Distilled water is a covalent compound. This classification is based on the fundamental chemical forces that hold the hydrogen and oxygen atoms together to form a single water molecule. Distilled water is defined as water purified through a process that removes nearly all dissolved solids, minerals, and salts, leaving behind essentially pure \(\text{H}_2\text{O}\) molecules. The nature of the chemical bond within this pure molecule dictates its categorization.
The Fundamental Difference Between Ionic and Covalent Bonds
Chemical compounds are categorized based on how their constituent atoms interact with electrons. This interaction falls primarily into two main types of chemical bonds: ionic and covalent. An ionic bond involves the complete transfer of one or more electrons from one atom to another, which typically occurs between a metal and a non-metal. The atom that loses an electron becomes a positively charged ion, while the atom that gains an electron becomes a negatively charged ion.
These oppositely charged ions are then held together by strong electrostatic attraction, forming an ionic compound, such as in the case of table salt, or sodium chloride (\(\text{NaCl}\)). In contrast, a covalent bond forms when two atoms share electrons rather than transferring them. This type of bonding usually occurs between two non-metal atoms, like the carbon and hydrogen atoms in methane (\(\text{CH}_4\)). The shared electrons orbit both atomic nuclei, effectively binding the atoms into a discrete unit called a molecule.
Understanding the Covalent Bond in \(\text{H}_2\text{O}\)
The water molecule, \(\text{H}_2\text{O}\), is formed when a single oxygen atom bonds with two hydrogen atoms. Oxygen and hydrogen are both non-metals, and they achieve stability by sharing electrons to complete their outer energy shells. Specifically, the oxygen atom shares one of its valence electrons with each of the two hydrogen atoms, and each hydrogen atom shares its single electron with the oxygen atom, forming two distinct chemical bonds.
Because the atoms share electrons rather than transferring them entirely, the internal forces holding the \(\text{H}_2\text{O}\) molecule together are classified as covalent bonds. These bonds are not perfectly balanced because the oxygen atom has a greater attraction for the shared electrons than the hydrogen atoms do. This unequal sharing of electrons creates a slight negative charge near the oxygen atom and a slight positive charge near the hydrogen atoms, classifying the bond as polar covalent.
Despite this internal charge distribution, the water molecule as a whole remains electrically neutral, held together by the shared electron pairs. This electron-sharing arrangement is the definitive reason why water is a covalent molecule.
Why Distilled Water is Non-Conductive
The confusion about water’s bond type often stems from the observation that ordinary water conducts electricity. Electrical conductivity in a liquid requires the presence of free-moving, charged particles called ions, which can carry an electrical current. Distilled water is created through a purification process that involves boiling water into steam and then condensing it back into a liquid, which effectively leaves behind nearly all dissolved minerals and salts.
These naturally occurring dissolved substances, like sodium, calcium, and chloride, are ionic compounds that dissociate into free ions when mixed with water. By removing these impurities, the distillation process strips the water of the charge carriers necessary for conduction. The resulting distilled water is a poor conductor of electricity because it is composed almost entirely of neutral \(\text{H}_2\text{O}\) molecules.
Pure water does exhibit a minimal level of self-ionization, where two water molecules can react to form a hydronium ion (\(\text{H}_3\text{O}^+\)) and a hydroxide ion (\(\text{OH}^-\)). However, this natural dissociation is extremely limited, with only about two out of every billion molecules ionized at any given time. At \(25^{\circ}\text{C}\), the concentration of these ions is only \(1.0 \times 10^{-7}\) molar.
This extremely low concentration of intrinsic ions results in a very low electrical conductivity. Therefore, the lack of conductivity in distilled water is not a sign of ionic bonding, but rather a direct consequence of the removal of external ionic impurities, confirming that the molecule itself is fundamentally covalent.