Back To CourseCollege Biology: Help and Review
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Wendy has taught high school Biology and has a master's degree in education.
There are many ways to describe water. It can be cool and refreshing in a swimming pool or warm and comforting in a bathtub. Water is thirst-quenching and life-giving. A universal solvent, it can wash away the dirt of life.
But would you ever use the word 'sticky' to describe water? Probably not. If anything, water is useful in washing stickiness away.
If you closely examine water in its smallest form of atoms and molecules, however, you'll find that it is 'sticky' in a sense. Not sticky like a child's hands after eating a lollipop, but sticky in a cohesive way. In this lesson, we'll examine cohesion in water and we'll take a closer look at the properties of water that make its molecules attracted to one another.
Cohesion in water has to do with properties of water molecules that make them 'stick' together. To better understand water cohesion, we need to zoom in on a teaspoonful of water. In this teaspoon, there are more than a hundred drops of water. And in each individual drop, we can find millions of water molecules. But what exactly makes up a water molecule?
A molecule of water is made up of one atom of oxygen and two atoms of hydrogen. As you may already know, its molecular formula is H2O. The oxygen and hydrogen atoms are held tightly together by bonds. But once again, to fully understand water cohesion, we must zoom in even further on a water molecule to examine its smallest components, the atoms.
An atom is the smallest building block of matter. Atoms are so tiny that they can't be seen by the naked eye. Yet, they make up every single thing in the universe. Atoms combine to form molecules, and molecules combine to form the elements that make up matter. You can think of an atom as the tiniest Lego piece in a set. If you attach several Legos together, you'll form a subunit of a larger structure. These subunits represent molecules of matter.
Atoms are composed of three smaller particles called protons, electrons, and neutrons. Protons carry a positive charge, electrons carry a negative charge, and neutrons are neutral. Neutrons and protons are found in the center, or nucleus, of the atom. Electrons are found orbiting around the nucleus so quickly that they move almost at the speed of light.
It's the electrons that are involved in the bonding, or attaching of atoms together, to make molecules. Electrons are found in different levels of orbit, depending on how many electrons are present in that atom. Sometimes atoms bond together by sharing the electrons in their outer levels. These are called covalent bonds.
Are you wondering how this relates back to water cohesion? Let's look again at a water molecule. One oxygen atom is bonded to two hydrogen atoms. As we mentioned before, the atoms are held together by a bond. This covalent bond is created when the oxygen atom shares its outer electrons with the electrons in the two hydrogen atoms.
Although the oxygen atom is sharing electrons with the two hydrogen atoms, the electrons tend to be more attracted to the oxygen atom. This is because oxygen is more electromagnetic than hydrogen, meaning it has a stronger pull for electrons. This creates an imbalance of charge within the molecule. If you were in a boat and everyone moved to one side, the balance of the entire boat would be thrown off. Likewise, the charge on a water molecule is unbalanced as the electrons lean toward oxygen. The side of the molecule with the oxygen atom is more negative, and the side with the hydrogen molecules is more positive.
Okay, this is the part where we finally relate back to the sticky water molecules! Imagine all of those water molecules floating around together. You probably already know that opposites attract. Therefore, a negative charge attracts a positive charge. What will happen when the slightly negative side of one water molecule comes into contact with the slightly positive side of another molecule? They'll be attracted and 'stick' together! This is the whole idea behind water cohesion.
The bonds formed by this attraction are called hydrogen bonds. They're very weak and only last a fraction of a second. However, a new bond forms with another molecule immediately after one is broken. And this increases the cohesiveness of water.
One way to see water cohesion at work is to examine the surface of a pond or lake. You might see small creatures called water striders skimming along the surface of the water. They're able to stay on the surface without falling through. This is because the water molecules at the surface are cohesive and create a condition called surface tension. As mentioned before, the hydrogen bonds keeping the water molecules together are weak, so larger creatures couldn't be held on water by its surface tension. But because water striders are small and light, the surface tension supports them.
Another way to observe water tension is to do a simple experiment involving a penny and an eyedropper. Using the eyedropper, carefully place one drop of water at a time onto the penny. As you continue to place drops on the penny, the water will actually form a mound before finally spilling off. This is due to water cohesion.
One more key example of water cohesion has to do with water transport in plants. In a plant, there are microscopic vessels through which water runs. As water evaporates from a plant's leaf, more water from the stem actually moves upward against the force of gravity to replace the lost water. This is because the water is cohesive, and each drop attracts the next, pulling it up. In this way, the different parts of a plant can receive the water they need.
Cohesion in water is a property of water that makes its molecules attracted to each other. A water molecule is made of one oxygen atom bonded to two hydrogen atoms. The molecule has an unbalanced charge, with the oxygen side being slightly more negative and the hydrogen side being more positive. This imbalance of charges makes opposites attract, and water molecules bond to one another with weak hydrogen bonds.
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Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons