Ammonia (\(\text{NH}_3\)) is a chemical compound recognized for its pungent odor and its use in household cleaning products and industrial applications. It is often encountered as a compressed gas or a solution dissolved in water. Understanding the temperature at which ammonia transitions from a liquid to a solid is important for its safe handling and commercial use.
The Freezing Point of Pure Ammonia
The temperature at which pure ammonia freezes is significantly lower than that of water. Anhydrous ammonia is a liquid only when kept under pressure or at a very low temperature. Under standard atmospheric pressure, liquid ammonia converts to a solid at \(-77.7^\circ\text{C}\).
This low-temperature transition converts the clear liquid into colorless crystals. This freezing point is more than 77 degrees Celsius below water’s freezing point of \(0^\circ\text{C}\). Pure ammonia has a wide temperature range in which it remains a liquid under specific pressures, as its boiling point is \(-33.35^\circ\text{C}\).
How Water Changes Ammonia’s Freezing Point
Ammonia is highly soluble in water, and this combination is often sold as household ammonia. When ammonia is dissolved in water, the freezing behavior of the resulting solution changes due to freezing point depression. This is a colligative property, meaning the change depends on the number of dissolved particles.
Adding ammonia molecules disrupts the formation of the ordered crystal structure that pure water needs to freeze. As small amounts of ammonia are added, the freezing point of the solution is lowered further than \(0^\circ\text{C}\). This effect is most pronounced up to the eutectic point, where the mixture has the lowest possible freezing temperature.
Most solutions, such as common household cleaners, contain only about 5 to 10 percent ammonia, meaning they are mostly water. Consequently, these dilute aqueous solutions freeze at a temperature much closer to that of pure water. This temperature ranges from about \(-7^\circ\text{C}\) to \(-10^\circ\text{C}\) for concentrated solutions, or closer to \(0^\circ\text{C}\) for weaker mixtures.
Why Ammonia Freezes So Low
The low freezing temperature of pure ammonia is rooted in its molecular structure. Ammonia (\(\text{NH}_3\)) is a polar molecule, featuring a nitrogen atom bonded to three hydrogen atoms. This polarity allows ammonia molecules to form strong intermolecular hydrogen bonds.
Despite forming hydrogen bonds, ammonia’s freezing point is much lower than water’s (\(0^\circ\text{C}\)). The difference stems from the number of potential hydrogen bonds each molecule can form. Water’s oxygen atom allows each water molecule to form up to four hydrogen bonds with neighbors, creating a highly stable and rigid lattice when frozen.
In contrast, ammonia’s nitrogen atom has only one lone pair, limiting the number of hydrogen bonds it can form with surrounding molecules. This results in a weaker overall network of intermolecular forces in solid ammonia compared to ice. Less thermal energy must be removed to lock the molecules into a solid structure, translating directly to a lower freezing temperature.
Practical Applications of Ammonia’s Low Freezing Point
The low freezing point of ammonia makes it valuable in industrial processes. Its capacity to remain liquid across a wide temperature range, combined with its high latent heat of vaporization, makes it an excellent refrigerant. Ammonia is widely used in large-scale industrial refrigeration, such as in cold storage warehouses and food processing plants.
In these systems, the low freezing point ensures that the liquid ammonia does not solidify even when used to achieve deep-freeze temperatures. The compound’s thermodynamic efficiency allows it to absorb large amounts of heat during its phase change. This provides cost-effective and energy-efficient cooling.
The low freezing point is also relevant to the storage and transportation of anhydrous ammonia, a major component in fertilizer. To keep this pure form liquid without excessive pressure, it must be refrigerated below its boiling point of \(-33.35^\circ\text{C}\). Since the liquid does not freeze until \(-77.7^\circ\text{C}\), this provides a safety margin for managing the liquid phase during distribution.