Calcium chloride (\(\text{CaCl}_2\)) is a common, white crystalline salt. Its properties are determined entirely by the nature of the chemical bonds holding its atoms together. Understanding whether calcium chloride is polar or nonpolar requires examining the type of bond it forms, rather than classifying it as a traditional polar molecule.
Defining Molecular Polarity
Polarity describes the distribution of electrical charge within a chemical bond. It applies primarily to covalent bonds, where atoms share electrons. This sharing is often unequal because different elements have varying attractions for electrons, a property called electronegativity.
When atoms with significantly different electronegativities bond, electrons spend more time near the more attractive atom. This unequal distribution creates a partial negative charge (\(\delta^-\)) and a partial positive charge (\(\delta^+\)). This separation of charge is known as a dipole moment, which makes the bond polar. A nonpolar molecule forms when electrons are shared equally, or when the molecule’s symmetric shape cancels out individual bond dipoles.
The Characteristics of Ionic Bonds
Calcium chloride does not fit the definition of a polar or nonpolar covalent molecule because it forms an ionic bond. This compound is formed between calcium (a metal) and chlorine (a non-metal). Ionic bonds are formed by the complete transfer of electrons from one atom to another, not by sharing.
Calcium (Ca) gives up its two valence electrons to two chlorine atoms. This transfer creates stable, fully charged ions: a calcium cation (\(\text{Ca}^{2+}\)) and two chloride anions (\(\text{Cl}^{-}\)). These oppositely charged ions are held together by strong electrostatic forces of attraction. They arrange into a rigid, repeating crystal lattice structure, characterized by full, separated electrical charges, unlike the partial charges seen in polar covalent bonds.
Why Calcium Chloride Dissolves in Water
The ionic nature of calcium chloride makes it highly soluble in polar solvents, such as water. Water molecules are strongly polar, possessing distinct negative and positive ends. When \(\text{CaCl}_2\) is introduced, the charged ends of the water molecules are attracted to the oppositely charged ions in the salt’s crystal lattice.
The water molecules surround and pull on the positive calcium ions and the negative chloride ions. This powerful attraction overcomes the electrostatic forces holding the lattice together, causing the salt to dissociate into individual ions (\(\text{Ca}^{2+}\) and \(\text{Cl}^{-}\)). Each ion is then encased by a layer of water molecules, known as a hydration shell. This dissolving process is often exothermic.
Because of the full electrical charges on the ions, \(\text{CaCl}_2\) interacts exceptionally well with polar solvents. Although it is an ionic compound, its behavior is similar to highly polar substances. Therefore, ionic compounds are often grouped alongside polar substances in the context of solubility.
Practical Applications of Calcium Chloride
The high solubility and strong affinity for water allow for several practical applications of calcium chloride. One common use is as a de-icing agent on roads and sidewalks during winter weather. When dissolved in water, the ions interfere with the formation of the crystal structure of ice, effectively lowering the freezing point of the water mixture to temperatures as low as \(-25^\circ \text{F}\) (\(-32^\circ \text{C}\)).
The compound is also widely used as a desiccant, a substance that absorbs moisture from the air. Its hygroscopic nature makes it useful in commercial dehumidifiers and in packaging to keep goods dry. Furthermore, in the food industry, calcium chloride is utilized as a firming agent, such as in canning vegetables or making cheese, and as an electrolyte source.