Scientists Say Chemistry

Scientists Say: Absorb and Adsorb

Many people confuse these similar-looking words, but one simple difference sets them apart

Researcher at the University of Granada, Raul Rojano (L) and Juan Manuel Martin, professor at the University of Granada, show an activated charcoal infiltrator used in the irrigation system that they have developed and patented to prevent olive trees from withering in Baena near of Cordoba on August 30, 2023. The infiltrator is a device that lasts over time and takes water directly to the root of the tree without moisture loss in the process

One common example of adsorption happens in air and water filters. In those devices, activated carbon adsorbs impurities onto its surface. Here, a researcher holds an activated carbon filtering device used in an irrigation system.

JORGE GUERRERO/Contributor/Getty Images Plus

By Katie Grace Carpenter

Absorb and adsorb (verb, “AB-sorb” and “AD-sorb”)

Absorb and adsorb both describe ways a material might soak up another substance. One big difference sets these words apart. Absorption occurs inside the material. Adsorption occurs at the surface of a material.

Absorption occurs when a substance enters inside and mixes with a material. For example, the fibers of a cotton towel absorb spills. They do this by pulling the liquid inside and holding it there. Absorption allows our bodies to draw nutrients from food into our blood. And when cooking pasta, water absorbs into the starchy noodles. Absorption changes noodles from stiff to soft.

In physics, absorption may also describe energy transformations. Energy — such as light waves or sound waves — might be absorbed. As in, a leaf absorbs sunlight for photosynthesis.

Adsorption, on the other hand, happens when a substance clings to a material’s surface. One everyday example of this happens on activated carbon. This material provides the purifying power in many water and air filters. In these filters, a layer of activated carbon is often sandwiched between other materials. Impurities in the air or water cling to the surface of the carbon. Chemists can even customize the particles to latch onto specific impurities.

Zooming into a pellet of activated carbon, you’d see the surface riddled with pores. These pores increase the amount of surface area exposed to the surroundings. That increases the pellet’s ability to cling to impurities. This is why it’s called “activated” carbon. Chemists sometimes “reactivate” carbon by exposing it to very hot steam. This technique can remove adsorbed material. That allows the carbon to be reused.

In a sentence

Virus particles adsorb onto microplastic surfaces but do not absorb inside.

Check out the full list of Scientists Say.

Power Words

More About Power Words

activated carbon: (also known as activated charcoal) A form of carbon that comes as a powder or small granules. It is used to filter out things that may be harmful, smelly or impart a bad taste. The impurities sorb (attach) to the carbon. The small size of the particles increases the carbon’s surface area. So does the particles’ rough surface, which — like a sponge — has lots of pores that offer even more places for the impurities to attach.

adsorb: To stick to — or adhere — to the outer surface of another material. Sunscreen stays on because it adsorbs onto your skin's surface.

adsorption: The process by which a substance sticks to, or adheres, to the outer surface of another material. Sunscreen stays on your skin because of adsorption.

carbon: A chemical element that is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

fiber: Something whose shape resembles a thread or filament. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fibers tend to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.

filter: (n.) Something that allows some materials to pass through but not others, based on their size or some other feature. (v.) The process of screening some things out on the basis of traits such as size, density, electric charge.

liquid: A material that flows freely but keeps a constant volume, like water or oil.

nutrient: A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

particle: A minute amount of something.

photosynthesis: (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

physics: The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

pore: A tiny hole in a surface. On the skin, substances such as oil, water and sweat pass through these openings.

sound wave: A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.

surface area: The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

About Katie Grace Carpenter

Katie Grace Carpenter is a science writer and curriculum developer, with degrees in biology and biogeochemistry. She also writes science fiction and creates science videos. Katie lives in the U.S. but also spends time in Sweden with her husband, who’s a chef.

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