The pH scale measures the acidity or alkalinity of a liquid, ranging from 0 (most acidic) to 14 (most alkaline), with 7 representing a neutral state. When water is subjected to boiling, its pH level typically changes. This alteration is a complex chemical interplay, usually resulting in a slight increase in the water’s alkalinity (the pH value rises). This change is primarily driven by the removal of dissolved gases, rather than the evaporation of water itself.
The Primary Effect Removing Dissolved Gases
Most natural water sources, including tap water, contain dissolved carbon dioxide (\(\text{CO}_2\)) absorbed from the atmosphere. This dissolved gas reacts with water molecules to form carbonic acid (\(\text{H}_2\text{CO}_3\)), a weak acid. The presence of this acid contributes hydrogen ions (\(\text{H}^+\)) to the solution, which lowers the pH and makes the water slightly acidic, often resulting in a pH below 7.0.
The application of heat dramatically reduces the solubility of gases in water, and when water reaches its boiling point, dissolved gases are driven out. This process is known as degassing or decarbonation, and it effectively reverses the formation of carbonic acid. As the \(\text{CO}_2\) escapes into the air, the chemical equilibrium shifts, removing the acidic component from the solution. The removal of carbonic acid leads to a decrease in the concentration of hydrogen ions, resulting in a measurable rise in the pH.
How Mineral Content Alters the Final pH
While the removal of \(\text{CO}_2\) is the primary driver of the pH increase, the non-volatile mineral content in the water introduces a secondary, counterbalancing effect. Tap water, especially “hard water,” contains various dissolved salts and ions, such as calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) bicarbonates. As water evaporates during boiling, the remaining water volume decreases, which leads to a concentration of these dissolved minerals.
An increase in the concentration of alkaline substances, such as bicarbonates, would logically tend to further increase the pH of the water. However, the boiling process also causes some of these minerals to precipitate out, forming “scale” or kettle fur. The removal of \(\text{CO}_2\) reduces the solubility of calcium and magnesium carbonates, causing them to convert from soluble bicarbonates to insoluble carbonates, which then settle out. This precipitation process removes some of the alkaline components from the solution, acting as a buffer against a runaway pH increase.
The final pH of the cooled, boiled water is a result of the competition between these two effects: the rise in alkalinity from gas removal and the counter-effect from mineral precipitation and concentration. In most tap water, the removal of \(\text{CO}_2\) is the dominant factor, leading to a net increase in pH. However, in water with extremely high mineral content, prolonged boiling might lead to a smaller net pH change or, in rare cases, a slight decrease in alkalinity.
Practical Considerations for Testing and Consumption
pH Measurement and Temperature Correction
For accurate scientific testing, the temperature of the water sample must be considered when measuring pH. The neutral point of water is exactly 7.0 only at 25 degrees Celsius. Because the ionization of water is temperature-dependent, pure water at 100 degrees Celsius will have a neutral pH closer to 6.14. Professional pH meters typically require temperature correction to provide an accurate reading referenced back to 25 degrees Celsius.
The act of boiling introduces temporary chemical instability that must be addressed before testing. The degassed water, once cooled, will immediately begin to reabsorb \(\text{CO}_2\) from the surrounding air. If the boiled water is left exposed to the atmosphere, the reabsorption of \(\text{CO}_2\) will cause the carbonic acid to reform, and the water’s pH will gradually drift back toward its initial level.
Impact on Consumption
For the average consumer, this temporary pH change has no impact on health or the taste of the water. Although some may find boiled water tastes “flat” due to the loss of dissolved gases like oxygen and \(\text{CO}_2\), the small shift in alkalinity is chemically insignificant for consumption. Boiled water used for drinking or cooking will quickly equilibrate with the air, meaning the temporary increase in pH is irrelevant for practical purposes once the water has cooled.