Deposition is the

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

in which gas transforms into

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

without passing through the liquid phase. Deposition is a thermodynamic process. The reverse of deposition is sublimation and hence sometimes deposition is called desublimation.

Applications

Examples

One example of deposition is the process by which, in sub-freezing air, water vapour changes directly to

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

without first becoming a

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

. This is how frost and hoar frost form on the ground or other surfaces, including leaves. For deposition to occur, thermal energy must be removed from a gas. When the air becomes cold enough, water vapour in the air surrounding a leaf loses enough thermal energy to change into a solid. Even though the air temperature may be below the dew point, the water vapour in the air alone may not condense spontaneously. This supercooled water vapour, when around a leaf, immediately begins to condense. Since it is already past the freezing point, the water vapour changes directly into solid frost. Soot accumulates as an aerosol deposition. In a fireplace, soot is deposited on the walls of the chimney. Soot particles are aerosolized within volatile organic compounds. These VOCs are in a hot and gaseous state, rising from the fire. When the VOCs come into contact with the walls, they cool, and change to the solid state, without formation of the liquid state. The soot particles also cling to the walls upon contact. This process is made use of industrially in combustion chemical vapour deposition.

Industrial applications

There is an industrial coating process, known as evaporative deposition, whereby a solid material is heated to the gaseous state in a low-pressure chamber, the gas molecules travel across the chamber space and then deposit to the solid state on a target surface, forming a smooth and thin layer on the target surface. Again, the molecules do not go through an intermediate liquid state when going from the gas to the solid. See also physical vapor deposition, which is a class of processes used to deposit thin films of various materials onto various surfaces. Deposition releases energy and is an exothermic phase change.

References

* Gaja, Shiv P., ''Fundamentals of Atmospheric Modeling,'' Cambridge University Press, 2nd ed., 2005, p. 525 * Moore, John W., et al., ''Principles of Chemistry: The Molecular Science,'' Brooks Cole, 2009, p. 387 * Whitten, Kenneth W., et al., ''Chemistry,'' Brooks-Cole, 9th ed., 2009, p. 7 * {{DEFAULTSORT:Deposition (Physics) Phase transitions