way. As a consequence, the Galactic centre, a place of great astronomical interest, is largely invisible and can only be studied by observing other wave lengths than visible light, in particular infrared which is hardly affected by dust absorption.