Question

How to determine whether a substance is liquid or vapour at certain temp

Answer 1

by looking its boiling and melting points

Answer 2

Here's your answer !!

Gases become more difficult to liquefy as the temperature increases because the kinetic energies of the particles that make up the gas also increase. The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied.