Hygroscopic vs Deliquescent
Difference between hygroscopic and deliquescent is in the extent to which each material can absorb moisture. This is because both of these terms are very much related to each other, and they refer to the property of absorbing and the retention of moisture from the air. However, they differ in the extent of absorption of moisture where hygroscopic materials absorb moisture, but not to the extent the original substance dissolves in it, which is the case with deliquescence. Therefore, deliquescence can be regarded as an extreme condition of hygroscopic activity.
What does Hygroscopic mean?
When materials are said to be hygroscopic, they tend to have the ability of absorbing moisture or more precisely water vapour from the environment and retaining that water vapour within them. It can be via a mechanism of ‘adsorption’ or ‘absorption.’ When it is ‘adsorbed’, the water molecules remain on the surface of the substance whereas, when it is ‘absorbed’, the water molecules are taken up through the molecules of the substance. This absorption of water vapour can give rise to various physical differences within the substance. Generally, its volume grows larger. But, there are instances where the temperature, boiling point, viscosity, and the colour could change, as well. Hygroscopic activity is different from capillary action, which is also a process where water is being taken up, but in the case of capillary action there is no absorption taking place.
Due to the nature of the hygroscopic materials, care should be taken when storing them. They are usually stored in air tight (sealed) containers. However, this characteristic is highly used in industries where it is needed to maintain moisture content within the products such as food, pharmaceutics, cosmetics, etc. In these preparations, materials used for their hygroscopic nature are referred to as ‘humectants.’ Sugar, caramel, honey, ethanol, glycerol are some commonly known humectants including many types of salts; table salt. Polymers such as cellulose and nylon are also considered as hygroscopic. Even nature has some fascinating examples and a common case is with germinating seeds. These seeds after passing their dry period, start absorbing moisture due to the hygroscopic nature of the peel.
Honey is hygroscopic
What does Deliquescent mean?
This is an extreme case of the hygroscopic activity where the materials absorb water vapour (moisture) from the air until the point that they dissolve in the absorbed water turning into a solution. This is a common scenario with salts. Examples include; calcium chloride, magnesium chloride, zinc chloride, sodium hydroxide, etc. These material have a very strong affinity to water than the other hygroscopic material and, therefore, absorbs a relatively large quantity of water.
Substances that undergo deliquescence are referred to as ‘desiccants‘ and come in handy in chemical industries where the removal water is needed after a chemical reaction. Deliquescence usually occurs when the air is sufficiently humid. Therefore, in order for a solution to form at the end, it is necessary that the vapour pressure of the solution is less than the partial pressure of water vapour in the air.
Magnesium chloride is deliquescent
What is the difference between Hygroscopic and Deliquescent?
• Hygroscopic materials absorb moisture from the air but do not dissolve in it, whereas materials that undergo deliquescence dissolves in the water vapour that is absorbed from the air, forming a liquid solution.
• Hygroscopic materials are called ‘humectants’ and materials undergoing deliquescence are referred to as ‘desiccants.’
• Desiccants have a higher affinity to water than humectants and, therefore, tend to absorb relatively large amounts of water.
Images Courtesy: Honey and Magnesium Chloride via Wikicommons (Public Domain)
