Deliquescence refers to that certain substances (mostly solids)atmosphereinabsorbperhapsadsorbentWater makes the surface become moist and smooth gradually, and finally the material willsolidBecome the substancesolutionPhenomenon.
Deliquescence refers to that certain substances (mostly solids)atmosphereinabsorbperhapsadsorbentWater makes the surface become moist and smooth gradually, and finally the substance will change from solid tosolutionPhenomenon.
Hydroscopy refers more broadly to the ability of a substance to absorb water.The common ones arecotton、paperetc.celluloseSubstances, andsugar、caramel、Honey、glycerol, ethanolmethanol、sulphuric acidSuch highly water-soluble substances, or substances that react with air components to produce water-soluble substances, such asphosphorus。Substances liable to deliquescence must be hygroscopic, while hygroscopic substances may not be deliquescent.
Physical interpretation
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Sodium hydroxide deliquescence
anhydrouscalcium chloride、Magnesium chlorideAnd solid sodium hydroxide are easily deliquescent in air.Some amorphous bodies form on the surface after deliquescenceSaturated solutionIt also turns into hydrate.If anhydrous calcium chloride becomes CaCl after deliquescencetwo·6HtwoO；Some form saturated solutions only on the surface, such as sodium hydroxide solids.Because the vapor pressure of the saturated solution of the compound is lower than the partial pressure of water vapor in the air at the same temperature, the material continuously absorbs water and deliquests.The water vapor pressure of the solution is related to the concentration of the solution (and of courseelectrolyteOfionizationOnly when the concentration of saturated solution is large enough can its water vapor pressure be small enough (less than the water vapor pressure in the air). Therefore, the substances that can deliquescence are those with very high solubility.Puresodium chlorideThe crystal is not deliquescent.At the same time, the occurrence of deliquescence is also related to the relative humidity of the air.Therefore, beakers should be used instead of paper when weighing NaOH.
Deliquescent substance
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Substances liable to deliquescence usually have high solubility in water, such as most nitrates（Ferric nitrate、Mercuric nitrateEtc.), most perchlorates（Copper perchlorate、Zinc perchlorateEtc.), some halides（Zinc chloride、Magnesium chlorideSome sodium salts, cesium salts (such asSodium permanganate、Cesium carbonateAnd so on).Some substances will accelerate deliquescence if they can react with components in the air to produce water, such assodium hydroxideAnd in aircarbon dioxideWhen reacting, water will be produced.[1]It should be noted that although some compounds have high solubility in water, they are not easy or will not deliquescence, such asLead nitrate。
Substances liable to deliquescence are often used as desiccants to absorb moisture from liquids or gases.NaOH is only used as a desiccant for neutral or alkaline gases.Substances liable to deliquescence must be stored under closed conditions;Drugs that are prone to deliquescence (especially APIs) should be stored under moisture-proof conditions to prevent mildew, rot and deterioration.
Sodium hydroxide is a white solid. After being exposed to the air (especially humid air), there is a layer of water droplets on the surface, and then the solid gradually dissolves[6]。[2]
Deliquescence is a common food problem, which will affect the quality of food and even cause deterioration, corruption and foodShelf lifeshorten.Brown sugar, white sugar, salt, milk, yogurt, milk powder, soybean milk powder, coffee, tea powderChicken essence, biscuits, melon seeds, nuts and other foods may affect product quality due to deliquescence.
HygroscopicOr deliquescence is due to theWater absorptionIt is strong and soluble in water. When it is in a high humidity environment, this kind of food will absorb waterDeliquescence phenomenon。[3]
Cobalt chlorideIt has hygroscopicity. Anhydrous matter absorbs moisture to obtain different hydrates, and the color changes.Use this property to makeDiscolored silica gel。
Hazard and prevention
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Chemically, deliquescence often deteriorates reagents. One is simple deliquescence, such asManganese nitrateThe manganese nitrate solution is obtained after the hydrate deliquescence. The second is the accompanying reaction during deliquescence, such asFerrous perchlorateDuring deliquescence, it will quickly react with oxygen to produce the basic formula of iron (III)Perchlorate。In order to prevent deliquescence, these reagents need to be sealed, dried and stored. If necessary, they need to be put into a dryer toDiscolored silica gelAs an indicator of water.
