Water like a stone

By December 21, 2016MyPhysicsFun

Christmas time is here again and one of my favorite Christmas songs is In the Bleak Midwinter.  One of the lyrics is “Earth stood hard as iron, water like a stone”.  During Job’s trial, God answered him out of the whirlwind, and said: “From whose womb comes the ice?  And the frost of heaven, who gives it birth?  The waters harden like stone, and the surface of the deep is frozen.” – Job 38:29-30.  Water and ice are so common, you’ve likely never thought about the special properties God established for them and upon which our lives depend.  Water is very unique among liquids because unlike most substances, when it freezes it becomes less dense.  If it were not so we would all die because the oceans would freeze from the bottom up killing all sea life.  When it’s warm, we love to swim in water and splash in it.  Kids run through sprinklers or cannon ball into the pool.  But drop the temperature about 45 degrees and it’s a whole different ball game.  Water turns hard as stone as ice forms.  Why is this?  We owe it all to the approximately 104.5 degree angle between the hydrogen atoms covalently bonded to an oxygen atom.

God designed H2O to be a ‘polar’ molecule so that opposite ends of the molecule would have a net charge which could attract the oppositely charged sides of other water molecules.  This electrostatic attraction is one of the four fundamental forces in our universe and explains what is called the ‘Hydrogen bond’ which holds all water molecules together.  It is fine and well that we have H2O molecules, but if they don’t bond with each other we have no swimming pools and no drinking water.  In fact we would all perish because plant life depends on capillary action to transport water and such cohesion of water molecules is courtesy of the hydrogen bond.

Hydrogen has only one proton and one electron while oxygen has eight of each.  Two hydrogens and one oxygen get together in a chemical reaction forming water when they share their electrons.  The water molecule is bent (the 104.5 deg angle) so that in the commonly used molecular model, the two hydrogen atoms sit atop the oxygen atom like the ears of Mickey Mouse.  This bent shape and sharing of electrons between the hydrogens and oxygen leaves an area around the hydrogen proton with no electrons making that area positively charged (because the proton is charge positive).  Likewise, the oxygen’s protons are outnumbered by the more than usual electrons making that side of the molecule negatively charged.  So the hydrogen side is positive and the oxygen side is negative which makes water a polar molecule.  An object with polarity has one side negative and the other positive like two terminals of a battery.  Water’s polarity explains why microwave ovens heat food, why water is a universal solvent and explains why ice freezes on the surface of a body of water instead of at the bottom.

In liquid water, hydrogen bonds are frequently forming and breaking due to thermal energy moving and agitating the molecules which breaks the bonds.  The battle between electrostatic attraction in hydrogen bonds and thermal energy forcing molecules to move determines the melting and freezing points of water.  As the temperature drops to the freezing point, the hydrogen bonds are reinforced because there’s less thermal agitation and those bonds can really set up shop.  Each water molecule bonds with four neighbors forming a tetrahedral structure (a three-dimensional pyramidal shape with four triangular faces) and, in the aggregate, form a larger lattice structure known as ice.  These hydrogen bonds now abound without the disruption of thermal energy, making the ice lattice very rigid.  Think of the guard rails on a roadway consisting of wooden posts with a steel rail connecting them.  One post by itself may be easily knocked over, but when all the posts work as one structure, connected by the rail, it can even withstand a collision with a moving automobile.

So far I’ve explained why ice is hard as stone but why does it float?  When water is frozen, the water molecules require a bit more space to achieve a tetrahedral arrangement.  The fully hydrogen bonded tetrahedron comprising ice is a more open structure than that of liquid water.  Liquid water is more randomly arranged because thermal energy causes molecules to move, continually breaking hydrogen bonds.   Ice’s tetrahedral structure is broken apart as it melts and water molecules pack more closely together increasing the density.

So next time you hear In the Bleak Midwinter and that lyric about water being hard as stone, you can thank God for bending the water molecule, making it polar, and able to form hydrogen bonds which facilitate the formation of ice with lower density than liquid water so all the fish in the ocean don’t die!

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