Water, Part III: The Solvent of Life

2 08 2009
Cool and refreshing, water is the stuff of life. Drink up!

Cool and refreshing, water is the stuff of life. Drink up!

It’s time for the thrilling conclusion of our series on water! Water Part III: The Solvent of Life! This one is pretty long, so if you need to, go grab a glass of cool refreshing water to tide you over.

Now, some of what follows isn’t going to make sense unless you’ve read Water Part I. So go do that now if you haven’t. Okay, ready? Great.

In the last post I talked about a few reasons why you should be thankful for hydrogen bonds, namely, that they keep that lovely liquid water around and that they enable plants to bring water up to their highest points. But this stuff doesn’t hold a candle to the #1 Reason You Need to Be Thankful For Hydrogen Bonds.

CLICK HERE TO READ MORE!

First we need a little background. You may be familiar with the term solution. A solution is just a mixture of some things in some other things. Well, okay, let’s be more specific. A solution is the mixture of a solvent (that’s the thing you’re putting stuff in), and one or more solutes (that’s the stuff you put in!). Solutions are also homogenous, which means that the solutes you put into the solvent are evenly distributed. If you take a sample from one region of the solution, it should contain the same amount of solute as if you took a similar sample from another region. Simple, right?

So if you’re one of those brainy types, you may have made the connection between the title (The Solvent of Life!) and that term solvent just introduced above. Well kudos to you, smarty pants. In fact, smarty pants is quite right and has hit on the theme of this article! Water is an excellent solvent! A superb solvent! A salubrious solvent! Indeed, there’s a good reason you use hot water to wash your dishes – it actually dissolves the food residues on your plates in the same way acetone dissolves nail polish. Pretty neat, huh?

Yes, pretty neat. But why does it dissolve those foods? Why does it dissolve sugar? Or salt? Or anything? Well if you’re the type to notice patterns, you’ve probably got the answer already. It’s hydrogen bonding.

The structure of crystalline table salt. The bigger spheres are Chlorine (atomic mass ~35.5), and the smaller spheres are Sodium (atomic mass ~23). Remember that sodium is positively charged and chlorine is negatively charged.

The structure of crystalline table salt. The bigger spheres are Chlorine (atomic mass ~35.5), and the smaller spheres are Sodium (atomic mass ~23). Remember that sodium is positively charged and chlorine is negatively charged.

The polar nature of the water molecule means that anything with a charge, whether other polar molecules or molecules with polar regions or ions (those are atoms or molecules that have a total, nonzero charge), will dissolve in water. Now, we should probably take a second to describe what it is we mean exactly by dissolve. By dissolve we just mean that if a solute is dissolved in a solvent, it moves freely about the solution.

In water, polar or otherwise charged solutes dissolve because water molecules surround that solute. So take for example common table salt, the chemical formula of which is NaCl. Table salt is an example of an ionic compound, which means that one of the atoms involved stole an electron from the other, and now they’re attached because of the attraction of opposite charges. In this case, the chlorine atom, Cl, stole an electron from the sodium atom, Na. This gives us Na+ and Cl-, which naturally attract, forming an ionic compound (ionic because its made of ions!). Check out the diagram at the right to see how NaCl looks in crystal form.

So when you dump salt crystals in some hot water, there’s an immediate attraction. The positively charged hydrogens on the water molecule are attracted to the negatively charged chlorine (Cl-), while the negatively charged oxygen of the water molecule is attracted to the positively charged sodium (Na+). As a result of this attraction, water molecules force their way in between the atoms in the crystal, prying them apart and wrenching them into the solution, among the other water molecules. Now free of their crystalline bondage, the ions are surrounded by water molecules. The coating of water molecules an ion or any other solute picks up in water is called a hydration shell, because it’s made of water (hydration) and it’s a shell (shell). Since the surface of each ion is covered in water, and the surface is the only part that can interact with other molecules, the ions behave in the solution as if they themselves were water! That is to say, they can slip and slide like the rest of the water molecules, having a good time, not sticking out, and blending right in. If only all of us were so popular.

The hydration shell that forms surrounding a dissolved sodium ion. Notice how it is the negatively charged oxygen, not the positively charged hydrogen, that faces the positive sodium ion.

The hydration shell that forms surrounding a dissolved sodium ion. Notice how it is the negatively charged oxygen, not the positively charged hydrogen, that faces the positive sodium ion.

Now, ionic compounds aren’t the only solutes that can dissolve in water. Anything with any polarity or polar region can dissolve. So for example take glucose. The chemical structure is shown at the bottom of this paragraph. The ringlike lines in the center represent a carbon ring. The corners of the ring are where carbon atoms are located, and the lines represent bonds between those carbons. But what I want you to notice are the multiple OH groups coming off of the glucose molecule. These OH groups (also known as hydroxyl groups) are polar, just like water, and for the same reason. The oxygen hogs up the electrons, leaving the hydrogen positively charged. When immersed in water, water molecules can hydrogen bond to these charged atoms and so dissolve the glucose.

The structure of the glucose molecule. Notice the HOs and OHs - these polar hydroxyl groups allow glucose to dissolve in water without being split apart like salt.

The structure of the glucose molecule. Notice the HOs and OHs - these polar hydroxyl groups allow glucose to dissolve in water without being split apart like salt.

Water can also dissolve bigger molecules, like proteins, as long as they have polar regions. Let’s take hemoglobin, a protein present in huge quantities in your blood. Hemoglobin is responsible for transporting oxygen around to your tissues, so that they can use it to produce energy. Hemoglobin has a molecular mass of 65,700. Comparing this to the molecular mass of glucose, which is about 180, or the molecular mass of water, which is about 18, we can see that hemoglobin is friggin huge, to use the technical term. Yet it dissolves in water, because regions of this tremendous protein complex have negative and positive charges that allow water to form a hydration shell.

So now you understand how solutes like sodium ions, glucose molecules, and even massive proteins can dissolve in water. It’s all a matter of having a charge (or having charged regions) that can serve as “hook points” for water molecules. But why did I bother typing all this up, and why should you be thankful for this?

Because you wouldn’t exist without it! Let me put it this way: Life, that is to say all living things, ranging from human beings in an apartment in New York City to bacteria living near hydrothermal vents on the ocean floor to the cuddly puppy currently chewing on my shoes in the next room, all consist of exquisitely orchestrated chemical reactions. We can go into detail further on this point in future posts, but the reason I bring this up is that chemical reactions can only occur if the involved molecules or atoms can come into contact with one another. So what water does is, it provides an environment in which the tremendous number of molecules in your body can interact.

If we took all your elemental constituents in powder form and mixed them together, we wouldn’t get the wonderful chemical reaction your mother loves so much. Those elements wouldn’t interact, because they’d stay put and wouldn’t react. But organize those elements in a watery environment, and presto! A face only a mother could love. Those molecules can slip and slide around the water molecules, slam into each other, and react, forming other, different molecules! In this way, water and its hydrogen bonding nature provides an arena for all manner of intermolecular reactions to occur, and since the sum of these reactions equals you (or at least the observable part of you), you can see why you should now be even more excited about water.

So now you can really appreciate water, and you can understand why it’s so critical for you to stay hydrated. If you don’t, the reactions you need to survive don’t have room to occur! So why not pour yourself another cool, refreshing glass of water, and top off those solvent levels.

-Neil

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