Hygroscopicity ( ancient Greek ὑγρός hygrós , wet 'wet' and σκοπεῖν skopein , watch '; hygroscopicity is synonymous and describe the same property) referred to in the chemistry and physics , the property of materials, moisture from the environment (usually in the form of water vapor from the humidity ). Many absorbing substances - insofar as they are solid substances - dissolve or clump together when they absorb water. An exception here are e.g. B. the zeolites .
Hygroscopy is often incorrectly referred to as hydroscopy ( ὕδωρ hydor 'water').
The moisture absorption capacity of textile fibers from the vapor phase is called hygroscopicity. The moisture absorption in percent depending on the ambient climate is used as a measurement variable for their identification.
This property is often undesirable in practice, for example when table salt clumps due to absorbed humidity. To prevent this, magnesium silicate is added as a food additive or release agent. This effect is also important in other areas. The moisture absorption of table salt is determined by the relative humidity. Above a relative humidity of about 75% rHd (deliquescent humidity), common salt absorbs so much water from the air that it goes into solution. The range between 35% rHd and 75% rHd is characterized by a single atomic occupation with water molecules.
In sugar confectionery (sweets and lollipops), the surface changes due to hygroscopicity: transparent products are matt, stick to smooth surfaces, shiny candy (eg gold nuts and silk cushions.) Lose their gloss (technical term "death") and chocolate becomes covered with sugar frost . Inferior goods with a high water content dissolve (technical term "cold flow"). Honey is hygroscopic and goes into fermentation if the water content is too high .
In the building industry, hygroscopic material is a disadvantage for weathered components, since water in the building material is converted to ice when there is frost. The ice crystals have a larger volume than the water and can lead to flaking or the destruction of internal structures of the material. Therefore, in the case of sensitive components, frost-resistant, i.e. H. non-hygroscopic materials required.
The brake fluid in vehicles is also hygroscopic. As the water content increases, the boiling point of the brake fluid drops. The heat generated during braking can cause the water to form vapor bubbles; since these can be compressed as gas, the braking driver steps “into space”. For this reason, it is recommended that the brake fluid be changed every two years.
Hygroscopy in building materials
The hygroscopicity of building materials , especially that of interior construction (such as plaster, floor coverings and wood), can have a decisive influence on the humidity of a room and thus on the room climate as a whole. The building materials, which absorb a lot of moisture from the air and can quickly release it again if necessary (hygroscopic building materials: largely all plant and animal building materials such as wood, sheep's wool or straw and porous mineral materials such as bricks , lime and clay ), are non-absorbent building materials (e.g. plastics , metals ) are particularly recommended for a pleasant room climate. See also " Breathing wall ", vapor barrier , as well as tightness against driving rain , water vapor diffusion resistance and water absorption coefficient of a building material .
Use of hygroscopic substances as drying agents in the laboratory
If small amounts of moist substances are to be dried at room temperature in the chemical laboratory, a closed, possibly evacuated desiccator , which has been provided with a desiccant , or a pipe filled with desiccant , is used to dry a gas stream. The desiccant is hygroscopic and continuously removes moisture from the gas phase. The substance or gas to be dried is gradually dried gently at ambient temperature - that is, without heating, which could lead to possible decomposition. The following are used as drying agents:
- Silica gel
- concentrated sulfuric acid
- granular calcium chloride
- Sodium hydroxide pellets
- Potassium hydroxide pellets
- Magnesium perchlorate
- di - phosphorus pentoxide (P 4 O 10 )
Use of hygroscopic substances to dry rooms
In the trade, sieve-shaped containers with an associated water container placed underneath are offered for drying rooms. They are filled with calcium chloride, which dissolves and drips off when the crystal water is absorbed. Their use only makes sense in uninhabited rooms. In inhabited rooms, the air you breathe when you breathe out and perspiration generate so much moisture that this method fails.
- Ralf-Dieter Reumann (Ed.): Test methods in textile and clothing technology . Springer, Berlin / Heidelberg / New York 2000, ISBN 3-540-66147-6 , pp. 671 .
- Walter Wittenberger: Chemical laboratory technology . 7th edition. Springer, Vienna / New York 1973, ISBN 3-211-81116-8 , pp. 135 .