By Claire Gillespie | Updated Aug 30, 2022
When an ionic salt dissolves, it splits into free ions and becomes an electrolyte that conducts electricity. Most covalent molecules remain undissolved in water and instead form a separate layer; only polar covalent substances like sugar dissolve, but they do not ionize.
Ionic compounds consist of positively and negatively charged ions held together by electrostatic forces. Covalent compounds are made of atoms that share electrons, typically forming discrete molecules. Ionic solids usually exhibit high melting and boiling points because a large amount of energy is required to break the strong ionic lattice. Covalent solids, being collections of individual molecules, have lower melting and boiling points and can separate more readily. Common ionic examples include sodium bromide, calcium chloride, and magnesium oxide, while ethanol, ozone, hydrogen, and carbon dioxide are classic covalent substances.
When an ionic compound contacts water, the polar water molecules attract the cations and anions, pulling them apart—a process known as dissociation. The separated ions become solvated by shells of water molecules, which stabilize them and prevent recombination. The resulting solution contains free-moving ions, making it an electrolyte that can conduct electric current.
Covalent molecules generally do not ionize in water because they lack charged fragments. Water’s polarity means it interacts strongly with polar molecules but poorly with nonpolar ones. Consequently, most covalent substances remain undissolved and form a separate layer on the liquid surface. Sugar, a polar covalent compound, is an exception—it dissolves in water as intact molecules, yet it never breaks into ions. Nonpolar covalent substances such as oil resist dissolution entirely.
