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Differences Between Cohesion and Adhesion

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  • 0:02 Importance of Water
  • 0:48 Why Is Water Sticky?
  • 1:41 Cohesion
  • 2:45 Adhesion
  • 3:32 Supporting Life
  • 4:10 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
This lesson explains the differences between cohesion and adhesion. Learn what each property is and how they affect life on Earth. Then take a look at the chemistry of water and why it has these two properties.

Importance of Water

Water is the basis of all life on Earth. Without water, even the smallest organisms could not survive. No plant life would grow. There would be no food for herbivorous animals, and thus there would be no food for carnivores. The entire food web would collapse! In fact, water is so important that scientists searching for life on other planets first look for signs of water, such as indications in rock that water was once present.

Two particular characteristics of water give it this ability to support life: cohesion and adhesion. These are both properties of water that make it sticky, adhering to itself and other things. Before we get into the details of cohesion and adhesion, let's look at why water is sticky, even though it appears wet and slick.

Why Is Water Sticky?

Water is made up of an oxygen molecule bonded to two hydrogen molecules. Each atom is made of a nucleus, or core, of protons and neutrons and surrounded by negatively charged electrons. Oxygen loves electrons. It pulls them close to its nucleus and never wants to let them go. We call this electronegativity, when an atom attracts a pair of bonded electrons. That's because when water is bonded to two hydrogen atoms it pulls their electrons a little closer to itself, giving water a partial negative charge. And since the hydrogen atoms are losing a negative charge, they become a little positive.

These partial charges create a polar, or charged, molecule. Positive charges love negative charges, so the partial charges on water allow it to stick to many things, including itself. Water sticking to itself is called cohesion, and now we'll look at the details of that process.

Cohesion

The prefix 'co-' means together and '-hesion' can be translated to adhere, or stick. So, cohesion is water molecules sticking to each other. This occurs because the partial negative charges on oxygen stick to the partial positive charges on hydrogen atoms of other water molecules.

The stickiness of water allows for a high surface tension, where water forms a barrier on the surface. Water striders are small insects that actually walk on water in ponds. They skim the surface because the surface tension creates a barrier they can walk on, despite them being heavy enough to sink.

You can try out this process at home as well! Fill a glass of water to the brim. You might notice the water can overfill the top of the glass and not spill. This is because the water molecules stick together, forming a tight barrier on top of the glass. Next, take a paperclip and gently place it on the top of the water. The paperclip will float, despite being much denser than the water. If you tap the surface of the water, disrupting the surface tension, the paperclip will sink to the bottom.

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