Hygroscopy

Hygroscopicity ( altgr. ὑγρός hygrostat, damp ',' wet ' and σκόπειν skopein, watch '; hygroscopicity is a synonym and describes the same property ) in chemistry and physics, the property of materials, moisture from the environment ( usually in the form of water vapor to bind from the humidity). Many receiving materials - as far as it concerns solids - melt or clump together by water absorption.

Hygroscopicity is often referred to incorrectly as Hydro copy ( altgr. ὕδωρ hydor 'water' ).

The moisture absorption capacity of the textile fiber material from the vapor phase is known as the hygroscopicity. This is measured to the mark, the moisture absorption is a percentage in relation to the ambient air.

Unwanted effects

The characteristic is often undesirable in practice, for example when sodium chloride due to clumped recorded humidity. To prevent this, magnesium silicate is added as a food additive or release agent. In other areas, this effect is of importance. The moisture absorption of salt is determined by the relative humidity. Above a relative humidity of about 75 % rHd ( Deliqueszenzfeuchte ) takes saline on so much water from the air that it becomes liquid. The range between 35 % and 75 % rHd rHd is characterized by a single atomic occupancy of water molecules.

In sugar confectionery (sweets and lollies ) changed by hygroscopic surface: transparent products are matt, stick smooth surfaces, shiny candies (eg, gold nuts and silk cushions ) lose the gloss ( technical term " withering away" ) and chocolate becomes covered with sugar frost. Inferior goods with high water content bleeds ( technical term " cold flow ").

Even in modeling the hygroscopic effect is undesirable because the fuel nitro methane used for Nitro engines contains, which is hygroscopic and is so often tainted by improper storage.

In the building industry is at hygroscopic material weathered components of a disadvantage, since water is converted in the building material in freezing to ice. The ice crystals have a larger volume than the water and can lead to flaking or destruction of internal structure of the material. Therefore, frost-resistant for sensitive components, that is not required hygroscopic materials.

The brake fluid is hygroscopic vehicles. With increasing water content of the brake fluid boiling point decreases. Due to the heat generated during braking, the water can form vapor bubbles; since these are compressible as a gas, the braking driver steps " into the void ". For this reason, a two-year change of brake fluid is recommended.

Desirable effects

In food

Hygroscopic moisturizers are used as plasticizers in foods by preventing the hardening ( for example sorbitol in marzipan ).

Hygroscopicity in building materials

The hygroscopicity of building materials, especially of interior finishes ( eg plaster, flooring and wood), can significantly influence the overall humidity of a room and therefore on the indoor climate. The building materials that absorb a lot of moisture from the air and can stop quickly as needed ( hygroscopic building materials: mostly all plant and animal materials such as wood or wool and porous mineral materials such as brick, limestone and clay ) are compared with non-absorbent materials ( z. , plastics, metals) for a pleasant room climate especially recommended. See also "Breathing Wall ", vapor barrier, and water tightness, water vapor diffusion resistance and water absorption coefficient of building material.

The use of hygroscopic substances as a desiccant in the laboratory

If you want to dry small amounts of moist substances at ordinary temperature in the chemical laboratory, one uses a closed desiccator evacuated if necessary, one has provided with a desiccant, or charged with desiccant tube to dry gas flow. The desiccant is hygroscopic, and continuously withdraws the gaseous phase moisture. Thus, the substance to be dried or the gas will gradually gently at ambient temperature (without heating, could lead to a possible decomposition) dried. Serve as a drying agent:

• Silica gel
• Concentrated sulfuric acid
• Granular calcium chloride
• Sodium hydroxide pellets
• Potassium hydroxide pellets
• Magnesium perchlorate
• Di - phosphorus pentoxide ( P4O10 )