Freezing is a physical change where a liquid converts into a solid state upon a reduction in temperature. This transformation occurs when molecules lose enough thermal energy to slow down and organize into a stable, highly structured arrangement known as a solid crystalline lattice. While most liquids solidify when cooled sufficiently, certain substances or mixtures resist this phase change, requiring conditions far below the freezing point of common liquids like water.
Understanding Freezing Point Depression
The primary scientific principle leveraged to prevent freezing in mixtures is freezing point depression (FPD). This phenomenon is classified as a colligative property, meaning the reduction in freezing temperature depends entirely on the number of solute particles dissolved in the solvent, not their chemical identity.
When foreign particles, or solutes, are introduced into a solvent like water, they interfere with the solvent molecules’ ability to align and connect into the orderly structure required for ice formation. The solute particles act as physical obstructions within the liquid.
To overcome this disruption and form a solid lattice, the mixture must be cooled to a lower energy state. This necessity to cool the solution further below the pure solvent’s freezing point is the depression effect. The greater the concentration of dissolved particles, the lower the final freezing point of the mixture will be.
Practical Applications of Non-Freezing Solutions
The mechanism of freezing point depression is used in numerous real-world solutions to maintain liquidity in cold environments. A common example is brine, a salt-water solution, used to de-ice roadways. Common rock salt (sodium chloride) can depress the freezing point of water down to about \(-21^\circ\text{C}\) at its eutectic point, where the mixture is saturated.
More sophisticated applications rely on glycol-based mixtures, notably in automotive and industrial settings. Ethylene glycol, the primary component in many car coolants, is effective in closed-loop systems due to its superior heat transfer efficiency and lower viscosity. A typical 60\% ethylene glycol solution can achieve a freezing point as low as \(-45^\circ\text{C}\).
Propylene glycol is a less toxic alternative often used in systems that may encounter food or potable water, such as in food processing or de-icing fluids for aircraft. Although propylene glycol is less efficient at heat transfer and has a higher viscosity than ethylene glycol, its low toxicity makes it the preferred choice where environmental or human contact is a concern. These mixtures prevent damage to engines and piping by ensuring the liquid coolant remains flowing well below the freezing point of pure water.
Pure Substances with Extremely Low Freezing Points
Not all liquids that resist freezing rely on a solute; some pure substances possess an intrinsically low freezing point due to their molecular characteristics. Mercury, a dense metal liquid at room temperature, is one example. Its freezing point is naturally low, at \(-38.83^\circ\text{C}\), due to its unique electron configuration and weak metallic bonds.
Pure ethanol also has an inherently low freezing point, solidifying only when the temperature drops to approximately \(-114^\circ\text{C}\). The molecular structure of ethanol, with its small size and relatively weak intermolecular forces compared to water, makes it difficult for its molecules to settle into a stable crystalline structure without extreme cooling.
A pure liquid can also temporarily exist below its standard freezing point in a state called supercooling. This occurs when the liquid is cooled without impurities or surface imperfections that could serve as a nucleation site, the initial point for crystal growth. Highly purified water, for instance, can remain liquid down to \(-48.3^\circ\text{C}\) before homogenous nucleation forces it to solidify. This resistance is an intrinsic property of the substance itself, unlike the solute-based effects of freezing point depression.
Liquid Helium: The Unfreezable Liquid
The substance that most definitively answers what liquid does not freeze is Helium-4, which exhibits behavior unlike any other element. Under normal atmospheric pressure, liquid helium will not solidify, even when cooled to absolute zero (0 Kelvin). This unique property is a direct consequence of quantum mechanics, specifically the principle of zero-point energy.
Even at the lowest possible temperature, helium atoms retain a minimum amount of kinetic energy, known as zero-point motion. This constant, inherent movement is significant enough to prevent the weak attractive forces between helium atoms from locking them into a fixed, solid lattice structure. The atoms are perpetually jostling, which keeps the substance liquid.
To force liquid helium to freeze, a significant amount of external pressure must be applied, typically around 25 times the pressure of Earth’s atmosphere. When this pressure is reached, the atoms are physically squeezed close enough to overcome the quantum zero-point energy, allowing the formation of a solid. This exotic, low-temperature liquid also exhibits superfluidity, a state of matter with zero viscosity, highlighting its unusual quantum nature.