deliquesce

1. 1. To melt away.
   2. To disappear as if by melting.
2. Chemistry. To dissolve and become liquid by absorbing moisture from the air.
3. Botany.
   1. To branch out into numerous subdivisions that lack a main axis, as the stem of an elm.
   2. To become fluid or soft on maturing, as certain fungi.

[Latin dēliquēscere : dē-, de- + liquēscere, to melt, inchoative of liquēre, to be liquid.]

The absorption of atmospheric water vapor by a crystalline solid until the crystal eventually dissolves into a saturated solution. This behavior is well known for certain salts such as hydrated calcium chloride, CaCl₂ · 6H₂O, and zinc chloride, ZnCl₂, but it is a property of all soluble salts in air of sufficiently high humidity.

Thermodynamically, the condition for deliquescence is that the partial pressure of the water vapor in the air exceeds the vapor pressure (aqueous tension) of the water in the saturated solution of the salt. The speed at which the process takes place depends upon the rate of diffusion of water vapor into the crystal lattice, crystal size, and other factors. The process will stop when the water vapor in the atmosphere is depleted to the point at which its partial pressure equals that of the saturated solution.

Crystalline solids also may absorb water by increasing their water of hydration if the dissociation pressure of the hydrated species to be formed is less than the partial pressure of the water vapor. It is this process, not deliquescence, which is the opposite of efflorescence.

Deliquescent substances can be used to remove water vapor from air, although they have no special advantage over substances which merely add water of hydration and remain crystalline. See also Desiccant; Efflorescence; Vapor pressure.

verb

To change from a solid to a liquid: dissolve, flux, fuse, liquefy, melt, run, thaw. See solid/liquid/consistency.

The condition of becoming moist or liquified as a result of absorption of water from the air.