Water has a fixed freezing point of \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) under standard atmospheric pressure, but this temperature changes when other substances are dissolved in it. Saltwater is a solution, typically containing dissolved salts such as sodium chloride, which alters the physical properties of the pure solvent. The presence of these dissolved particles changes the temperature at which water molecules transition into their solid, crystalline state. This phenomenon means that saltwater remains liquid at temperatures below the freezing point of fresh water. Understanding this temperature difference is important because it dictates everything from ocean currents to winter road maintenance.
The Specific Freezing Point Range
The temperature at which saltwater freezes is not a single fixed number but is directly dependent on its salinity, or the amount of dissolved salt. For standard ocean water, which maintains an average salinity of about 35 parts per thousand (\(\text{ppt}\)), the freezing point is lowered significantly below \(0^\circ\text{C}\). This average salinity causes the water to begin freezing at approximately \(-1.8^\circ\text{C}\) to \(-2.0^\circ\text{C}\) (\(28.8^\circ\text{F}\) to \(28.4^\circ\text{F}\)). This depression is enough to keep large portions of the planet’s oceans liquid even in contact with freezing air temperatures. For example, while a freshwater lake would be frozen solid at \(-1^\circ\text{C}\), a typical ocean bay would still be entirely liquid.
The Science of Freezing Point Depression
The lowering of the freezing point is a measurable effect rooted in the principles of physical chemistry, specifically known as freezing point depression. This is categorized as a colligative property, meaning the effect depends on the number of solute particles dissolved in the water, not on the chemical identity of those particles. When salt, such as sodium chloride (\(\text{NaCl}\)), dissolves in water, it dissociates into separate ions, \(\text{Na}^+\) and \(\text{Cl}^-\). These charged ions act as interference when water molecules attempt to organize themselves into the highly ordered, hexagonal lattice structure of solid ice.
The formation of ice requires water molecules to slow down and align perfectly, but the dissolved salt ions actively disrupt the necessary intermolecular forces. Because the ions physically block the formation of the crystal structure, the water requires a lower temperature to freeze. This lower temperature reduces the water molecules’ kinetic energy, allowing them to overcome the disruptive presence of the solute particles and lock into the crystalline structure. The more dissolved particles present, the greater the disruption and the lower the temperature must drop for the water to solidify.
Factors Influencing the Exact Temperature
The most significant variable determining the exact freezing temperature is the concentration of salt, known as salinity. As the amount of dissolved salt increases, the freezing point continues to drop linearly up to a specific limit. For sodium chloride, the lowest possible freezing point is reached at the eutectic point, which is approximately \(-21.1^\circ\text{C}\) (\(-6^\circ\text{F}\)) at a concentration of \(23.3\%\) salt by mass. Beyond this concentration, the salt begins to crystallize out of the solution alongside the ice.
Other Influencing Factors
Other factors also contribute to minor shifts in the freezing point. The immense pressure found in the deep ocean, for instance, can slightly lower the freezing point further. The type of salt also matters because different salts dissociate into varying numbers of particles. For example, calcium chloride (\(\text{CaCl}_2\)) dissociates into three ions (\(\text{Ca}^{2+}\) and two \(\text{Cl}^-\) ions), giving it a greater freezing point depression effect than \(\text{NaCl}\), which only produces two ions.
Real-World Applications
The principle of freezing point depression in saltwater is used extensively in both natural systems and human engineering. In the marine environment, this phenomenon is responsible for the formation of sea ice in polar regions. When seawater freezes, the ice crystals formed are almost entirely pure water, a process known as brine rejection. This process forces the salt out into the surrounding liquid, increasing the salinity of the remaining unfrozen water. This, in turn, lowers its freezing point even further, creating dense, cold water masses that drive deep-ocean circulation patterns.
Winter Road Maintenance
On land, the application of this concept is most visible in winter road maintenance. Salt is spread on icy roads to create a saltwater solution with the existing ice and snow. The resulting solution has a freezing point below \(0^\circ\text{C}\), causing the ice to melt and remain liquid at temperatures where pure water would be solid. This de-icing method is effective only as long as the ambient temperature remains above the eutectic point of the salt solution being used. If the temperature drops below \(-21.1^\circ\text{C}\), common rock salt becomes ineffective, and other chemical compounds are required to maintain a liquid state.