Core Concepts
Water is one the most abundant substance on Earth, covering 71% of its surface. In this tutorial, you will learn the polarity of water and how it gives rise to some of water’s most important properties.
Topics Covered in Other Articles
- What is a Polar Covalent Bond?
- Polar vs. Non-Polar Bonds & Molecules
- What is Electronegativity?
- VSEPR Theory
- What is a Solute? Solvent vs Solute with Examples
What does it mean for a molecule to be polar?
By definition, a polar molecule has a partially positive end and a partially negative end. The molecule achieves this by having an uneven distribution of electrons between its atom. That is, electrons gather more closely to one atom than the other, thus making one atom slightly more negative than the other. Moreover, these atoms must also be arranged in a way that their partial charges do not cancel each other out.
Is water polar?
As stated above, to be polar, a molecule must have polar covalent bonds and be asymmetrical.
Does water have polar covalent bonds?
The electronegativity of oxygen is 3.5, and the electronegativity of hydrogen is 2.1, making the difference in electronegativity between them 1.4. Atoms in polar covalent bonds generally have a difference in electronegativity between 0.4 and 1.7. Thus, the O–H bonds in water molecules—where the O end is partially negative, and the H end is partially positive—are polar covalent bonds.
Is water asymmetrical?
The oxygen atom in a water molecule bonds to 2 hydrogen atoms and 2 lone pairs, giving it the steric number of 4. According to VSEPR theory, a steric number of 4 means the electron geometry of water is tetrahedral. Moreover, because that steric number includes 2 lone pairs, the molecular geometry of water is bent. A bent structure gives the water molecule its asymmetry.
In conclusion, water is polar because it has polar covalent bonds and is asymmetrical.
What comes out of water’s polarity?
Water as the universal solvent
Recall the golden rule of solubility: like dissolves like. Therefore, water, as a polar solvent, readily dissolves polar solutes. How it dissolves is as follows: the partially positive hydrogen atoms of water latch onto the negatively charged atoms of the solute, while the partially negative oxygen of water attracts the positively charged atoms of the solute. In doing so, water dissociates the covalent or ionic bonds of the solute, separating the solute’s atoms from each other.
However, water can’t dissolve nonpolar solutes like oil or wax (remember the golden rule), which makes its title of the “universal solvent” a little bit misleading.
Hydrogen bonding
The polarity of water allows it to form hydrogen bonds, a stronger version of the usual dipole-dipole intermolecular force (and NOT a type of covalent or ionic bond as its name might imply). Hydrogen bonding occurs between a partially positive H atom (usually bonded to an N, O, or F atom) of one molecule and a partially negative atom (usually N, O, or F) of another molecule. In the case of water, hydrogen bonding occurs between an H atom of one water molecule and an O atom of another water molecule.
Hydrogen bonding gives water cohesion, adhesion, and a high boiling point.
Cohesion
Cohesion refers to the attraction between molecules of the same type. In other words, molecules with strong cohesive forces like to stick to each other. Thanks to its ability to form strong hydrogen bonds, water is one such molecule.
Water molecules like to stick to each other, and such stickiness (i.e. cohesive forces) gives rise to the surface tension of water. Surface tension is the ability of the surface of a liquid to resist an external force by behaving like an elastic film. In the case of water, its surface tension allows small insects to move across it without the need to swim or float, like the water strider pictured below.
Adhesion
Adhesion refers to the attraction between molecules of different types. The polarity of water allows it to stick to other polar molecules. Thus, the more polar a molecule, the better water will stick to it.
The adhesive and cohesive forces of water allow water to perform capillary action. During capillary action, water flows through a narrow space without the help of, or even against, gravity. This is important because many processes in everyday life rely on capillary action. Capillary action helps water to reach the roots of some plants. Capillary action allows paper towels and sponges to soak up water. Moreover, some scientific techniques like thin-layer chromatography utilize capillary action.
