HYGROSCOPICITY: CONCEPT, HYGROSCOPIC SUBSTANCES, EXAMPLES - PHYSICAL - 2025

The hygroscopicity is the property possessed by certain substances absorb or expel water molecules to or from the surrounding environment. A hygroscopic substance has the ability to absorb (or expel) water vapor from the air around it, until a certain equilibrium humidity is reached or until the substance becomes saturated.

Each substance has a characteristic equilibrium humidity. When the environment has this degree of humidity, the speed with which the substance captures environmental molecules of water equals the number of molecules released.

Figure 1. Silica gel is considered a hygroscopic substance. Source: Pixabay.

For its part, air humidity is due to atmospheric water vapor from evaporation from various sources such as the sea, rivers, water in the ground, respiration and transpiration of plants and animals, among other sources.

The amount of vapor molecules retained in the air depends on the air temperature. The higher the temperature, the greater the retention of water molecules in the air. But if the ambient humidity is above the equilibrium humidity of a certain material, then the material will pick up water molecules from the environment.

Materials and substances that have a low equilibrium humidity are used as room driers, this is the case of the silica gel shown in figure 1 and of calcium chloride.

Hygroscopic substances

The substances that capture the water vapor from the air or the liquid that surrounds them are hygroscopic substances and are used as environmental desiccants, as we have said.

As for the mechanism of action of hygroscopic substances, there are basically two types:

-The water molecules are trapped in the crystalline structure of the substance, without reacting chemically with it.

-A chemical reaction occurs between the water molecules and the substance in question.

In the first case there is sodium sulfate, while in the second case there are alkali metals and hydrides, which react strongly with water.

Other hygroscopic substances or materials are:

-Paper

-Cotton fibers

-Wood

-Sugar

-Honey

-Some alcohols such as ethanol and methanol

-Sales such as sodium hydroxide, calcium chloride, sodium chloride.

Some of the hygroscopic substances tend to dissolve in the water they absorb, such as salts, sugar or honey. A substance that dissolves in the water taken up by itself is called a deliquescent substance.

Coefficient of hygroscopic expansion

Materials or substances that trap moisture can expand, in this case causing stresses or strains on the surrounding materials. Such is the case of old batteries that capture ambient humidity, expanding and bursting their packaging.

Another similar case occurs with the laminated covers of books, which consist of a cardboard face that absorbs moisture, while the plastic film does not. In a very humid environment the cardboard absorbs water and expands, resulting in the lid sagging outwards.

The characteristic described in the previous paragraph, of the differential dilation by hydration of two different materials, has been used for the construction of instruments to measure environmental humidity, such as the hygrometer.

The hygrometer

The hygrometer is the instrument used to measure ambient humidity. Instruments designed for this purpose make an indirect measurement of ambient humidity.

For example, it may be a mechanical variation due to the absorption of moisture by the substance that serves as a sensor.

Figure 2. Bilayer spiral coil hygrometer (metal - paper impregnated with salt). Source: Wikimedia Commons.

The electrical capacity of two metal plates separated by air may change slightly, due to variations in humidity in the environment.

The electrical resistivity of certain materials can also be sensitive to changes in ambient humidity. It is these properties that are used as humidity sensors.

There is a particular type of hygrometer called a psychrometer that calculates humidity based on the temperature difference of two thermometers: one with the dry bulb and the other with the wet bulb.

RH

Hygrometers generally measure the relative humidity of the air. This is defined as the quotient between the humidity of the air divided by the humidity of the saturated air and multiplied by 100. Therefore, the relative humidity is conveniently expressed as a percentage.

A formula that allows the determination of relative humidity is the following:

In this expression Pv is the vapor pressure and Pvs the saturation vapor pressure.

To ensure that the air is saturated with water vapor, the dew point is searched. This consists of reducing the temperature to such a point that the air does not admit more water vapor and begins to condense on cold objects such as metals and glass.

The humidity measurement at the dew point is 100% on the relative humidity scale.

Specific humidity

Another very useful quantity is the specific humidity. It is defined as the number of grams of water vapor per 1000 grams of humid air and the following relationship is used to determine it:

- Example 2 (Home experiment)

The following experiment is a practical and visual demonstration of the hygroscopicity of wood, that is, of its ability to absorb water and produce expansion and forces.

Take five wooden toothpicks and split them in half, but without completely separating. They are then arranged in a circle like the one shown in the following figure:

Figure 3. Initial arrangement of the dry and broken toothpicks in their center. Source: raulexperimentos.blogspot.com

A drop of water is placed in the center. As soon as the drop of water falls in the center of the arrangement of wooden sticks, it tends to expand due to the adsorption (not to be confused with absorption) of the water through the fibers of the wood.

In a short time the wood seems to come to life due to its gradual expansion and after approximately 1 minute, the result is as seen in the following figure.

Figure 4. Final disposition of toothpicks due to moisture adsorption. Source: raulexperimentos.blogspot.com

References

1. Biostudy. 2 keys of natural materials. Hygroscopicity and vapor diffusion. Recovered from: mirencaballerobioestudio.com
2. raulexperiments. Wood, anisotropy and hygroscopy: a star is born. Recovered from: raulexperimentos.blogspot.com
3. TIS. Hygroscopicity / surprising behavior. Recovered from: tis-gdv.de
4. Wikipedia. Hygrometer. Recovered from: es.wikipedia.com
5. Wikipedia. Hygroscopicity. Recovered from: es.wikipedia.com
6. Wikipedia. Absolute humidity. Recovered from: es.wikipedia.com
7. Wikipedia. Air humidity. Recovered from: es.wikipedia.com
8. Wikipedia. RH. Recovered from: es.wikipedia.com
9. Wikipedia. Hygroscopy. Recovered from: en.wikipedia.com