By Claire Gillespie | Updated Mar 24, 2022
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The pH scale quantifies how acidic or alkaline a solution is. Pure, distilled water is neutral, registering a pH of 7. When water is heated, its pH decreases ever so slightly—an effect so subtle that standard pH strips usually miss it.
In pure water, temperature increases lower pH and decreases raise it, but the shifts are too small for most routine pH testing.
The pH value reflects the concentration of hydrogen ions (H⁺) in a solution. Each whole-number step on the scale represents a ten-fold change in H⁺ concentration. Thus, a shift from pH 5 to 6 indicates a ten-fold reduction in hydrogen ions.
Le Châtelier’s principle states that a system at equilibrium will shift to counteract a disturbance. Increasing the temperature of water pushes the self-ionization equilibrium toward the end products—more H⁺ and OH⁻. Consequently, the overall pH drops. For example, raising the temperature from 0 °C to 10 °C reduces the pH of pure water by about 0.2 units. Lowering the temperature produces the reverse effect.
A lower pH does not mean the water becomes more acidic; the concentrations of hydrogen and hydroxide ions remain equal, keeping the solution neutral. At 25 °C, pure water has a pH of 7. When heated to 100 °C, its pH falls to 6.14—still neutral on the scale, just slightly lower.
