Yes, distilled water is a polar substance. The fundamental structure of the water molecule, \(\text{H}_2\text{O}\), dictates its polarity, and the process of distillation does not alter this inherent molecular characteristic. Polarity refers to the uneven distribution of electrical charge across a molecule. This unique electrical nature is the reason water possesses many of its most well-known and important properties.
The Science of Molecular Polarity
Molecular polarity originates from the concept of a covalent bond, where two atoms share electrons. Polarity arises when this sharing is unequal, driven by a property called electronegativity. Electronegativity is essentially an atom’s measure of its ability to attract a shared pair of electrons toward itself in a chemical bond.
The difference in electronegativity determines the nature of the bond. In water, oxygen (3.44) is significantly more “electron-hungry” than hydrogen (2.20), drawing the shared electrons closer to its nucleus.
This unequal distribution creates a separation of charge, forming a dipole moment. The oxygen atom acquires a partial negative charge (\(\delta-\)), while the hydrogen atoms acquire a partial positive charge (\(\delta+\)). This unequal sharing forms a polar covalent bond, which is necessary for molecular polarity.
Why Water’s Structure Creates a Dipole
While the individual hydrogen-oxygen bonds are polar, the overall polarity of the water molecule depends on its geometry. Water is not linear; it has a bent, or V-shaped, geometry with the oxygen atom at the center. This shape is determined by the four electron domains surrounding the central oxygen: two bonding pairs and two non-bonding “lone pairs.”
These electron pairs repel each other, pushing the hydrogen atoms closer together. This repulsion results in a bond angle of approximately 104.5 degrees. This bent shape is crucial because it prevents the individual bond dipoles from canceling each other out.
For example, in a linear molecule like carbon dioxide (\(\text{CO}_2\)), the dipoles pull in opposite directions, neutralizing the overall charge. Water’s structure, however, spatially separates the negative charge near the oxygen and the positive charges near the hydrogens. The net result is a strong, permanent molecular dipole moment, making the \(\text{H}_2\text{O}\) molecule highly polar.
Addressing the “Distilled” Qualification
The term “distilled” refers to a purification process, not a change in the fundamental nature of the water molecule itself. Distillation involves boiling water into steam and condensing it back into a liquid, leaving behind nearly all dissolved minerals, salts, and impurities. This process removes conductive ions, such as calcium, magnesium, and chloride, which makes distilled water an electrical insulator, or non-conductive.
This lack of conductivity sometimes confuses people into thinking the water is non-polar. However, polarity is an intrinsic property of the individual \(\text{H}_2\text{O}\) molecule, defined by its atoms and bent structure. Distillation does not break the covalent bonds or alter the molecule’s geometry. Every molecule of \(\text{H}_2\text{O}\) remains highly polar; distilled water is simply closer to pure \(\text{H}_2\text{O}\), lacking the dissolved solutes found in tap water.
Practical Consequences of Water’s Polarity
Water’s polarity has profound effects, most notably its designation as the “universal solvent.” This is explained by the rule of “like dissolves like”: polar solvents dissolve polar solutes and ionic compounds. When an ionic compound like table salt (\(\text{NaCl}\)) is placed in water, the partially negative oxygen end of the water molecule is strongly attracted to the positive sodium ions (\(\text{Na}^+\)).
Simultaneously, the partially positive hydrogen ends are attracted to the negative chloride ions (\(\text{Cl}^-\)). This strong attraction overcomes the forces holding the salt crystal together, allowing water molecules to surround and separate the ions, dissolving the salt. Water also dissolves polar molecular substances like sugar through similar dipole interactions.
The polarity of water also enables it to form special intermolecular attractions with other water molecules, known as hydrogen bonds. The partially positive hydrogen atom of one molecule is attracted to the lone pair electrons on the oxygen atom of a neighboring molecule. Hydrogen bonding is a relatively strong force that links water molecules together, contributing to properties such as high surface tension and cohesion. This cohesion is what allows water to be drawn up through the narrow vessels of plant stems against the force of gravity.