What is the phase in chemistry where a gas chnges to a solid?
A "vapor" in chemistry is always the gas phase of a substance that is more familiar in one of its condensed phases: liquid or solid. Therefore, iodine vapor is the gas phase of the element iodine, and has the same molecular composition as the solid, I2. Iodine is a solid at standard temperature and pressure, but sublimes directly to gas phase, without any intermediate liquid phase, at only moderately higher temperatures than the standard.
For most substances, the solid phase is more dense than the liquid and gas phases. The liquid phase is less dense than the solid phase but more dense than the gas phase, and the gas phase is less dense than either the solid or liquid phases. Water is an exception. Its solid phase (ice) is less dense than the liquid phase.
There is a branch of chemistry called physical chemistry, which deals with phase changes (the phases being solid, liquid, or gas). Clouds involve phase changes. Liquid water evaporates to produce clouds which then condense back into liquid to produce rain, or freeze to produce snow. Chemistry can shed light on exactly how these things happen.
The three phases of matter (as classically defined) are solid, liquid and gas. Most phase changes that begin with solid and end with gas pass through the liquid phase on the way. Likewise, phase changes that begin with gas and end with solid pass through liquid on the way. Think of water... usually, from solid ice it passes first to liquid water, and only then does it evaporate to gaseous water vapor. It is sometimes…