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What Makes Water Water?

A brief study in the properties of water.

Michael Rees

on 13 September 2010

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Transcript of What Makes Water Water?

What are the properties of water? There are seven: Water has a high heat capacity Water is a solvent Water has a high heat of vaporization Water is cohesive Water is adhesive Ice is less dense than water Water has a high surface tension Let's take a closer look at each: So...what does that mean?? First off, we have to know about hydrogen bonds.
Hydrogen bonds are caused by attraction between the water molecules. You see, water molecules are slightly polar. Electrons are not evenly shared between the oxygen and the hydrogen atoms--the oxygen attracts the electron towards itself. This gives the oxygen atom a negative charge, since electrons have a negative charge, and the hydrogen a positive charge, since the proton's charge is not negated by the electrons. Hydrogen bonds occur because the positive hydrogen atoms attract the negative oxygen atoms--and vice versa. Although the bonds are not as strong as covalent bonds (where the atom has a full outer "shell" of electrons) or ionic bonds (where electrons are shared between atoms), hydrogen bonds are still strong enough to cause water to "stick" together. How does this property apply to the world?
Whenever you skip rocks across a pond, you are watching the surface tension of the water create a strong enough surface for the rocks to bounce across.
When a leaf falls on the surface of a lake, it floats across instead of sinking to the bottom. It simply doesn't have enough mass to break the hydrogen bonds.
Insects are able to walk across the surface of a body of water because of the strength of these bonds. Closely related to the last one... The cohesiveness of water means that water will attract other water. This is caused by our old friend hydrogen bonds. This property is important for: Plants
Because water is cohesive, once water is evaporated from the stomata in a plant, it pulls more water into the leaves. This same process allows the water to be pulled up from the roots through the stem. Any animal with a closed circulatory system
Because of cohesion, water will fill a tubular vessel.
This allows blood (which is 90% water) to travel throughout the blood vessels. Blood is important to us because it carries nutrients and other resources throughout the body. You can observe cohesion by dropping water on a sheet of wax paper. Even though water is a liquid, and, by definition, should take the shape of its container, cohesion prevents it from doing so. It becomes a ball instead of a puddle. What's the difference between this and cohesiveness? Well, cohesiveness means water sticks to other water, while adhesiveness... ...means water sticks to other substances. Again, this is important to plants, because:
When water is being pulled up the stem (due to cohesiveness), the water has to stick to something or it the column of water would slip down the sides of the xylem. Adhesiveness prevents this from happening. Adhesiveness is caused by the polarity of water. Because of the positive/negative charges of the atoms, water is attracted to other surfaces. In other words, water has a high specific heat. Specific heat is the energy needed to raise one gram of water one degree Celsius. Because the specific heat of water is so high, quite a bit of energy is needed to change its temperature. This is the reason cities near large bodies of water, such as Los Angeles, have such mild climates. The ocean stores heat without changing its own temperature, then releases and absorbs this energy as the climate changes. The ocean acts as a regulator of the temperature. This property is also important for life, because it keeps the internal temperature of animals (who are 70%-90% water!) from constantly swinging. What causes this? Once again: hydrogen bonds. These links help water absorb this energy without changing temperature. Covalently bonded materials do not have these hydrogen bonds; therefore, their specific height is not nearly as high. Again, this is closely related to the high heat capacity of water. Imagine a pot of water over a fire. It heats up and gets hotter and hotter, and finally it boils. But what is the water like just before it boils? Water boils at 100 degrees Celsius. Just before this point, one gram of water needs 540 calories of heat energy to increase the temperature by one degree. Ever had something explode in the freezer? Now you know why. When water freezes, the molecules group into a hexagonal shape, with quite a bit of empty space in the middle. This empty space was not in the liquid form. In other words, the liquid expands when it becomes a solid! It's also interesting to note that water is densest at four degrees Celsius. This explains why large bodies of water never freeze. Water at the surface can cool down to four degrees, but then it sinks to the bottom (because it's denser). Only if all of the water reaches this level can a body of water freeze. The fact that ice floats (because its density is less than that of water's) is very important. If ice sank, there would be serious problems for anything living in water--such as freezing. For proof...look at the oceans. Ever had a mouthful? But why would water dissolve a substance like salt? Salt is composed of two elements: Sodium (Na) and Chlorine (Cl). Together, they make Sodium Chloride (NaCl). NaCl is ionic. When it enters water, the polarized oxygen and hydrogen atoms literally tear it apart. The negative oxygen attracts the positive sodium, and the positive hydrogen, the negative chlorine. Molecules like salt that can become solutes with water are called "hydrophilic" (literally "water-loving"). Molecules such as oil that cannot mix with water are known as "hydrophobic" ("water-fearing"). So...I hope I've answered the question:
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