# What’s the difference between absorb and adsorb? **URL:** \ **Category:** Factual Questions\ **Created:** [April 8, 2003, 8:24pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387 "2003-04-08T20:24:19Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [April 8, 2003, 8:24pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/1 "2003-04-08T20:24:19Z")
According to [this](http://www.pioneerair.com/htdocs/glossary.htm) site, > [@](#): > > **Absorb** > To take in and hold, not just on the surface, but deep into the material, usually forming a mixture out of two or more substances. > > **Adsorb** > To take in and hold liquid and gas on the surface of a solid but porous material without causing a change in the basic structure of the material. So does that mean a sponge does not absorb water? That it instead _adsorbs_ water? Have I been using the term “absorb” wrong all my life? [This](http://www.tomgriffin.com/aquasource/filtersdopt3.shtml) site gives opposite definitions. Is it incorrect? ---
**Author:** ![Surreal](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@Surreal](https://boards.straightdope.com/u/Surreal)\ **Post date:** [April 8, 2003, 8:34pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/2 "2003-04-08T20:34:55Z")
I don’t agree with those definitions. Absorb is used to describe a situation where a substance is mechanically drawn into something else, i.e. capillary action. Adsorb is used to describe a situation where something is drawn into something else via some type of _chemical_ attraction. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [April 8, 2003, 8:43pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/3 "2003-04-08T20:43:57Z")
Thanks **Surreal** , but it looks like we have dueling definitions here… These sites say “a **d** sorb” is when a material is “deeply entrenched” or “molecularly bonded” to another, while “a **b** sorb” is when a material simply fills the voids of another: [http://www.tomgriffin.com/aquasource/filtersdopt3.shtml](http://www.tomgriffin.com/aquasource/filtersdopt3.shtml) [http://www.nanfa.org/archive/nanfa02/nanfajul02/0496.html](http://www.nanfa.org/archive/nanfa02/nanfajul02/0496.html) These sites say **just the opposite** , i.e. : “a **b** sorb” is when a material is “deeply entrenched” or “molecularly bonded” to another, while “a **d** sorb” is when a material simply fills the voids of another: [http://www.greaseinspectors.com/pictures\_info/gi\_roof/roof\_info.lasso](http://www.greaseinspectors.com/pictures_info/gi_roof/roof_info.lasso) [http://www.pioneerair.com/htdocs/glossary.htm](http://www.pioneerair.com/htdocs/glossary.htm) [http://www.agresearch.co.nz/scied/search/tools/Elisa/answers.htm](http://www.agresearch.co.nz/scied/search/tools/Elisa/answers.htm) The score is 3 to 2 with the above links. Your definition is all-together different. I’m really confused. So where do you go for accurate info nowadays?? ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 8, 2003, 9:01pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/4 "2003-04-08T21:01:39Z")
Surreal, that’s absolutely wrong. Crafter\_Man, bread absorbs gravy, sponges absorb water, and silica gel absorbs water. CO adsorbs onto the surface of platinum, mercury adsorbs onto the surface of gold, and just about everything adsorbs onto activated charcoal. Absorption is deep, but not necessarily strong, and adsorption is on the surface, but not necessarily weak (usually pretty strong, as weak interaction with a surface is usually called “touching” 😃 ) Either chemical interaction, mechanical trapping, or both might be involved in either process. The absorption of water into silica gel (or phosphorous pentoxide, or other desiccants) is a matter of chemical interaction (hydration), whereas a sponge relies on a more mechanical process, but both involve the deep penetration of water and intimate comingling with the sorbent molecules (sponges, unlike most sorbents, does not dissociate when saturated; instead, the polymer molecules “stretch”). Adsorption, on the other hand, is a purely surface phenomenon. This is apparently the only thing the sites cited 😃 understand about it. Adsorption can occur by simple trapping of material on the surface (I am reminded of water “wetting” almost any surface; there’s an electrostatic attraction between water and all hydrophilic materials, and if the material is porous, it will adsorb water), or by a more chemical sort of interaction, as when a crystal surface, already irregular, accepts atoms of a doping agent. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [April 8, 2003, 9:07pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/5 "2003-04-08T21:07:28Z")
I just found this in my ASHRAE Handbook in the “Sorbents and Desiccants” chapter: > [@](#): > > The process of attracting and holding moisture is described as either a_d_sorption or a_b_sorption, depending on whether the desiccant undergoes a chemical change as it takes on moisture. **Adsorption** does not change the desiccant, exception by the addition of the mass of water vapor; it is similar in some ways to a sponge soaking up water. **Absorption** , on the other hand, changes the desiccant. An example of an absorbent is table salt, which changes from a solid to a liquid and it absorbs moisture. The ASHRAE Handbook is rather authoritative. Should I use this definition for my paper? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [April 8, 2003, 9:10pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/6 "2003-04-08T21:10:53Z")
