Deicing substances are materials used to melt existing ice or snow, or to prevent ice from forming on surfaces like roads, walkways, and aircraft. Deicers do not work by heating the ice significantly, but rather by altering the physical properties of water itself. The process relies on a fundamental chemical principle that forces water to remain liquid even when temperatures drop below its normal freezing point.
Understanding How Ice Forms
Water molecules are composed of one oxygen atom and two hydrogen atoms, held together by strong hydrogen bonds in the liquid state. As the temperature of liquid water begins to drop, the kinetic energy of the molecules decreases, causing them to slow down. At the standard freezing point of 32°F (0°C), water molecules attempt to arrange themselves into an organized, rigid, six-sided crystalline structure.
This orderly arrangement, known as the hexagonal crystal lattice, is what defines solid ice. For water to freeze, the molecules must align perfectly, and the process often requires an initial seed or nucleus around which the crystal can begin to grow. If this alignment is disrupted, the water cannot solidify, even if the temperature falls below the zero-degree mark.
The Science of Freezing Point Depression
Deicers operate by exploiting a colligative property of water known as freezing point depression (FPD). This principle states that adding a solute (the deicer chemical) to a solvent (the water) lowers the temperature at which the solution can freeze. When a deicing agent dissolves in the thin layer of liquid water present on an ice surface, the solute immediately separates into ions.
These dissolved ions then physically interfere with the water molecules’ ability to join together and form the rigid structure of ice. The presence of the foreign particles blocks the water molecules from achieving the necessary, precise alignment for the hexagonal lattice to form. Essentially, the ice crystal growth process is obstructed by the solute particles.
The water molecules now require a much lower temperature to overcome the disorder caused by the dissolved ions and successfully crystallize. This action creates a brine solution that remains liquid at temperatures colder than the freezing point of pure water. As more ice melts into this brine, the solution spreads, melting the ice layer from the bottom up. The degree to which the freezing point is lowered depends directly on the concentration of the deicer and the number of ions it releases when dissolved.
Common Deicing Chemicals
The most widely used deicing agent is sodium chloride, commonly known as rock salt, favored for its low cost and abundance. When dissolved, sodium chloride releases two ions: one sodium and one chloride. This chemical is effective but loses its melting ability when pavement temperatures fall below approximately 15°F (-9°C).
Calcium chloride and magnesium chloride are alternative chloride-based deicers that are more effective in colder conditions. Calcium chloride releases three ions—one calcium and two chlorides—and its dissolution is an exothermic reaction, generating heat that speeds up melting. Magnesium chloride is also used, often as a liquid brine, and is effective at lower temperatures than rock salt.
Non-chloride options include chemicals like potassium acetate and ethylene or propylene glycol, frequently utilized in aviation for deicing aircraft. These materials are chosen because they are less corrosive to metals and infrastructure than standard salts. Compounds are selected based on their specific temperature performance and environmental or corrosive concerns.
Temperature Limits and Practical Application
Although deicers lower the freezing point of water, this ability is not limitless, and each chemical has a practical temperature threshold. Rock salt, for instance, is not recommended for use below 15°F because the rate of ice melting becomes too slow to be effective. Stronger deicers, such as calcium chloride, maintain their melting capability down to temperatures as low as -20°F (-29°C).
The application of these agents is categorized into two distinct strategies: deicing and anti-icing. Deicing is the reactive process of applying the chemical to a surface to melt ice that has already formed. Anti-icing, by contrast, is a proactive strategy that involves applying a liquid brine solution to a pavement surface before a storm begins.
The purpose of anti-icing is to create a barrier layer that prevents snow or ice from bonding firmly to the road surface. This allows for easier mechanical removal later, and the proactive application is often more efficient, requiring less chemical than a reactive deicing effort.