![]() ![]() Solid carbon dioxide ( dry ice) sublimes everywhere along the line below the triple point (e.g., at the temperature of −78.5 ☌ (194.65 K, −109.30 ☏) at atmospheric pressure, whereas its melting into liquid CO 2 can occur along the solid-liquid line at pressures and temperatures above the triple point (i.e., 5.1 atm, −56.6 ☌). Examples Carbon dioxide Dry ice subliming in air Water, on the other hand, passes through a liquid phase at 1 atmosphere. As dry ice is heated, it crosses this point along the bold horizontal line from the solid phase directly into the gaseous phase. Comparison of phase diagrams of carbon dioxide (red) and water (blue) showing the carbon dioxide sublimation point (middle-left) at 1 atmosphere. ![]() Sublimation is sometimes used as a generic term to describe a two-step phase transition ― a solid-to-gas transition (sublimation in a more precise definition) followed by a gas-to-solid transition ( deposition). Similarly the combustion of candles, containing paraffin wax, to carbon dioxide and water vapor is not sublimation but a chemical reaction with oxygen. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction. The term sublimation refers specifically to a physical change of state and is not used to describe the transformation of a solid to a gas in a chemical reaction. Solid-to-gas transition is always called sublimation in both corresponding cases. While vaporization (from liquid to gas) is divided into two types vaporization on the surface of the liquid is called evaporation and vaporization at the boiling point with formation of bubbles in the interior of the liquid is called boiling, there is no such distinction for the solid-to-gas transition. While the definition of sublimation is simple, it is often confused as to what counts as a sublimation. The enthalpy of sublimation (also called heat of sublimation) can be calculated by adding the enthalpy of fusion and the enthalpy of vaporization. ![]() Since the process requires additional energy, sublimation is an endothermic change. Sublimation is caused by the absorption of heat which provides enough energy for some molecules to overcome the attractive forces of their neighbors and escape into the vapor phase. This is because the pressure of their triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid, is very high. water ice just below 0 ☌).įor some substances, such as carbon and arsenic, sublimation from solid state is much more achievable than evaporation from liquid state and it is difficult to obtain them as liquids. Thus, any solid can sublime if its vapour pressure is higher than the surrounding partial pressure of the same substance, and in some cases, sublimes at an appreciable rate (e.g. atmospheric) pressure of the entire system. The pressure referred to is the partial pressure of the substance, not the total (e.g. In these cases, the transition from the solid to the gas state requires an intermediate liquid state. At normal pressures, most chemical compounds and elements possess three different states at different temperatures. Īll solids sublime, though most sublime at extremely low rates that are hardly detectable. The reverse process of sublimation is deposition or desublimation, in which a substance passes directly from a gas to a solid phase, without passing through the liquid state. ![]() The product obtained by sublimation is also called the sublimate. The verb form of sublimation is sublime, or less preferably, sublimate. Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Not to be confused with subliminal stimuli.ĭark green crystals of nickelocene, sublimed and freshly deposited on a cold finger ![]()
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