Can Water Stay Liquid Below Zero Degrees Celsius?

Introduction

Yes, water can indeed remain in a liquid state even when the temperature drops below zero degrees Celsius. This phenomenon can occur due to several factors, primarily involving pressure, the presence of additives, and the purity of the water itself.

The Role of Pressure

The phase of a material, whether it is a gas, liquid, or solid, is influenced significantly by temperature and pressure. For most liquids, increasing pressure tends to raise the temperature at which the liquid freezes into a solid. This is because applying pressure forces the molecules closer together, allowing them to form stable bonds that solidify them.

Water, however, has unique properties. When water molecules form a solid crystalline structure (ice), they spread out, making ice less dense than liquid water. This characteristic results in ice floating on water. Interestingly, applying pressure to water lowers its freezing point. If sufficient pressure is applied, liquid water can exist several degrees below $0 \, \text{°C}$.

The Effect of Additives

Another way to keep water in a liquid state below its usual freezing point is by adding substances like salt. Salt, composed of sodium and chlorine ions, interferes with the chemical bonding necessary for water to freeze. When salt is dissolved in water, the water molecules are more inclined to bond with the salt ions rather than with each other, delaying the freezing process.

As more salt is introduced, the freezing point of water can decrease significantly. For instance, salt can lower the freezing point of water down to as low as $-21 \, \text{°C}$. This property is why salt is often sprinkled on icy sidewalks; it lowers the freezing point of the ice, causing it to melt. However, this method is ineffective if the ambient temperature falls below $-21 \, \text{°C}$.

Supercooling: Pure Water’s Ability to Stay Liquid

Even without applying pressure or adding substances, water can remain liquid below $0 \, \text{°C}$ in a state known as “supercooled” water. For water to freeze, it typically requires nucleation centers—starting points for the crystallization process. In most environments, impurities, dust, or slight vibrations provide these nucleation centers.

However, in very pure and still water, there may be an absence of these nucleation centers. As a result, it is possible to cool pure water well below $0 \, \text{°C}$ without it freezing. At standard pressure, pure water can be supercooled to approximately $-40 \, \text{°C}$. If nucleation centers are introduced (for instance, through vibrations), the supercooled water will freeze rapidly. A common natural occurrence of supercooled water is freezing rain, where the rain remains liquid until it contacts surfaces, providing nucleation centers that trigger freezing.

Conclusion

In summary, water can exist as a liquid below $0 \, \text{°C}$ through mechanisms involving pressure, the addition of substances like salt, or by existing in a supercooled state. These fascinating behaviors emphasize the complex interactions between temperature, pressure, and molecular dynamics in determining the state of a substance.