Cohesion and Adhesion (Water) — Properties & Examples

What Causes Cohesion and Adhesion?

Hydrogen bonding allows for cohesion, the attraction between molecules of the same type. Water really likes to be close to other water molecules, even going as far as beading up on smooth surfaces. Dew droplets are excellent examples of this. Cohesion is also why some insects can walk on water.

Similar to cohesion, adhesion is the attraction between molecules of different substances. Water uses adhesion when its attraction to another substance is greater than the water's attraction to itself. If you have ever dropped a cup of water on a hardwood floor, you know that it spreads out instead of forming beads. This is adhesion.

Capillary Action

The cohesive and adhesive properties of water allow for another interesting phenomenon: capillary action. If you place a thin straw in a glass of water, the water will be attracted to it. At the same time, however, the water wants to stick together with other water molecules. If the adhesive attraction between the water and straw is stronger than the water's cohesive attraction with itself, the water will move up the straw unassisted. This is how plants bring water from the soil up into their leaves.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Polarity}}$ : Partial positive and negative charges caused by some atoms wanting electrons more than other atoms in a molecule

$\color{#8925AE}{\text{Hydrogen Bond}}$ : Positively charged hydrogens of one molecule are attracted to the negative charge on another molecule

$\color{#8925AE}{\text{Cohesion}}$ : The intermolecular (between-molecules) attraction between molecules of the same type

$\color{#8925AE}{\text{Adhesion}}$ : The intermolecular attraction between different types of molecules

$\color{#8925AE}{\text{Density}}$ : Water's liquid state is much denser than its solid state, causing ice to float on top of water

$\color{#8925AE}{\text{Specific Heat}}$ : The amount of heat energy needed to increase the temperature of a specific amount of a substance to a specific temperature. The lower the specific heat, the easier it is to heat a substance up

$\color{#8925AE}{\text{Solvent}}$ : Water is a universal solvent. Polar molecules dissolve very quickly in it

Surface Tension and Capillary Action

Recall that water ($H_{2}O$) is a polar molecule because oxygen atoms pull more strongly on electrons than hydrogen atoms do. This creates two poles on the water molecule: one with a positive charge and one with a negative charge.

The importance of water's polarity can be boiled down to the phrase "opposites attract". Positive-negative attraction dictates how water behaves in some really important ways:

(Video) Surface Tension and Adhesion

by khanacademymedicine

Needles sitting on liquid water. Water defying gravity. Tents that keep you fully dry in the rain. Are these magic tricks? Nope, just cohesion (surface tension) and adhesion. Watch this video to find out more about how these amazing properties of water work.