Liquid chlorine does freeze, but the temperature at which it solidifies depends entirely on the specific form of chlorine. When people refer to “liquid chlorine,” they usually mean the water-based solutions used for pools or household cleaning, which freeze at temperatures far different from the pure, elemental form. Understanding this distinction is crucial for safe storage and handling during cold weather.
The Freezing Point of Pure Elemental Chlorine
The purest form of chlorine, known as elemental chlorine (\(\text{Cl}_2\)), is a substance whose properties are vastly different from its common household compounds. In its natural state at room temperature, it is a yellowish-green gas. It is typically converted to a liquid by applying pressure or cooling it to extremely low temperatures for industrial storage and transport.
This pure liquefied chlorine solidifies at a low temperature, approximately \(-101^\circ \text{C}\) or \(-149.5^\circ \text{F}\). This temperature is far below any natural environmental cold experienced, meaning it will not freeze in typical storage environments. This form of chlorine is a cryogenically stored liquid or pressurized gas and is not the product sold to general consumers.
How Aqueous Chlorine Solutions React to Cold
The liquid chlorine used in pools, spas, and household bleach is an aqueous solution of sodium hypochlorite (\(\text{NaClO}\)) dissolved in water. This solution contains a percentage of the active chemical in a water base, which is why its freezing behavior is closer to saltwater than pure chlorine. The presence of the dissolved sodium hypochlorite and the salt byproduct (sodium chloride) lowers the freezing point of the water.
The specific freezing point of the solution depends directly on its concentration, which can vary from about \(6\%\) for household bleach to \(12.5\%\) or more for commercial pool liquid chlorine. A typical \(6\%\) sodium hypochlorite solution will freeze around \(18.5^\circ \text{F}\) (\(-7.5^\circ \text{C}\)), while a stronger \(12.5\%\) solution can resist freezing down to about \(5^\circ \text{F}\) (\(-15^\circ \text{C}\)). These temperatures are well within the range of many cold climates, making freezing a practical concern for consumers.
When these solutions freeze, the water turns to ice, but the dissolved chemicals are forced into the remaining liquid fraction, creating a highly concentrated brine. This concentration can lead to the precipitation of salt crystals, especially in solutions over \(10\%\). While the solution can often be thawed without total destruction of the chemical, the separation of ingredients and crystallization can reduce the overall effectiveness of the chlorine as a sanitizer.
Cold Weather Storage and Handling Safety
Since common liquid chlorine solutions freeze well above the temperature of pure elemental chlorine, proper winter storage is an important consideration. To maintain maximum effectiveness, sodium hypochlorite solutions should be stored in an area where the temperature remains consistently above their specific freezing point. For most commercial pool chlorine, this means keeping the product above \(5^\circ \text{F}\) to \(10^\circ \text{F}\).
Allowing the solution to freeze and thaw can lead to a loss of product potency, as the freeze-concentration effect accelerates the degradation of the active hypochlorite ion. The expansion of the freezing liquid can also cause the storage container to crack or rupture, leading to a hazardous chemical spill. If a container is found to be frozen, it must be thawed gradually in a protected area away from direct heat sources, and adequate ventilation must be ensured.