Salt melts ice by initiating two distinct physical processes. Salt does not melt ice by introducing heat; instead, it changes the physical properties of water, forcing the ice to melt at a temperature lower than the standard freezing point of \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). This melting process inherently causes the resulting liquid solution to become colder. The mechanism begins when salt dissolves in the thin layer of liquid water always present on the ice surface.
Freezing Point Depression
The primary mechanism by which salt melts ice is called Freezing Point Depression (FPD), which relates directly to the number of dissolved particles in the water. Pure water freezes at \(0^\circ\text{C}\) when its molecules align to form a stable crystal lattice structure.
When an ionic compound like sodium chloride (\(\text{NaCl}\)) is spread on ice, it dissolves into the liquid film and breaks apart into its constituent ions (\(\text{Na}^+\) and \(\text{Cl}^-\)). These foreign particles disperse throughout the water and physically interfere with the ability of water molecules to bond together. The ions act as obstacles, blocking the water molecules from rejoining the rigid lattice structure necessary for solidification.
This interference forces the water molecules to lose more kinetic energy to overcome the obstruction and form a stable ice crystal. Consequently, the temperature of the mixture must drop below \(0^\circ\text{C}\) for freezing to occur, effectively lowering the freezing point of the solution. The degree to which the freezing point is lowered depends only on the concentration of the dissolved particles.
Since the surrounding temperature is often above this new, lower freezing point, the ice structure cannot maintain its solid state and begins to melt. For common rock salt, the maximum freezing point depression, known as the eutectic point, is reached when the solution is about 23\% salt by mass. This lowers the freezing point to approximately \(-21^\circ\text{C}\) (\(-6^\circ\text{F}\)). If the temperature drops below this eutectic point, the salt-water mixture itself will freeze solid, and the de-icing agent becomes ineffective.
The Temperature Drop During Melting
The mixture becomes colder during the melting process because the dissolution of sodium chloride in water is an endothermic process. This means it absorbs thermal energy from its immediate surroundings, a phenomenon referred to as the heat of solution.
Dissolution requires energy input to break the salt’s ionic bonds and separate the water molecules. This necessary energy is pulled directly from the surrounding materials, including the liquid water, the ice, and the pavement.
This absorption of heat energy causes the temperature of the resulting brine solution to fall significantly below the temperature of the original ice and water. The combined effect of Freezing Point Depression and the endothermic dissolution creates a very cold, liquid brine that continues to melt the ice, provided the ambient temperature remains above the solution’s new, depressed freezing point.
Comparing De-Icing Agents
The effectiveness of de-icing agents is determined by their eutectic temperature and whether their dissolution is endothermic or exothermic. Sodium chloride (\(\text{NaCl}\)) is the most common and least expensive option, but its practical melting limit is around \(-6^\circ\text{C}\) (\(20^\circ\text{F}\)) because its melting rate slows drastically near its eutectic point.
Calcium chloride (\(\text{CaCl}_2\)) and magnesium chloride (\(\text{MgCl}_2\)) offer greater performance in colder conditions. They dissociate into three ions per molecule, compared to \(\text{NaCl}\)‘s two, resulting in a greater freezing point depression effect. Magnesium chloride has a lower eutectic point of about \(-33^\circ\text{C}\) (\(-28^\circ\text{F}\)).
Calcium chloride is the most effective common de-icer, boasting a eutectic point of approximately \(-51^\circ\text{C}\) (\(-60^\circ\text{F}\)). Unlike \(\text{NaCl}\), the dissolution of \(\text{CaCl}_2\) is an exothermic process, meaning it releases heat into the surrounding environment as it dissolves. This released thermal energy accelerates the melting process, allowing the agent to work more quickly and at much lower temperatures.