Deposition [phase transition]

Deposition is the phase transition in which gas transforms into solid without passing through the liquid phase. The reverse of deposition is sublimation and hence sometimes deposition is called desublimation. Deposition releases energy and is an exothermic phase change (thermodynamic process).

One example of deposition is the process by which, in sub-freezing air, water vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground. Another example of physical deposition is the artificial process of lamaw physical vapor deposition, used to deposit thin films of various materials onto various surfaces.

Unlike sublimation, which is an endothermic transformation in which the solid absorbs heat, deposition is an exothermic process in which the gas must give up heat to pass to the solid phase. The physical causes of deposition, as always, are to be found in the microscopic world and in particular in molecular interactions.

In the gaseous state, molecules move freely and have an intense kinetic energy. This means that the average speed of agitation of molecules is high, to the point of preventing the formation of intermolecular bonds. As the temperature decreases, however, the kinetic energy of the molecules decreases, and it can happen that intermolecular bonds typical of a solid start to form. Little by little the molecules are no longer free to move as before and they can only vibrate around their equilibrium position, inside the crystal lattice of the newly formed solid.

Temperature, pressure and latent heat of depositon

As for other state transitions, also deposition can occur not only with a decrease of the gas temperature, but also with an increase of the pressure value. As usual, and exactly as for all the other phase changes, for a certain substance we cannot simply talk about deposition temperature or deposition pressure. It is necessary to consider the couple of values (pressure, temperature) and, once one of them is fixed, to establish if there is a value of the other value for which the deicing occurs.

The values of pressure and temperature for which the phenomenon can be observed change according to the particular substance under examination, and can be deduced from the relative phase diagram.
In general, the brininess of a gas can occur as long as the temperature is equal or lower than the melting (or solidification) temperature of the substance.

To have a clearer idea let’s consider a specific example: the phase diagram of water. In the case of other substances we will have different values, but similar considerations will apply.

Let’s take as reference the sublimation curve, or equivalently the deposition curve of water. First of all we observe that there are a specific value of temperature and pressure above which the gas cannot frost (those corresponding to the triple point).

If we set a pressure value lower than that of the triple point and we consider the corresponding horizontal line on the state diagram, the intersection between this line and the deposition curve identifies the deposition temperature that corresponds to the chosen pressure value. Similarly, if we consider a temperature value lower than that of the triple point and we consider the corresponding vertical line, the intersection between that line and the deicing curve determines the deicing pressure corresponding to the chosen temperature value.

Finally, the latent heat of deposition is nothing more than the latent heat of sublimation \(L_s\): \(Q=mL_s\) where \(Q\) is the heat that the gas must give up to the external environment and \(m\) is its mass. Let’s remember that the latent heat of sublimation depends on the substance, pressure and temperature, and it is the amount of energy that must be given to 1 kilogram of substance in order to sublimate completely. In the case of deposition we will use the same formula, thus calculating the amount of energy that must be subtracted from the gas in order for it to frost completely.

Examples of deposiotion

The term dew comes (obviously) from the word frost, which forms naturally on plants and is in fact the main example of dew. In winter the water vapor in the air comes into contact with bodies that are at a temperature less than or equal to 0 °C and as a result of the low temperature it frosts, forming small ice crystals that are deposited on leaves or branches until they form small, clearly visible layers of ice.

It is easier for frost to form on clear nights, when the temperature reaches colder values; on the contrary, on cloudy nights the clouds tend to retain the heat that is absorbed during the day from the ground and then radiated during the night, thus maintaining higher temperatures.

Sometimes it is possible to observe the phenomena of sublimation and frost simultaneously. For example it is possible to perform a small experiment in the laboratory: small iodine crystals are placed in a transparent container, which is then closed. The container is then placed on a stove so that the iodine on the bottom can heat up. As the iodine gains heat, it sublimates, emitting clearly visible purple vapors that rise from the crystals. As it rises, the vapor comes into contact with the top of the container, which is not directly exposed to the heat source: the vapor cools down and starts the reverse process, i.e. the frosting, with the formation of small solid crystals of iodine. With a single experiment it is possible to observe the passage solid → gaseous → solid.

Related keywords

  • Deposition (chemistry)
  • Deposition (geology)
  • Deposition (aerosol physics)

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