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How Water Shapes Our World

October 11, 2014 at 4:10 pm (Abiotic, grab-bag) (Chemistry, hydrogen bonding, Water)

This was originally an Outside Story piece, but the editor decided it didn’t fit the theme of the column, which is pretty region-specific, as closely as we hoped it would when I pitched the idea. We both felt that it was still a solid bit of writing, so here it is for your reading pleasure:

If we had to choose one thing as defining life as we know it, it might have to be water. Water shapes our world from weather to cells. Because water permeates our lives and experiences from the moment we’re born, it’s easy to overlook all the ways it’s unique.

Let’s start our exploration of why water is the way it is, and how that shapes our world, by zooming all the way in on a single water molecule. You’ve probably heard water called by it’s chemistry name: H2O. That name is really a description saying a single water molecule is made up of two hydrogen atoms attached to one oxygen atom.

Atoms attach to each other by sharing their electrons, but not all atoms are good at sharing. Oxygen is notoriously greedy. In a water molecule, oxygen doesn’t share the electrons nicely with the hydrogens, so the electrons spend more time with the oxygen atom while the hydrogen atoms feel electron-neglected. This means they look for electron love in other places and will cozy up to other molecules’ oxygen atoms, acting like magnets. Chemists call molecules like this ‘polar’.

That polar attraction between the oxygen side of a water molecule and the hydrogen side of another water molecule is called a hydrogen bond. You can picture hydrogen bonding as water molecules holding hands like a group of happy hippies. Like hand-holding, the hydrogen bonds keep the molecules together, but also like hand-holding, the bonds are easy to break.

Hydrogen bonding explains a lot about water. It is often called a “universal solvent”, because so many substances will dissolve in it. All those polar water molecules, with their positive and negative sides, happily interact with other polar molecules or charged ions. As far as life is concerned, it is water’s ability to dissolve many substances that make it so awesome. Think about all the stuff that you’re made of: proteins, enzymes, sugars, salts, hormones, antioxidants, fats. All floating around in the water that makes up 55 to 60% of your body.

Another neat feature of water is it’s strong surface tension. Surface tension comes from the molecules at the surface holding on to molecules beneath them through hydrogen bonds, but water can’t hold on to air. The net effect is that water molecules at the surface behave like a skin. Many small animals, most famously the waterstrider, use this effect to walk on water. Surface tension is also what is at work in the capillary action that allows plants to “suck up” water, going against gravity to get it all the way from their roots to their leaves.

The cohesion—that tendency of water to stick to itself—that creates surface tension also allows water to hold a lot of heat before noticeably changing temperature. It takes more heat to raise the temperature of water than other materials. Think of how long it takes to boil even a small amount of water compared to how long it takes for the metal pot you’re using to get hot enough to burn you.

This high heat capacity means water acts like it’s saving heat for later—good news for our weather! During hot northeastern summers water soaks up the heat, mellowing the temperature, only to slowly release it as the seasons cool. This is why the coasts, like areas of Maine and New Hampshire, have milder season shifts than inland.

Water does something else odd that we see all the time here in the north, as the seasons change into winter: it floats when it freezes. Usually solids are more dense than liquids, so they sink, but because of those hydrogen bonds, water forms a spacious crystal when it freezes, making it less dense and allowing it to float. This is more good news for us, because if ice sank, rivers and lakes would slowly freeze solid from the bottom up over the course of a northeast winter, and might never warm enough in the summer to unfreeze. Now that’s a chilly thought!

Some good resources on water:

How stuff works: water properties. http://science.howstuffworks.com/environmental/earth/geophysics/h2o7.htm

Water properties, US geological survey. http://water.usgs.gov/edu/waterproperties.html

Intermolecular bonding – hydrogen bonds. http://www.chemguide.co.uk/atoms/bonding/hbond.html

Elmhurst college, Intermolecular bonding – hydrogen bonds. http://www.elmhurst.edu/~chm/vchembook/161Ahydrogenbond.html

Hydrogen bonding. http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/Atomic_Theory/Intermolecular_Forces/Hydrogen_Bonding

About.com Chemistry: Why does ice float? http://chemistry.about.com/od/chemistryfaqs/f/icefloats.htm

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