At one point I was a chemistry major in college, and at no point in my school career was I good at chemistry. I always thought the subject was interesting, but I wasn’t great at math or balancing equations; two pretty vital skills if one plans to pursue this field. I revisited my chemistry past recently when I was asked a question about freeze-breaks in plumbing systems: What happens at the molecular level inside a pipe filled with water? At the atomic level, water is a sticky thing, which can wreak havoc on your plumbing system.
Water will freeze below 32 F at the range of pressures you are likely to experience on earth. Hard stop. (Side note: If you plan on building a house near the planet Jupiter, the freeze temperature may deviate from 32 F, because the pressure is much different.) Regardless of the vessel holding the water, H2O will transition from a liquid to a solid state below freezing. Insulation around a pipe may buy you some time before the water cools and freezes, but it doesn’t prevent freezing. You could have R-50 insulation around a pipe, and it would eventually freeze if you left it stagnant in sub-freezing conditions. Insulation is a thick jacket; it isn’t magic.
Your options to prevent freezing are fairly simple: keep the pipe warm, keep displacing cold water with warmer water or drain the pipes. Beyond that, if you can find a piping material that can expand 10 percent without breaking, you might be okay freezing. Where does the 10 percent number come from? This is where the molecular plot thickens.
As water molecules cool down, they begin to shrink, just like you might do when you are watching a football game in the bleachers of a cold stadium. Oddly, at 39 F, the water molecule gets larger again. Picture it curling up into a ball, then springing out into a jumping-jack position. (This expanded molecule shape causes ice to float in liquid water instead of sinking to the bottom.) By the time it completely freezes, it is 9 percent bigger than when it is a liquid at ambient temperature and pressure. That is why, when frozen, water might break out of your metallic pipes like the Kool-Aid man.
A stretchy material, like a water balloon, may be able to withstand a freeze, because the material is elastic enough to expand with the water. In some cases, flexible materials like PEX pipe can survive a freeze, where copper may not. That doesn’t make PEX freeze-proof, however. Generally, rigid fittings are the limiting point. The PEX might expand with the ice, while the fittings might not. Reducing the amount of fittings you use adjacent to outside walls with PEX piping might help survive a brief freeze, but the only sure-fire way to avoid a break is to avoid dipping below 32 F.
Solid water isn’t a single molecular shape. The ice you have dropped into a drink and seen floating down to the ground as a snowflake are both classified as Ice I. At extreme pressures and temperatures, such as in outer space or in laboratory experiments, water molecules can form different solid molecular structures. Scientists know of or have predicted the shape of 17 different molecular ice arrangements, classified by roman numerals. Ice on a planet far away could be 500 F, but still form a solid because the pressure is so high that the molecule is crushed into a cube.
Why is water considered sticky? Water molecules are polar, like a magnet. The oxygen side of the molecule has a negative charge, and the side with the pair of hydrogen atoms has a positive charge. If you put a drop of water on a piece of wax paper, the water will hold on to the other water molecules around it, forming a bead. A non-polar molecule example is gasoline, which disperses instead of beading.
I listened to a podcast episode of BBC In Our Time on the topic of water. They gathered three water experts and had them nerd out about H2O. This is where I heard the ‘water is sticky’ phrase: “Water is sticky toward all kinds of other materials … When your finger touches a table, it turns out that it is not actually your finger touching the table, but it is the water molecules on your finger, which are touching the water molecules on the table.” Water is stuck to everything.
This polar stickiness, by way of hydrogen bonding between water molecules, also gives water surface tension. Surface tension allows a lizard or a water bug to run across a lake without falling in. It also increases the boiling point of water. This high boiling point makes water a great molecule to use for hydronics. Water stays in its liquid form over a large temperature range, so it can carry BTUs without hitting a phase change and flashing to steam. It would be hard to operate a boiler system with hydrogen sulfide, even though the molecular structure is similar to water. The H2O reset curve would have to be somewhere in the range of -115 F and -75 F to keep the molecules in a liquid state.
The term universal solvent is another nickname given to water. A USGS article describes the way water can dissolve molecules like sodium chloride:
The positively-charged side of the water molecules are attracted to the negatively-charged chloride ions, and the negatively-charged side of the water molecules are attracted to the positively-charged sodium ions. Essentially, a tug-of-war ensues with the water molecules winning the match. Water molecules pull the sodium and chloride ions apart, breaking the ionic bond that held them together. After the salt compounds are pulled apart, the sodium and chloride atoms are surrounded by water molecules.
If water wasn’t so good at dissolving salts, scale wouldn’t be as much of a problem in a plumbing system. Because water breaks apart salt molecules containing sodium or calcium so well, it transports them into your house, and they come out of solution, adhering to hot electrical elements, heat exchangers or the walls of metal pipes. Hard water slowly paints the inside of your plumbing components with salts that might have come from an aquifer below your neighborhood years before.
The stickiness of water is a blessing and a curse. With our current knowledge of this interesting little molecule, we can minimize scale and freezing risks but not stop them completely. The properties of water that make this planet livable are also the same forces that can ruin your plumbing system. l
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