The temperature at which chlorine evaporates from water does not have a single, simple answer because the process is not like boiling a pure liquid. Instead, the release of chlorine is a continuous process of volatilization, or off-gassing, which occurs at all temperatures above freezing. This rate is heavily influenced by heat but is not defined by a specific temperature point. Understanding this process requires examining the chemical forms chlorine takes once dissolved and how its escape rate is accelerated by environmental factors.
The Chemical Forms of Chlorine in Water
When chlorine is added to water, such as for disinfection, it immediately reacts to form “free chlorine.” Free chlorine consists of two main chemical species that exist in equilibrium: hypochlorous acid (\(\text{HOCl}\)) and the hypochlorite ion (\(\text{OCl}^{-}\)). This chemical balance determines how easily chlorine can escape into the air.
Hypochlorous acid (\(\text{HOCl}\)) is a neutral molecule and is the form primarily responsible for rapid off-gassing. Because it lacks an electrical charge, it readily passes from the water into the atmosphere. The hypochlorite ion (\(\text{OCl}^{-}\)), however, carries a negative charge. This negative charge holds it securely within the water, making this ionic form stable and less prone to volatilization.
How Temperature Drives the Off-Gassing Rate
The rate at which chlorine off-gasses is directly related to the water’s temperature. Temperature provides the necessary kinetic energy for the volatile hypochlorous acid molecules to break the surface tension and escape. The chlorine loss rate can nearly quadruple with an increase of just \(11^{\circ}C\) in the water temperature.
At room temperature (around \(20^{\circ}C\) or \(68^{\circ}F\)), chlorine off-gassing from a container may take up to 48 hours for significant reduction. When water is heated for a shower or brought to a boil, the rate increases dramatically. Boiling water (\(100^{\circ}C\) or \(212^{\circ}F\)) is highly effective at driving off hypochlorous acid quickly. This effectiveness is due to the extreme increase in molecular energy and the steam generated.
The Role of pH and Other Environmental Factors
Temperature is a major factor, but the water’s pH level plays an equally important role in determining the off-gassing rate. The pH dictates the ratio between the volatile hypochlorous acid (\(\text{HOCl}\)) and the stable hypochlorite ion (\(\text{OCl}^{-}\)). At a lower, more acidic pH (below 7.5), the equilibrium shifts to favor the formation of the volatile \(\text{HOCl}\), which accelerates the escape of chlorine.
Conversely, at a higher, more alkaline pH (above 7.5), the equilibrium shifts toward the stable, non-volatile \(\text{OCl}^{-}\) ion. This results in a much slower rate of off-gassing, as the primary form of chlorine is trapped in the water.
Other Factors Affecting Volatilization
Beyond temperature and pH, other factors also increase the rate of volatilization. These include a larger water surface area exposed to the air and any form of agitation or aeration, such as pouring or stirring, which constantly disrupts the surface layer.
Applying the Science to Everyday Water Use
The understanding of chlorine volatilization translates into practical actions for daily water use. In a hot shower, the combination of high temperature, large surface area from mist and droplets, and aeration causes chlorine to rapidly off-gas into the enclosed space. The resulting inhalation of chlorine gas is a concern for individuals seeking to remove it from their bathing water.
For removing chlorine from drinking water, simply leaving tap water in an open container at room temperature will cause the chlorine to dissipate over a day or two. Boiling water for a few minutes will accelerate this removal process significantly due to the heat. Conversely, maintaining chlorine in a swimming pool requires controlling the pH to keep the chlorine in the more stable hypochlorite ion form, slowing down its loss.