Hygroscopicity (from other Greek. Ὑγρός "wet" + σκοπέω "I observe") - the ability of some substances to absorb water vapor from the air . Examples of hygroscopic substances are: honey , ethanol , methanol , glycerin , concentrated sulfuric acid , concentrated sodium hydroxide solution , anhydrous calcium chloride . Due to the presence of water vapor in the atmosphere , hygroscopic materials must be stored in sealed containers. For storage of hygroscopic substances in the laboratory, you can use a desiccator .
The hygroscopicity of silica gel is used to lower the humidity of the nearby objects: electronics, clothing, shoes.
Different materials and compounds have different hygroscopic properties, which can lead to harmful effects, such as stress concentration in composite materials. The influence of ambient humidity on materials or compounds can be taken into account by the coefficient of hygroscopic expansion (GSR) or the coefficient of hygroscopic compression (CHS) - the difference between them is determined by the ability of substances to change the volume under the action of humidity and is taken into account in the formulas in the form of a sign.
A common example on which to demonstrate this phenomenon is paperback books. In a relatively damp place, the cover of the book will curl. This is due to the fact that the non-laminated side of the cover absorbs more moisture than the laminated side, and its area increases. This causes tension that bends the cover towards the lamination. The analogy can be seen in bimetallic plates .
Content
Gunpowder and explosives
Gunpowder
Black powder has a slight hygroscopicity, since its main component is potassium nitrate . Sodium nitrate is energetically and economically profitable, but due to its high ability to absorb moisture when air humidity is more than 70% (it dries with less moisture), the production of gunpowder is limited, and potassium nitrate with low hygroscopicity has found the most application.
Nitrocellulose , unlike nitrate, is not hygroscopic. The appearance of smokeless powder on its basis has accelerated the development of semi-automatic and automatic firearms , since it does not clog the mechanisms and does not change the physical properties when exposed to humidity. The hygroscopicity of some components of the cartridges , primarily the igniting compositions of blasting caps , is compensated by their high sensitivity to ignition.
Explosives
The hygroscopicity of explosives and explosive compositions largely determines the time and conditions of their storage. Moisture has a particularly significant effect on industrial nitrate-containing explosives, which can either lose the necessary physical and explosive characteristics, or, conversely, acquire increased sensitivity to external influences. There are water-containing explosives, the characteristics of which depend on the hygroscopicity and the effect of moisture to a small extent.
Construction
Hygroscopic materials play an important role in construction ; for example, wood is very hygroscopic. Such materials are affected by moisture contained in the building. The higher the relative humidity, the more vapor is adsorbed. However, many types of wood begin to rot , if the relative humidity for a long time more than 80%.
Most lightweight porous wall stones (lightweight ceramic stones [1] , aerated concrete and foam concrete , claydite concrete , limestone ) are very hygroscopic - the figure can reach 30%, and some limestones from Cyprus gain moisture to the state of "wet" wall.
In addition, on the raw base they work like a kerosene lamp wick, due to the capillary effect of its porous structure. All lightweight wall stones [2] , require an airtight waterproofing cut-off - from all abutments to walls and monoliths with high humidity - the wall cut-off should be only of film type, flexible, with full waterproofness. Usually, the ground floor and the first floor are cut off from all “wet” structures - the basement , the basement, the underground part of the ground floor .
The cutoff of high-quality cement mortar generally accepted in the USSR does not work - initially it does not completely limit the moisture in the dry wall - over time, the freezing and thawing cycles open and expand the capillaries in the solution. A constant inflow of water into the thickness of the building wall begins, new portions of moisture finally wash out and open the capillaries.
A light brick is less susceptible to capillary effect , but in the absence of a cut-off it can be soaked up to a height of several floors, right up to the roof itself.
Biology
The seeds of some herbs expand when the humidity changes, allowing them to scatter on the ground.
Notes
- ↑ POROTHERM and KERAKAM, are made of foamed clay.
- ↑ Despite the statements of manufacturers about the stability of products to moisture, the capillaries are always there, and due to the winter frosts and the many defrost cycles, their number increases dramatically.