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Why is Water the Molecule of Life? Ever wondered why the human body 60% water?
Why can fish survive under frozen lakes during winter? Or, why do we sweat when we feel hot?
Water is often called the "Molecule of life" not only because it exists in all three states
naturally and floods 71% of our planet, but also because it is a universal solvent, forms
ice with lower density than water, and has a high heat of vaporization.
Many of the unique properties of water result from a special kind of intermolecular force
called "Hydrogen Bonding". But before we get into that let's take a closer
look at the water molecule, which as everyone knows has the chemical formula H2O. Since
the oxygen atom is missing 2 e- and the hydrogen atoms each has an extra e- in order to complete
their octets, the oxygen forms 2 covalent bonds with 2 hydrogens.
What's more, since oxygen likes e- more or is "electronegative compared to hydrogen",
the oxygen atom pulls the bonding electrons closer and acquires a partial negative charge,
while the hydrogen atoms are left as bare protons with partial positive charges. This
makes water a polar molecule. There's a saying that "Like dissolves like".
This means that charged can dissolve easily in water because they form ion-dipole and
dipole-dipole interactions with the water molecules, while "Hydrophobic" or "water-fearing"
substances like oils do not dissolve and clump together.
In the human body, fluids such as blood is mostly made up of water and contain lots of
particles like sugars or salts that provide us with energy or chemical messengers, and
also proteins such as red blood cells that transport oxygen.
The charged sugars and salts are dissolved so they can be easily transported to and be
taken up by cells. The string of amino acids that make proteins,
on the other hand, are folded in such a way that the hydrophobic, non-polar parts in inside
the charged parts. If our blood was not made of water, proteins will change shape, lost
their functions and we will die. So what happens when a bunch of water molecules
come together, like inside a lake? Because unlike charges attract and like charges
repel, the molecules will orient themselves in such a way that each O is close to another
molecule's H. The dotted line represents the intermolecular
interaction between them, which is called a "Hydrogen Bond".
This little line has many implications. When water freezes and forms a lattice, the molecules
become more spread out and decrease the density of ice relative to water. Why? Because in
liquid water the molecules are constantly sliding past each other and making new H-bonds,
but when temperature decreases and they jiggle less and less, the molecules settle into a
fixed position. Because each molecule has a bent shape caused by the nonbonding electron
pairs on oxygen, there's a lot of empty space. So ice can float on a lake and helps to insulate
the warmer and denser water below. Imagine if ice were denser than water, like many other
substances: then ice formed on the top of the lake would sink to the bottom and eventually
the entire lake freezes solid. If you are a fish, yikes!
H-bonding is a relatively strong force between covalent compounds, so it takes a lot of energy
to break the bonds. This can be beneficial when we want to keep our bodies cool during
summer. Simply use the excess body heat to evaporate tiny droplets of water on our skin
-- so we sweat. When water evaporates, they break free from
each other, forming gaseous water vapour. The energy needed to change 1 mole of a liquid
to its vapour is called its "Heat of Vaporization". For water, this value is 44.02kJ/mol at room
temperature. This means that for every 1kg of sweat that
evaporates, our body gets rid of 2444J of energy in the form of heat, equivalent to
39 teaspoons of sugar. No wonder exercising is a good diet strategy!