In chemistry, “adhesion” refers to the tendency of some substances to cling to other substances. It comes from the root words “ad” for “other” and “hesion” for “to stick to.” Adhesion is usually caused by interactions between the molecules of the two substances.
The most important adhesive substance on Earth is water.
Water molecules are highly polar – meaning that one part of the molecule has a slight negative charge, while the other has a slight positive charge.
This is because the nucleus of water’s oxygen atom attracts the molecule’s electrons more strongly than the nuclei of its hydrogen atoms. As a result, the oxygen atom is slightly negatively charged, while the hydrogens are slightly positively charged.
In the image above, the “δ” symbol indicates a partial charge due to unequal electron sharing.
This allows water to interact strongly with any other charged or polar substances. Because atoms of opposite charges are actively attracted to each other, different parts of water molecules are attracted to both positive and negative ions and molecules.
On a large scale, this means that water will stick to many substances.
Water molecules also stick to each other for the same reason, because all water molecules are polar. This property, called “cohesion,” works with adhesion to make some life functions possible.
Examples of Adhesion
Examples #1: Capillary Action
You have seen capillary action if you have ever dipped the edge of a paper towel or toilet paper square into a puddle of water.
You’ll notice that the water races up the paper towel quite fast, eventually soaking the whole thing even if you have only dipped a corner into the liquid!
That is the adhesion of water in action: water molecules stick to charged molecules within the paper.
To make them absorbent, paper towels and toilet paper are specially made with narrow channels that encourage water to “climb” up them until they run out of channels to fill.
This is the same mechanism that allows trees to draw water and minerals up from the soil to nurture their leaves. Trees are made of the same materials as paper (or rather, paper is made from trees), and trees have tiny channels inside of them that suck up water in just the same way as a paper towel.
If water were not polar and did not have the property of adhesion, trees as we know them could not exist!
Examples #2: Dissolving Solutes
The tendency of water molecules to stick to other molecules also allows water to dissolve substances that are important to life, such as ions, DNA, and many proteins.
This is necessary for life because molecules that have been dissolved in water or another polar solvent interact with each other more readily. So, the polarity of water also makes chemical reactions that are important to life possible!
Related Biology Terms
- Capillary action – The mechanism by which trees draw water up through their roots to feed their leaves.
- Cohesion – The tendency of molecules to stick to other molecules like themselves.
- Polarity – A property of some molecules where one part of the molecule has a slight positive charge, while another has a slight negative charge.
Test Your Knowledge
1. Adhesion happens when a molecule is ionic.
2. The molecules making up trees and paper towels are nonpolar.
3. In addition to being important for capillary action, water’s adhesive properties help cells to function.