Salt significantly lowers the freezing point of water, a phenomenon known as Freezing Point Depression. This means that water containing salt must be cooled below the standard \(0^\circ \text{C}\) or \(32^\circ \text{F}\) before it can solidify into ice. This property is a colligative property, meaning it depends only on the number of solute particles dissolved, not on the nature of the particles themselves.
How Salt Disrupts Water’s Structure
Water molecules naturally arrange themselves into a highly ordered, hexagonal structure as they transition from a liquid to a solid state, forming the crystal lattice recognized as ice. When a substance like table salt, or sodium chloride (\(\text{NaCl}\)), is dissolved in water, it dissociates into its component ions: a positively charged sodium ion (\(\text{Na}^+\)) and a negatively charged chloride ion (\(\text{Cl}^-\)).
These foreign ions disperse throughout the water, physically interfering with the ability of water molecules to bond and achieve the rigid arrangement necessary for ice formation. Because the ions disrupt this process, the water requires a greater reduction in thermal energy, or a lower temperature, to successfully force the molecules into a stable solid form. This interference stabilizes the liquid state, making it harder for the water to transition into its solid phase.
The Role of Concentration in Lowering Temperature
The extent to which the freezing point is lowered is directly proportional to the concentration of dissolved particles in the water, not the mass of the solute. For instance, calcium chloride (\(\text{CaCl}_2\)) is more effective than sodium chloride (\(\text{NaCl}\)) because it dissociates into three ions (\(\text{Ca}^{2+}\) and two \(\text{Cl}^-\) ions), while \(\text{NaCl}\) yields only two ions.
As more salt is added, the freezing point continues to drop, but this effect has a hard limit known as the eutectic point. For common sodium chloride, the eutectic point is reached when the solution contains about \(23.3\%\) salt by weight. At this maximum concentration, the freezing point of the brine solution is approximately \(-21.1^\circ \text{C}\) (or \(-6^\circ \text{F}\)). If the concentration of salt is increased beyond this point, the freezing temperature will begin to rise again.
Everyday Applications of Freezing Point Depression
De-Icing Roads and Sidewalks
One of the most recognizable applications of freezing point depression is the use of salt to de-ice roads and sidewalks during winter weather. When salt is spread over a road, it mixes with the thin layer of liquid water present on the surface of ice or snow. This forms a salt-water solution, or brine, which has a lower freezing point than the surrounding ice.
As the salt dissolves, it prevents the liquid water from refreezing, causing the existing ice to melt. This method is effective only when the ambient temperature is above the eutectic point of the salt being used; for example, standard rock salt (\(\text{NaCl}\)) becomes ineffective once the temperature drops below about \(-21^\circ \text{C}\).
Making Homemade Ice Cream
Another practical application is the traditional method of making homemade ice cream. The ice cream mixture is placed in a container surrounded by a slurry of ice and salt. Adding salt to the ice causes it to melt, forming a brine solution with a temperature significantly lower than \(0^\circ \text{C}\). This super-cold bath rapidly draws heat away from the ice cream mixture, allowing it to freeze quickly and achieve a smooth texture.