Spring Entropy 3/23/23

Science with Richard Bleil

My friend suggested that, now that spring is upon us and our roads are falling into chaos, I should revisit the concept of entropy.  Well, honestly, I do take requests in my topics, and entropy happens to be one of my favorite topics, so here we go.

Mechanically, winter is very hard on roads.  In essence, a concrete rode is just an attempt to create one long continuous smooth flat rock.  For the most part, concrete is made up of small rocks held together by cement, which is largely some form of limestone and a source of silica and alumina like clay.  Limestone and clay solidifies into a kind of rock, holding onto one another and trapping the small rocks (filler so it’s not all cement and therefore less expensive) in its lattice.  Limestone is a carbonate, so it can be eroded with acid rain which might explain why portions of the sidewalk on my house are crumbling from the top down, although, honestly, it was probably just poorly applies.

For the most part, potholes and cracks are because of water.  Although a lot of care is taken to try to drain away any water that might get underneath the concrete, such as a layer of stone underneath the concrete and water sealing layers, there are still times that water will get underneath the concrete without draining away.  As it freezes, it expands.

Water is the only compound known to science that literally expands on freezing (there is also one element).  This is because of the incredibly strong intermolecular force called “hydrogen bonding” (the topic of my personal research today) and the intricate tetrahedral arrangement of the water molecules creating a crystal lattice that is very open.  Density is the measure of mass of a fixed volume of substance, and as water cools (as with any liquid), the density increases as the molecules lose kinetic energy (the energy of motion) and pull closer to one another.  For water, the maximum density at 4 degrees Celsius (about 39 degrees Fahrenheit).  This is roughly the temperature of a refrigerator.  Scuba divers are very familiar with this fact as, on cold days, they hit an “isotherm” near the bottom of the body of water.  This is water at 4 degrees Celsius, denser than the water above it, so it just sits there with all water underneath being this same maximum density.  Suddenly, the water just gets noticeably cold as it has not yet warmed up.

As the temperature continues to drop, the water molecules undergo a kind of “self-assembly”, rearranging themselves to create hydrogen bonds with four water molecules around them.  Each water molecule wants these four hydrogen bonds, so the lattice opens up a bit creating cavities.  This more open lattice means the density of water will decrease, which is why ice floats.  Curiously enough, if water did not do this, complex life wouldn’t be possible because the ocean floor would permanently be covered in a layer of ice too far from the surface to ever get the light necessary to melt. 

During thaws, water can get trapped underneath the concrete and asphalt in its liquid form, filling any cavity completely because it is a liquid.  When the temperatures drop, this water will freeze and expand.  We are familiar with the forces possible as water expands (a tribute to the strength of the hydrogen bond) especially if we’ve ever personally experienced a pipe bursting in our homes.  The same thing happens.  The ice under the concrete expands pushing up on the road and breaking the concrete apart.  In the thaw, the ice melts and drains away, leaving cavities creating potholes and cracks for the drivers to skillfully dodge while putting other oncoming drivers at risk of collision. 

This is in part the second law of thermodynamics.  It’s a natural process that results in chaos (disorder if you prefer).  What was once a long continuous smooth stone is moving towards rubble and destruction.  Left alone, those potholes and cracks will let more water in next winter, and the road will just get worse (not to mention mechanical stress every time somebody drives over those cracks and potholes).  What most people don’t understand about the second law of thermodynamics is that, yes, entropy really can be reduced.

Many people have learned and believe that entropy can never be reduced, which is true but only for the universe as a whole.  For a local system, such as the roads, entropy can be reduced by performing work on the system.  This is the construction crews putting “bandages” on the road by merely filling in the cracks and potholes, or repairing the road by taking up the concrete and laying a new road in its entirely (as they did in my neighborhood a couple of years ago).  Work requires energy, so in essence, they are using energy to undo the entropy caused by the water, energy in the form of petrol fuel and food which the construction workers eat to give themselves energy.  The byproducts of both of these is carbon dioxide and other gases which, in turn, increase the entropy of the surroundings as they work.  As the entropy of the road decreases, the entropy of the surroundings increases.  Because the entropy of the universe cannot ever decrease, the entropy of the surroundings is increasing more than the entropy of the road is decreasing. 

Ultimately, all of the spring road construction that we all love so very much is because of the second law of thermodynamics.  And, honestly, as much as we love to hate road construction, if it never occurred, the roads would quickly become impossible to drive. 


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