Difference Between Deliquescent Efflorescent and Hygroscopic

Main Difference – Deliquescent vs Efflorescent vs Hygroscopic

Some substances can undergo physical changes when they are kept in an open place. This is due to absorption or adsorption of water vapor or release of water molecules from their structure. There is about 0-4% of water vapor in the air, depending on the location and the time of the day. Deliquescent substances are solids that can get dissolved by absorbing water vapor. But this absorption depends on the humidity of the environment. Efflorescent substances are crystals that can lose water molecules that are already present in their molecular structure. Hygroscopic substances are another type of solid matter that can either absorb or adsorb water vapor from the atmosphere. But these substances do not dissolve after the absorption. The main difference between deliquescent efflorescent and hygroscopic substances is that deliquescent substances form an aqueous solution by absorbing water vapor while efflorescent substances do not absorb water vapor and hygroscopic substances can absorb water vapor, but they do not form an aqueous solution.

Key Areas Covered

1. What is Deliquescent
      – Definition, Process, Examples
2. What is Efflorescent
      – Definition, Process, Examples
3. What is Hygroscopic
      – Definition, Process, Examples
4. What is the Difference Between Deliquescent Efflorescent and Hygroscopic
     – Comparison of Key Differences

Key Terms: Deliquescence, Deliquescent Substance, Efflorescence, Efflorescent Substances, Hygroscopic Substances, Hygroscopy, Water Vapor

What is Deliquescent

Deliquescent substances are solid matter that can get dissolved by absorbing water vapor. The resulting solution is an aqueous solution.  This process is known as deliquescence. These deliquescent substances have a high affinity to water.

The atmosphere has 0-4% of water vapor, depending on the location and the time of the day. 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.

What is Efflorescent

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. Examples include Na₂SO₄, 10H₂O, Na₂CO_3, 10H₂O, and FeSO₄.  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.

What is Hygroscopic

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. This causes the volume of the substance to increase. Hygroscopy can result in changes in the physical properties of the hygroscopic substances; such properties include color, boiling point, viscosity, etc.

Figure 3: Zinc Chloride Powder

Most examples of hygroscopic substances include salts. Some examples are Zinc chloride (ZnCl₂), sodium chloride (NaCl) and sodium hydroxide (NaOH). There are also some other common substances we know as hygroscopic. These compounds include honey, silica gel, germinating seeds, etc.

Difference Between Deliquescent Efflorescent and Hygroscopic

Definition

Deliquescent: Deliquescent substances are solids that absorb moisture from the atmosphere until they dissolve in the absorbed water and form solutions.

Efflorescent: Efflorescent substances are solids that can undergo spontaneous loss of water from hydrated salts.

Hygroscopic: Hygroscopic substances are solids that can absorb or adsorb water from its surroundings.

Water Vapor Absorption

Deliquescent: Deliquescent substances can absorb a high amount of water vapor.

Efflorescent: Efflorescent substances do not absorb water vapor.

Hygroscopic: Hygroscopic substances can either absorb or adsorb water vapor.

Other Names

Deliquescent: Deliquescent substances are called desiccants.

Efflorescent: Efflorescent substances are crystals.

Hygroscopic: Hygroscopic substances are called humectants.

Affinity for Water

Deliquescent: Deliquescent substances have a very high affinity for water.

Efflorescent: Efflorescent substances have no considerable affinity for water.

Hygroscopic: Hygroscopic substances have a less affinity for water.

Formation of a Solution

Deliquescent: Deliquescent substances form an aqueous solution by absorbing water vapor.

Efflorescent: Efflorescent substances do not form a solution.

Hygroscopic: Hygroscopic substances do not form a solution, but absorb water vapor.

Conclusion

Some compounds can absorb water vapor whereas some compounds can release water as water vapor. This ability depends on the molecular structure of the compound and environmental factors. According to this ability, substances can be divided into three different groups as deliquescent substances, efflorescent substances, and hygroscopic substances. Deliquescent substances form an aqueous solution by absorbing water vapor, and efflorescent substances do not absorb water vapor whereas hygroscopic substances can absorb water vapor but they do not form an aqueous solution. This is the basic difference between deliquescent efflorescent and hygroscopic.

References:

1. Helmenstine, Anne Marie. “Hygroscopic Versus Hydroscopic.” ThoughtCo, Available here.
2. “Efflorescence.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 12 Apr. 2007, Available here.
3. Helmenstine, Anne Marie. “Chemistry Glossary Definition of Deliquescence.” ThoughtCo, Available here.

Image Courtesy:

 1. “SodiumHydroxide” By Walkerma – Own work (Public Domain) via Commons Wikimedia
2. “Calcium sulfate efflorescence” By Eurico Zimbres (CC BY-SA 2.5) via Commons Wikimedia
3. “Zinc chloride” By User:Walkerma – Own work (Public Domain) via Commons Wikimedia