Hygroscopic: What it Means, What You Need to Know

Hygroscopic

Hygroscopic is a compound word that starts with ‘hygro’- “relating to moisture or water”. In simple words, hygroscopic materials “pick up water”.

Products that are hygroscopic are often dry powders, but may also be liquids. To find out if a material you are working with is hygroscopic, consult available data for the product. This data may be found on Product or Safety Data Sheets, online, or in chemical reference books.

 

In this article:

  • What makes something hygroscopic?
  • Is hygroscopic a problem?
  • All water-hungry molecules are NOT created equally.
  • How to handle hygroscopic chemicals.
  • Examples of materials.

 

What makes something Hygroscopic?

 

Whether or not a material is hygroscopic has to do with the elements in the chemical or product itself and the way they’re organized. Both the atoms themselves, and the way they’re arranged into molecules, create the tendency to strip water out of the atmosphere. Hygroscopic molecules are “more comfortable” when hydrated, or have a certain amount of water molecules associated with them. This creates an affinity for, or attraction to, water.

The air around us always carries a certain amount of water naturally, even when it is not raining. This is called humidity. Warm air is able to hold more water than cold air. This is why humidity levels are higher in the summer than in the winter. The higher the humidity, the more water available for molecules to pick up.

Water-hungry molecules pull water out of the air surrounding them – and presto – the phenomenon of hygroscopy!

 

Is Hygroscopic a Problem?

 

When a dry product or chemical is exposed to the air and absorbs water, is this a problem? Ultimately it depends what the product is being used for, and whether or not it messes it up for your application. It certainly can be a problem- and often is. The water can cause both physical and chemical changes to the substance.

Physical: Moisture may lead to clumping, bricked-up product, gummy product, a pasty/putty mess, agglomeration, or the like. This can make it hard to work with (i.e. it is very hard to measure and scoop bricked-up Caustic Soda Beads…!).

Chemical: When certain chemicals come into contact with water, they may react with the water and cause a chemical change. Chemical by-products may be created unintentionally. There may be some hazard associated with this as well.

 

All Water-Hungry Molecules are Not Created Equally

 

If you hear that a certain chemical or product you are working with is hygroscopic, it is important to know that this water-absorbing characteristic varies depending on the product. Not all molecules are created equally! Some chemicals will be mildly hygroscopic, and some aggressively so.

Different words may be used to describe the phenomenon. Depending on which word is used, these may help you know how hungry for water the material you’re working with really is:

Hygroscopic (or Hydroscopic): These two words may be confused and used interchangeably. ‘Hygro’ and ‘Hydro’ both mean ‘water’. However, a hydroscope is, by definition, an instrument used for viewing objects below the surface of water – this is technically an incorrect usage of the word. Materials described with these words may choose to pick up just enough water to where the molecules are surrounded in a better way by the electrons or shared electrons of other molecules; and they’re comfortable, or satisfied.

Deliquescent vs Hygroscopic: Deliquescent is a word that is used to describe a material that will keep absorbing water until the (dry) product is literally liquid.

Is it possible to reduce the hygroscopic nature of a substance? No, this is not possible.

 

How to Handle Hygroscopic Chemicals

 

Chemicals that absorb water from the atmosphere must be stored and handled in a way that always minimizes exposure to air. Proper storage will help to avoid unwanted chemical or physical changes.

When in storage, make sure bags and/or containers are properly closed. While using or handling, minimize exposure time to air by closing containers as quickly as possible after use, especially in highly humid environments.

It is also important to pick your supplier of hygroscopic chemicals carefully. Careless handling of bags from a supplier, i.e. placing bags on pallets with nails sticking up, can cause large quantities of chemicals to be essentially ruined for your application upon purchase.

Can I remove moisture from my chemical or product after it has already been absorbed into it? Yes, it is technically possible to remove moisture from product. However, the process involves subjecting the chemical to a lot of heat to dry the product (and to do so safely). Most people would not be able to do this, and then keep the moisture from accumulating back onto the product after it cools and they get time to package it. Since this is not something most consumers would ever be able to do, we don’t recommend it.

 

Examples of Materials