What factor can cause the pH of pure water to deviate from 7 at room temperature?

What factor can cause the pH of pure water to deviate from 7 at room temperature?

• A. Temperature changes
• B. Atmospheric pressure changes
• C. Humidity changes
• D. Dissolved CO2 only at high altitude

Answer: (A)

The pH of pure water is determined by the autoionization of water: H2O ⇌ H+ + OH-. The product of the hydrogen and hydroxide ion activities, Kw, depends on temperature. At a common room temperature, Kw gives [H+] = [OH−] = 1×10−7 M, so the pH is 7. But Kw changes with temperature: as temperature rises, Kw increases, producing more H+ and OH− ions, which lowers the pH (water becomes more acidic). Conversely, cooler temperatures lower Kw, reducing ion concentrations and pushing the pH above 7 (water becomes more basic). So, changes in temperature directly cause the pH of pure water to deviate from 7. While factors like dissolved CO2 from air can also affect pH, the primary, direct factor among the options is temperature change.

The pH of pure water is determined by the autoionization of water: H2O ⇌ H+ + OH-. The product of the hydrogen and hydroxide ion activities, Kw, depends on temperature. At a common room temperature, Kw gives [H+] = [OH−] = 1×10−7 M, so the pH is 7. But Kw changes with temperature: as temperature rises, Kw increases, producing more H+ and OH− ions, which lowers the pH (water becomes more acidic). Conversely, cooler temperatures lower Kw, reducing ion concentrations and pushing the pH above 7 (water becomes more basic). So, changes in temperature directly cause the pH of pure water to deviate from 7. While factors like dissolved CO2 from air can also affect pH, the primary, direct factor among the options is temperature change.