From the vast icebergs found near the North and South poles to the minute cubes of ice found in a refreshing drink served in a bar, why ice floats on water has remained a mystery for a very long time. Why is it that this substance behaves so differently to others?
We must first understand that a solid which has a higher density than the liquid it is placed in will always sink, as shown by Archimedes’ principle. A cube of solid bromine will sink in a beaker filled with liquid bromine, because solid bromine is denser than liquid bromine, as is the case with most other substances. This occurs due to the kinetic energy of molecules increasing as they are heated, resulting in faster moving molecules and more collisions. This causes the molecules to push outward from each other, as the kinetic energy rises above the energy of the intermolecular forces of attraction between the molecules. This means that on average, the molecules will take up more space, hence resulting in a lower density.
But why is this not the case with water and ice? In order to explain this, a closer look at the structure of water molecules is required.
As shown above, water consists of two hydrogen atoms each bonded covalently to one oxygen atom. However, as shown by the delta symbols, this molecule is polar, with the hydrogen atoms having positive dipoles and the oxygen atom having a negative dipole. This is a result of the separation of electric charge due to atoms having different tendencies to attract bond pair electrons in covalent bonds (electronegativity).
While polarity is present in many other compounds, this pairing between hydrogen and oxygen, an element more electronegative than hydrogen, is unique. This results in a stronger intermolecular bonding interaction known as hydrogen bonding. The only elements capable of this interaction with hydrogen are nitrogen, oxygen, and fluorine, all of which are more electronegative than hydrogen.
As shown above by the segmented lines, the hydrogen bond involves one oxygen atom bonding to two hydrogen atoms in a different water molecule. As a liquid, the hydrogen bonds in water are broken more quickly than they can be formed due to the high kinetic energies of the molecules, resulting in random, chaotic movements.
As water is cooled down, its density increases; the molecules no longer move with as much kinetic energy. However, at around 4 degrees C, its density starts to decrease. This is because the average kinetic energy of the molecules is no longer greater than the energy required to overcome the hydrogen bond interactions. This results in hydrogen bonds being formed more frequently than they are broken. This causes the formation of a crystal lattice structure, in which the water molecules are held further apart by the hydrogen bonds.
This crystal lattice structure is much more organised than the chaotic movements of liquid water. This hence results in the ice molecules taking up more space on average, meaning that the substance is less dense than liquid water, resulting in its ability to float.
If it weren’t for this phenomenon, life on Earth would not be possible. Ice’s ability to float on water allows sea life to survive in winter, as lakes and rivers freeze from top to bottom, insulating the water underneath. The icebergs in the Antarctic and the Arctic would have also sunk, reducing the amount of light reflected back into space, leaving the Earth to be baked by the sun’s scorching radiation.
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