Changing a substance from its physical state of gas to the physical state of a solid requires the removal of thermal energy. A gas has particles that have a larger amount of kinetic energy or moving energy, they are vibrating very rapidly.
A solid has particles with lower amounts of kinetic energy and they are vibrating slower without changing position. This change of state from a gas to a solid is not a very common phase change but is referred to as deposition.
It is called deposition because the particles in the gas form are depositing into a solid form.
gas to solid is called – Deposition
Deposition is the phase transition in which gas transforms into a solid without passing through the liquid phase. Deposition is a thermodynamic process. The reverse of deposition is sublimation and hence sometimes deposition is called de-sublimation.
Deposition is the process by which a gas skips the gaseous state and changes directly into a solid. For example, iodine vapors on cooling solidify into iodine crystals.
As stated earlier, vapor deposition is the opposite of sublimation. Deposition is when a substance in gas form changes states to become a solid. The gaseous substance gets deposited (usually as crystals) bypassing the intermediate liquid state.
An example of deposition is when water vapor in the atmosphere changes directly into ice, such as the formation of frost.
Example Of Deposition
The most typical example of deposition would be frost. Frost is the deposition of water vapor from humid air or air containing water vapor onto a solid surface.
Solid frost is formed when a surface, for example, a leaf, is at a temperature lower than the freezing point of water and the surrounding air is humid. Snow is also deposition. The water vapor in the clouds changes directly to ice and skips the liquid phase entirely. This can only occur in freezing temperatures.
As is typical with a transition from a gas state to liquid or solid, thermal energy must be removed so the molecules will begin to slow down and come closer together. For the gas to change to a solid, the molecules must be able to lose a lot of thermal energy.
In clouds, water vapor is subject to extremely cold temperatures, and loses its thermal energy before it comes down. As it comes down to the surface of the earth, it becomes snow. Another example is the frost that is found on the windows of the car on a very chilly day.
Examples of Gas to Solid:
Under certain circumstances, gas can transform directly into a solid. Some examples of gas to Solid include:
1. Frost Formation.
Frost formation occurs when humid air encounters a surface whose temperature is less than the freezing temperature of water (273 K), and is less than the dew point temperature, so that water vapor goes from a gaseous to a solid state.
Frost usually forms on objects like cars, windows, and plants that are outside in air that is saturated, or filled, with moisture.
2. Dry Ice Formation.
Dry ice is nothing but the solid form or the frozen form of Carbon dioxide (CO2).
Generally, Carbon dioxide exists in the gaseous state. When Carbon dioxide (CO2) is kept at a temperature of -78o C and then compressed, it gets converted into solid directly from the gaseous state, without getting converted to liquid.
3. Snowflakes.
Snowflakes form in clouds where the temperature is below freezing (less than 0ºC, or 32ºF). The ice crystals form around tiny bits of dirt that have been carried up into the atmosphere by the wind. As the snow crystals grow, they become heavier and fall toward Earth.
Different types of snowflakes form in different conditions. Temperature determines if the crystals become a flat plate, a long column, or a prism shape.
4. Iodine Crystals.
Iodine is heated in a beaker to cause the iodine to sublimate. The iodine vapor formed in the beaker can undergo deposition on the walls of the beaker and the round flask filled with ice water on top of the beaker. Solid iodine crystals form that are very shiny.
5. Camphor Crystals.
However, there are some substances which on heating directly turn into gases from solids. camphor iodine crystals and solid carbon dioxide are such substances. Such substances are called sublimatory substances and this process of conversion is known as sublimation.
6. Freeze-Drying.
Freeze drying, also known as lyophilization or cryodesiccation, is a low-temperature dehydration process that involves freezing the product and lowering pressure, removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat.
7. Airplane Contrails.
Contrails (condensation trails) formed by airplanes at high altitudes can lead to ice crystals directly forming from the water vapor present in the engine exhaust gases.
8. Helium Snow.
At extremely low temperatures, helium gas can undergo deposition and transform into a solid. This is often referred to as “helium snow.”
Industrial applications
There is an industrial coatings 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.