Deposition, by definition in chemistry, refers to a phase transition in which matter transitions directly from a gaseous state into a solid state without passing through an intermediate liquid phase. Deposition is the opposite of sublimation, a phase transition in which a solid transitions directly into a gas.
Deposition and sublimation are 2 of the 6 first-order phase transitions, the other 4 being: freezing (liquid to solid), melting (solid to liquid), evaporation (liquid to gas) and condensation (gas to liquid).
“All matter originates and exists only by virtue of a force which brings the particle of an atom to vibration and holds this most minute solar system of the atom together.” — Max Planck
In order for deposition to occur, the chemical system must release energy into the environment. This makes deposition an exothermic reaction as it involves the release of heat into the environment. Some common examples of deposition include the formation of frost on a cold surface and the formation of ice crystals in clouds. In both cases, water vapor is converted from a gaseous state directly into solid water ice without passing through a liquid phase.
Phase Transitions And State Changes
Deposition is a phase transition. A phase transition refers to the process during which a substance changes from one state of matter into another: solid, liquid, or gas. Every substance can undergo a phase transition and the state of matter that a substance takes is dependent on the surrounding temperature and pressure. Phase transitions are physical reactions, not chemical reactions, as they do not involve the breaking and forming of chemical bonds. The phase diagram of a substance is a graphical representation of the thermodynamic properties of a substance and shows the temperatures and pressures at which that substance exists in one of the three classical states of matter.
For example, consider a phase diagram for water. Water is a good test case for understanding phase transitions as water seems to be the only known substance that naturally exists in all three classical states of matter on Earth. The 3 regions S, L, and V represent the temperature and pressure at which water exists in a solid, liquid, or gaseous state, respectively. The point TP represents the triple point which is the temperature and pressure at which a substance exists in equilibrium in all 3 states of matter. The lines dividing those regions represent the phase boundary; the temperature and the pressure at which water transitions from one state to the next. The point CP stands for the critical point, the temperature and pressure at which water begins to exist in a liquid/gaseous hybrid state called a supercritical fluid. For sake of convenience, phase transitions are typically described at standard pressures of 1 atm, or the average pressure of the Earth’s atmosphere.
“The rain drop that fell on my head, After a long cycle of evaporation and condensation, Was meant for me.” — Shivesh Shukla
Each phase transition, evaporation, melting, boiling, etc. requires either the introduction or release of latent heat from a system. For example, the latent heat required to evaporate liquid water into vapor is 697.3 cal/g; it requires an input of 697.3 calories of heat energy to evaporate 1 gram of water. If the latent heat value is positive, the reaction is endothermic, because it must absorb heat from the environment. If the value is negative, the reaction is exothermic as the system must release that much energy to change state.
So, according to a phase diagram, deposition can be understood as a physical change in which the state of a system crosses over the line that separates the S and V regions without passing through the L region. In order for deposition to occur, a system must release latent heat into the environment. Removal of heat from water vapor causes the irregularly arranged gaseous molecules to take on a definite shape and structure. The latent heat required for the transition of water vapor into solid ice is -677.0 cal/gram. Therefore, deposition is an exothermic reaction as it requires the release of 677 calories of heat to depose one gram of water vapor into one gram of ice.
Examples Of Deposition
The most obvious example of deposition is the formation of snow in clouds from water vapor. Water from liquid sources on Earth is evaporated into the Earth’s atmosphere. Sub-freezing temperatures in the atmosphere cause the water vapor to depose directly into ice crystals, which then precipitate to Earth. This is also the same mechanism by which frost forms directly on cold surfaces, like windows or metals. Water vapor in the atmosphere comes into contact with a cold surface. Heat in the water vapor is absorbed by the cold surface, and frost forms.
Various chemical vapors can also depose to form films on surfaces. For example, bug bombs used to rid houses of parasites release a gaseous pesticide into the house. The pesticide comes into contact with household surfaces and deposes into a sticky film that traps and kills pests. The same logic is behind various industrial chemical coating processes. The coating substance is placed in a chamber as a solid and is heated to a gaseous state. Gaseous molecules come into contact with the surface to be coated and depose into a uniform layer. The same process is commonly used to purify chemical materials for the laboratory. The deposition of a compound is used to split the compound into pure samples of its chemical constituents.
Deposition is also used to form dry ice from carbon dioxide. Carbon dioxide gas is placed in a chamber and the temperatures and pressure are modified. Gaseous carbon dioxide will depose into a solid at around 5.1 atm and -79°C. The sublimation of carbon dioxide back into gas produces dry ice’s characteristic fog effect.
“If I melt dry ice, can I swim without getting wet?” — Steven Wright
Deposition is also the main mechanism responsible for the obstruction of the lungs resulting from inhaled pollutants. Deposition of cigarette smoke, for example, forms the tarlike residue found in the lungs of COPD patients. Certain new COPD treatments seek to make use of the process of deposition create drugs that can be directly administered through the lungs.