Alkaline water is typically defined as having a \(\text{pH}\) level above 7, sometimes intentionally increased to 8 or 9 by manufacturers for perceived health benefits. Consumers are interested in this water for general hydration or to help neutralize acidity in the body. The core question is whether boiling alkaline water alters its properties, specifically, does boiling change its \(\text{pH}\)?
Understanding pH and Alkaline Water
The \(\text{pH}\) scale measures the concentration of hydrogen ions (\(\text{H}^+\)) in a solution, determining its acidity or basicity on a logarithmic scale from 0 to 14. A lower \(\text{pH}\) indicates greater acidity, while an alkaline solution has a lower concentration of these ions. Alkalinity is distinct from \(\text{pH}\); it refers to the water’s buffering capacity, or its ability to neutralize acids and resist \(\text{pH}\) changes. This capacity is largely due to dissolved alkaline substances like bicarbonates (\(\text{HCO}_3^-\)) and carbonates (\(\text{CO}_3^{2-}\)).
Alkaline water sold commercially typically achieves its elevated \(\text{pH}\) in one of two ways. Some producers add alkaline minerals, such as calcium, magnesium, or potassium, to raise the \(\text{pH}\). Other methods involve electrolysis, where water is run over platinum and titanium plates to separate the water into acidic and alkaline streams, producing water with a higher concentration of hydroxide ions (\(\text{OH}^-\)). The presence of these dissolved minerals and ions is what makes the water chemically reactive to heat.
The Chemical Reaction: Why Boiling Changes pH
Boiling alkaline water initiates two competing chemical processes that directly impact the final \(\text{pH}\). The first mechanism involves the loss of dissolved gases. As water is heated, dissolved carbon dioxide (\(\text{CO}_2\)) is driven out and escapes into the atmosphere. Since \(\text{CO}_2\) forms carbonic acid (\(\text{H}_2\text{CO}_3\)) in water, its removal causes the chemical balance to shift, leading to a slight, temporary increase in the water’s \(\text{pH}\).
The second, more significant mechanism in highly alkaline or hard water is mineral precipitation. Alkaline water often contains high levels of dissolved calcium and magnesium bicarbonates. Boiling promotes the chemical conversion of soluble bicarbonate ions (\(\text{HCO}_3^-\)) into less soluble carbonate ions (\(\text{CO}_3^{2-}\)), which then combine with calcium to form calcium carbonate (\(\text{CaCO}_3\)). This solid calcium carbonate precipitates out of the solution, forming a white residue known as limescale.
The removal of these carbonate and bicarbonate ions causes the \(\text{pH}\) to decrease. The final \(\text{pH}\) of the cooled, boiled alkaline water is determined by the balance between two effects: \(\text{CO}_2\) loss, which tends to raise the \(\text{pH}\), and mineral precipitation, which tends to lower it toward neutral. In very hard or highly mineralized alkaline water, the precipitation effect is often dominant, resulting in a net \(\text{pH}\) drop back toward 7.0.
Practical Effects: Hardness, Scale, and Taste
The chemical changes that occur during boiling have tangible effects on the water’s physical properties. The precipitation of calcium and magnesium carbonates directly reduces the water’s temporary hardness—the portion of hardness that can be removed simply by boiling. This process is essentially a form of water softening.
The physical evidence of this chemical process is the formation of limescale, a hard, chalky deposit that adheres to the surfaces of kettles and pots. This scale is the solidified calcium carbonate removed from the water through boiling. While not harmful, its buildup can reduce the efficiency of heating elements in appliances and may require regular cleaning.
Boiling also affects the water’s taste profile. The expulsion of dissolved gases, including oxygen and carbon dioxide, can make the water taste “flat” compared to unboiled water. Conversely, if the water is highly concentrated with certain alkaline minerals after boiling, it may retain a slightly bitter taste.
Relevance for Drinking and Cooking
The slight shift in \(\text{pH}\) after boiling has minimal practical significance for drinking. While the \(\text{pH}\) may drop toward neutral, the intended health benefits often claimed for alkaline water, such as neutralizing body acidity, are largely unaffected by this small change. The strong acid in the stomach, hydrochloric acid, is highly effective at neutralizing any ingested water, regardless of whether its \(\text{pH}\) is 7.5 or 9.0.
For cooking, the \(\text{pH}\) change is usually a secondary concern. The primary practical application is the reduction of temporary hardness, which can improve the clarity of certain beverages like tea or coffee. However, the trade-off is the scale buildup in appliances, the most noticeable consequence of boiling highly alkaline or hard water. Boiling remains a common and safe method of preparation, with mineral precipitation being the main factor to consider.