Super Powered Water: Cohesion and Adhesion
Ask Science exposes water’s secret identity. Learn about water’s superpowers in Part 1 of this aquatic series.
Lee Falin, PhD
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Super Powered Water: Cohesion and Adhesion
Confession time. When I was a kid, I was pretty sure I had some latent superpower that had yet to manifest itself. I knew that if I waited long enough, something 
would happen to trigger these hidden abilities and I would be off on a lifelong mission to fight crime and save innocent lives from destruction. Then, after a lot of patient waiting, something happened that I thought was going to be the key to my life of superhuman adventure.Â
I was taking a shower and I noticed that the water would run down my arm, fly off my outstretched fingers, and shoot through the air. Surely this was it! All that was needed was to develop my powers further through training and dedication. Was I going to be Water Boy? Aquaman? Maybe I would be able to freeze water too, like Iceman. Unfortunately it wasn’t long before I discovered that not only was my superpower not getting any more super, but it turned out that I wasn’t the only one this happened to.Â
Sadly it turned out that this superpower didn’t belong to me at all. It was just one of water’s many extraordinary abilities. Before we explore some of water’s many amazing talents, let’s take a look at the source of it’s powers.Â
Hydrogen Bonds
Water doesn’t derive its many abilities from exposure to cosmic rays, a radioactive spider bite, or from a rare genetic mutation. The secret to water’s greatness lies in its ability to form hydrogen bonds.Â
In a previous episode on atomic bonds, I mentioned that water molecules are formed when one oxygen atom forms covalent bonds with two hydrogen atoms. The oxygen atom shares one electron with each hydrogen atom, which in turn share one electron each with oxygen.
We also mentioned that since oxygen is more electronegative than hydrogen, the pair of shared electrons tend to spend more time hanging around the oxygen side of the water molecule than they do around the hydrogen side. Since the electrons spend so much more time around oxygen, it gets a sort of partial negative charge. Likewise, since hydrogen spends so much extra time without the shared electrons, it has a partial positive charge.
As you might have heard, “opposites attract.” While this may or may not be true in the world of romance, it is definitely true in the world of chemistry. When two water molecules drift by each other, the partial positive charge of a hydrogen atom on one molecule pulls it towards the partial negative charge of the oxygen atom on the other molecule. These tenuous relationships are called polar bonds or hydrogen bonds and it turns out that these bonds are the source of water’s amazing powers.
Tension is Rising
The first superpower of water we’ll look at is called cohesion. While in common usage cohesion means sticking together, in science cohesion specifically means things of the same type sticking together. So water’s ability to form hydrogen bonds with itself results in cohesion.Â
While each hydrogen bond is extremely short lived (lasting an average of only a hundred-billionth of a second), there are such a large number of bonds being formed and reformed, that they create strong cohesion.Â
Cohesion is responsible for the property of water known as surface tension. At the surface of a body of water, all of the water molecules on top are hydrogen bonded to other water molecules underneath. These bonds allow some neat effects. Such as the fact that small objects can sit atop of water, even though they might be dense enough that they should sink. This is how insects such as water striders are able to walk across the surface of water.Â
Let’s Stick Together
Another important power granted to water by its hydrogen bonds is adhesion. While cohesion is the ability of water to form hydrogen bonds with itself, adhesion is the ability of water to form hydrogen bonds with other things.Â
One of the great things about adhesion is how plants use it to get water. Water pulls itself upwards through a plant thanks to adhesion. The water molecules form hydrogen bonds with the inside walls of the root, causing the water to be pulled through via a process called capillary action.
Do Try This at Home
If you want to expose water’s secret identity and see some of its amazing powers at home, here are some things you can try.Â
First, fill a glass with water. As the water reaches the very top of the glass, very slowly pour additional water into the glass until the water level is higher than the top of the glass. If you look closely, you’ll see the water has formed a rounded edge around the top of the surface. That’s surface tension in action. Cohesion is keeping the water from spilling over the side. Of course cohesion can only do so much, and if you keep adding water, it will eventually spill.
Another thing you can do to see hydrogen bonding at work is to take a glass of water and tip it very slowly as if you were pouring it out. When the water starts to pour over the edge, you’ll likely see water running back along the bottom of the glass instead of just falling off the side. This is adhesion in action. The water molecules are forming hydrogen bonds with the glass, trying to hang on as gravity pulls them downwards.
Tune in next week when we discuss water’s other superpowers.Â
If you liked today’s episode, you can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see answered in a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
Water drop image, Karen Blaha at Flickr. CC BY SA-2.0. Water molecule image courtesy of Shutterstock.
