Water (H2O) is a colorless, odorless, and tasteless compound that is vital for every known living process. Water is a polar molecule, meaning that it has a net dipole moment across its structure due to the uneven spatial distribution of its constituent electrons.
The polarity of water explains many of its unique physical properties, such as its high boiling point/specific heat capacity, surface tension, and solvent capabilities.
Electronegativity And Polarity Of Water
The polarity of a chemical compound is due to the polarity of its individual constituent chemical bonds. Every chemical element has an electronegativity. An element’s electronegativity can be seen as a measure of how “hungry” an atom of that element is for electrons. Atoms with few electrons in their outer shell tend to be less electronegative and atoms with many electrons in their outer shell tend to be very electronegative (except for noble gases, which have a full outer electron shell and are thus very inert).
Whether or not two elements will form a polar bond depends on the relative difference between those element’s electronegativities. If the electronegativities are identical, the bond will be completely non-polar. A completely polar bond involves the more electronegative element taking electrons from the less electronegative element, and is more properly called an ionic bond. Thus, the term “polar” is mostly reserved for covalent bonds that have a relative difference of electronegativities that falls between .05–2.0.
In polar bonds, the more electronegative element will exert an unequal pull on the electrons of the molecule. This causes the electrons to tend towards one side of the molecule. The net motion of the electrons creates a dipole moment in the molecule, with a positive end and a negative end. So for example, water is a molecule made of two hydrogen atoms and one oxygen atom. Oxygen is more electronegative than hydrogen, so when the atoms bond, oxygen exerts a stronger pull on the molecule’s electrons. Because of this pull, the electrons undergo a net movement towards the oxygen end of the atom. The movement creates a dipole moment with two positively charged nodes at the hydrogen ends and one negatively charged node at the oxygen end. The uneven pull of electrons also explains the geometric shape of a water molecule; a bent triatomic molecule with a bond angle of 104.5°.
Polarity And The Physical Properties Of Water
The polarity of water is responsible for many of water’s unique physical properties. First, the polarity of water explains its solvent properties. A sample of liquid water is capable of dissolving a number of ionic compounds like salts, polar organic compounds like ethanol (alcohol), and acids. The polar water molecules exert a pull on any ions or other polar molecules, pulling them apart from their larger structure and dissolving them. Because it can readily dissolve ionic compounds, water is able to serve as a good conductor of electricity. Despite common knowledge, pure water by itself is a poor conductor of electricity. When water dissolves a small amount of an ionic compound (like table salt) though, it becomes an electrical conductor. Almost every living organism relies on the solvent capabilities of water to survive.
Water’s polarity also allows it to engage in a special kind of intermolecular bonding called hydrogen bonding. Hydrogen bonding occurs when hydrogen is bonded to a more electronegative element (e.g. oxygen, nitrogen, fluorine) and is in the presence of another polar molecule or a lone electron pair. The positively charged hydrogen end of a water molecule attracts the negatively charged oxygen ends, creating a partial electrostatic bond between different molecules of water. A single molecule of water is able to engage in up to 4 hydrogen bonds with neighboring water molecules.
Because electrostatic attraction is proportional to the squared distance between two charged bodies, the closer a hydrogen atom can get to a neighboring molecule the stronger the bond will be. Since hydrogen atoms are so small, they are able to get extremely close to the neighboring oxygen atoms and form relatively strong electrostatic bonds.
The strength of hydrogen bonds between water molecules explains water’s high boiling point (100°C) compared to other compounds with a similar chemical structure (hydrogen sulfide). The hydrogen bonds require a large amount of kinetic energy to break, which also explains water’s relatively high specific heat capacity (4.2 J/Kg). Hydrogen bonds are also responsible for water’s high surface tension. Molecules on the surface of a sample of water will form stronger bonds with their neighboring water molecules than with molecules in the air directly above. The cohesion of the water molecules creates a “film” on the surface which is capable of supporting some weight. A handful of animals, such as water striders and some geckos, have evolved to take advantage of this high surface tension and can actually walk on top of water. This molecular cohesion also explains why water has a tendency to go against gravity and climb up the walls of narrow tubes. Vascular plants take advantage of this capillary action to pump water from the roots to the body of the plant.
The polarity and hydrogen bonds are responsible for water’s unique multi-phasic nature. Water is the only compound known to commonly exist in all three phases of matter (solid, liquid, gas) under natural conditions. Because water is polar and engages in hydrogen bonds, it has a relatively stable physical profile over a wide range of temperature and pressure conditions. Additionally, the presence of hydrogen bonds explains why water expands when it freezes. All compounds get denser when they are cooled, but once water is cooled to about 4°C, it begins to expand. The slow movement of molecules in a cooled sample of water makes it easier for hydrogen bonds to form and arrange the molecules in a crystalline structure. The result is that the volume of a sample of frozen water increases by about 9%, hence why your can of soda will explode in the freezer
In summation, water is a polar compound formed from two hydrogen atoms and one oxygen atom. The more electronegative oxygen atom exerts an unequal pull on the molecule’s constituent electrons, causing a net dipole moment across the molecule. The polarity of water explains many of water’s unique properties and explains why it is a necessary ingredient for life.