Hygroscopy

Parade to determine the higroscopicity of a fertilizer. Nitrogen Fixing Research Laboratory, ca. 1930.

The hygroscopy (from the Greek ύγρος hygros 'damp, wet' and σκοπειν skopein 'observe, look'), also hygroscopy or hygroscopicity, is the ability of some substances or materials to absorb moisture from the surrounding environment. It is also synonymous with hygrometry, which is the study of humidity, its causes and its variations (in particular, atmospheric humidity).

For each substance there is an equilibrium humidity, that is, such a humidity content of the atmosphere at which the material neither captures nor releases humidity to the environment. If the latter is less than this equilibrium value, the material will dry out, if the ambient humidity is higher, it will become humid. Thus, certain minerals such as calcium chloride are capable of capturing water from the atmosphere in almost any condition, because their equilibrium humidity is very low. Substances like these are used as desiccants. Other examples are sulfuric acid, silica gel, etc.

Hygroscopic compounds

Hygroscopic are all compounds that attract water in the form of vapor or liquid from their environment, and for this reason they are often used as desiccants.

Some of the hygroscopic compounds react chemically with water, such as hydrides or alkali metals. Others trap it as water of hydration in their crystalline structure, as is the case with sodium sulfate. Water can also be physically adsorbed. In these last two cases, the retention is reversible and the water can be desorbed. In the first case, having reacted, it cannot be recovered in a simple way. These processes are exothermic.

Some examples of the best-known hygroscopic compounds are:

• sulfuric acid (H₂SO₄)
• calcium chloride (CaCl₂)
• magnesium chloride (MgCl₂)
• sodium chloride (Halita)(NaCl)
• silica gel
• sodium hydroxide (NaOH)
• hydroxylamine (NH₂OH)
• Honey
• copper sulfate (II) (CuSO₄)
• Phosphorus oxide (V) (P)₄O₁₀)
• calcium oxide (living calcium) (CaO)

For each substance, there is a humidity that is called equilibrium humidity, that is, it absorbs humidity from the environment. If the ambient humidity is less than this equilibrium humidity value, then the material will dry out; on the other hand, if the environmental humidity is higher, the material will become humid. Thus, certain minerals, such as calcium chloride, are capable of capturing water from the atmosphere in almost any condition, because their equilibrium humidity is very low. Substances like these are used as desiccants. Other examples are sulfuric acid and silica gel.

Hygroscopic nuclei and the generation of artificial rain

Hygroscopic nuclei are the particles of salts or droplets of saline solutions, mainly from the seas and oceans, on which the humidity of the air in the atmosphere condenses. Once in the atmosphere, they begin by coming into contact with moisture droplets and dissolve in them. As the vapor pressure of the solution is lower than that of pure water, the vapor condenses in it, until it forms larger drops that end up precipitating as rain. One of the methods to artificially cause rain consists precisely in disseminating hygroscopic nuclei within the clouds.

Automobile brake fluid is also a hygroscopic fluid. Care should be taken not to touch it and avoid spillage. In the vehicle it has a stripping effect on the body paint. It has a high boiling point, so it can withstand high temperatures (300 °C). It is usually formed by alcohols or glycerin.

Deliquescence

Deliquescent materials (from the Latin deliquescere, to become liquid) are substances (mostly salts) that have a strong chemical affinity for moisture and that absorb relatively large amounts of water if exposed to the atmosphere, forming a liquid solution. Examples of deliquescent substances are: calcium chloride, ferric chloride, magnesium chloride, zinc chloride, potassium carbonate, potassium hydroxide and sodium hydroxide.

Due to their strong affinity for water, these substances are often used as desiccants.