Water’s Cohesive and Adhesive Properties

Advertisements
Advertisements

Related Posts:


A photograph shows a needle floating at the surface of a glass of water.  Though the needle floats, it appears to be slightly sinking below the surface.
A needle’s weight pulls the surface downward. At the same time, the surface tension pulls it up, suspending it on the water’s surface preventing it from sinking. Notice the indentation in the water around the needle. (credit: Cory Zanker)

OpenStax Biology 2e

Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass.

– What is the action or property of like molecules sticking together, being mutually attractive?

Cohesion allows for surface tension, the capacity of a substance to withstand rupturing when placed under tension or stress. This is also why water forms droplets when on a dry surface rather than flattening by gravity. When we place a small scrap of paper onto a water droplet, the paper floats on top even though paper is denser (heavier) than the water. Cohesion and surface tension keep the water molecules’ hydrogen bonds intact and support the item floating on the top. It’s even possible to “float” a needle on top of a glass of water if you place it gently without breaking the surface tension.

– What is the tendency of liquid surfaces to shrink into the minimum surface area possible?

A thin hollow tube sits in a beaker of water. The water level inside the tube is higher than the water level in the beaker due to capillary action.
The adhesive forces exerted by the glass’ internal surface exceeding the cohesive forces between the water molecules themselves causes capillary action in a glass tube. (credit: modification of work by Pearson-Scott Foresman, donated to the Wikimedia Foundation)

Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for transporting water from the roots to the leaves in plants. These forces create a “pull” on the water column. This pull results from the tendency of water molecules evaporating on the plant’s surface to stay connected to water molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water from their roots to their leaves. Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider use the water’s surface tension to stay afloat on the water’s surface layer and even mate there.

– What is the tendency of different particles or surfaces to cling to one another?

Photo shows an insect with long, thin legs standing on the surface of water.
Water’s cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat. (credit: Tim Vickers)

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

https://en.wikipedia.org/wiki/Cohesion_(chemistry)

Advertisements
Advertisements