Efflorescent substances include most hydrated salts. A common example of efflorescence is drying of cement. However, when these water molecules are lost from the hydrated salt, the salt shows a powdery surface due to the loss of water. Eventually, the salt crystals will remain in the container. The phase of water is changed to the gaseous phase. Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature.
This process occurs when the aqueous vapour pressure of the hydrate is greater compared to the partial pressure of the water vapour in the air. Certain substance absorbs moisture from the atmosphere at ordinary temperature but does not dissolve in it.
These are called hygroscopic substances and the property is known as hygroscopy. These substances are generally used as a drying agent. When water vapour is absorbed, the water molecules are taken by the molecules of the hygroscopic substance, which results in physical changes like an increase in volume, boiling point, temperature, and changed viscosity.
Eg: Zinc chloride, sodium chloride , sodium hydroxide crystals, silica gel, honey, nylon, and ethanol are hygroscopic in nature. Germinating seeds are also examples of hygroscopic. Once the seeds are dried, their outer coating becomes hygroscopic and it begins to absorb the moisture required for germination. Efflorescent Substances. Deliquescent Substances. Since there are many other gases and vapors in the atmosphere, water vapor has a partial pressure. Deliquescence happens when the vapor pressure of the solution that is going to form is less than the partial pressure of water vapor in the air.
Humid environments are highly concentrated with water vapor. Therefore, deliquescent substances can easily undergo deliquescence and form solutions by absorbing a high amount of water vapor when they are placed in a humid environment. Figure 1: NaOH pellets can absorb water vapor from the air. Most common examples of deliquescent substances include some salts; for example, sodium hydroxide, potassium hydroxide, ammonium chloride, sodium nitrate, calcium chloride, etc.
These substances can be used as desiccants. When the water vapor inside a container has to be removed in order to stop a particular chemical reaction, these substances can be kept inside the container.
Then the deliquescent substances will absorb a high amount of water and prevent the interferences coming from water vapor. Efflorescent substances are solids that can undergo spontaneous loss of water from hydrated salts. Hydrated salts are inorganic salts containing water molecules combined in a definite ratio. These salts can lose these water molecules when kept outside.
This process is known as efflorescence. Efflorescence occurs when the aqueous vapor pressure of the hydrate is greater than the partial pressure of the water vapor in the air.
Efflorescent substances include most hydrated salts. A common example of efflorescence is drying of cement. Figure 2: Calcium Sulfate Efflorescence. However, when these water molecules are lost from the hydrated salt, the salt shows a powdery surface due to the loss of water.
Eventually, the salt crystals will remain in the container. The phase of water is changed to the gaseous phase. Hygroscopic substances are solids that can absorb or adsorb water from its surroundings. When water vapor is absorbed by hygroscopic substances, the water molecules are taken into the spaces of the crystal structure.
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