**Nametag** : Appreciate the response. But does your definition jive with ASHRAE’s definition as it applies to desiccants? If not, might ASHRAE be wrong? ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [April 8, 2003, 9:30pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/7 "2003-04-08T21:30:21Z")
I think **Nametag** ’s got it. Adsorption involves some form of chemical bonding to a surface. Absorption when one substance is drawn into another substance, and may or may not involve a chemical change. I think part of the confusion comes because adsorbent materials are usually porous - the better to increase their surface area. [So do sponges absorb or adsorb water?] ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [April 8, 2003, 9:35pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/8 "2003-04-08T21:35:54Z")
_Van Nostrand’s Scientific Encyclopedia_ gives the following definitions. I’ll give adsorption first, since that’s the easier one. > [@](#): > > Adsorption is a type of adhesion which takes place at the surface of a solid or a liqid in contact with another medium, resulting in an accumulation or increased concentration of molecules from that medium in the immediate vicinity of the surface. Absorption has several definitions, depending upon whether it is energy, physiology, or a process. > [@](#): > > Absorption (Energy). The process whereby the total number of particles emerging from a body of matter is reduced relative to the number entering as a result of interaction of the particles with the body. Also, the process whereby the kinetic energy of a particle is reduced while traversing a body of matter… > Absorption (Physiology). The process by which materials enter the living substance of which the organism is composed. Materials including food and oxygen are taken into special organs by ingestion and respiration, but they must pass through the cell wall to become an integral part of the organism by absorption. The basic physical forces involved are those of osmosis and diffusion. > Absorption (Process). Absorption is commonlyused in the process industries for separating materials, notably a specific gas from a mixture of gases; and in the production of sol;utions such as hydrochloric and sulfuric acids. … The absorption medium is a liquid in which (1) the gas to be removed, i.e., absorbed is soluble in the liquid, or (2) a chemical reaction takes place between the gas and the absorbing liquid. ---
**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\ **Post date:** [April 8, 2003, 9:36pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/9 "2003-04-08T21:36:51Z")
Absorption is used very loosely by all scientific disciplines. Adsorption is not used that loosely but still used somewhat loosely. I can see ASHRAE’s definition to be not entirely correct. But again it works for a moisture system - from a Chemical Engineer’s standpoint it’s wrong. Here are somethings to note : **Absorption** 1. Usually refers to a physical phenomenon where liquids or gases fill up pores due to surface tension. It also refers to soluble gases dissolving in liquids. Having said that, sometimes low entropy change reactions may occur during absorption - like absorbing CO2 in a NaOH solution. 2. Refers to gases absorbed in liquid. And liquids absorbed in solids. 3. The entropy change is usually not much. 4. When designing a absorption system - you are concerned with the solubility of the gas/liquid being absorbed and hence the volume of the liquid needed. **Adsorption** 5. Usually is a surface activity - surface means not only what you see but both macro and microscopic pores. It is almost a chemical bond. 6. Usually refers to gases/vapors absorbed on a solid surface. 7. Adsorption is very selective - only particular gas/vapor molecules will be absorbed in a mixture. 8. Usually reversible - and reversed by changing pressure/temperature. 9. When designing a Adsorbing system, you are concerned about how much surface area is available for adsorption. I can point out many other differences 😃 if you want. Oh another thing a adsorbing system is much more expensive than a absorbing system. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [April 8, 2003, 10:20pm UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/10 "2003-04-08T22:20:58Z")
ADsorb means something adheres to the surface of something else. ABsorb means something is drawn into the interior volume of something else. ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [April 9, 2003, 1:46am UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/11 "2003-04-09T01:46:54Z")
Not getting into how a discipline may have adapted (correctly or incorrectly) these words in a specialized way, in general, we can say: The prefix ad- means **to** or **toward**. And so adsorb would be like adhesion. Material X sticks **to** Material Y. Water vapor adsorbs **to** cold surfaces. Cold surfaces adsorb water vapor. The prefix ab- means **away** or **away from**. And so absorb would be like suction. Material Y absorbs Material X **from** Medium Z. Sponges absorb water **from** counter tops. Water is absorbed **from** counter tops by sponges. Hope that helps. ## Peace Now, for adductors and abductors… ---
**Author:** ![Apollyon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/apollyon/32/4238_2.png) [@Apollyon](https://boards.straightdope.com/u/Apollyon)\ **Post date:** [April 9, 2003, 4:39am UTC](https://boards.straightdope.com/t/what-s-the-difference-between-absorb-and-adsorb/167387/12 "2003-04-09T04:39:57Z")
[Merriam-Websters](http://www.m-w.com/) concurs: **Absorb** , from Latin _ab- + sorbEre, to suck up_: to suck up or take up \ \ \ **Adsorption** (noun form of adsorb): adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids) to the surfaces of solid bodies or liquids with which they are in contact.