Pure water at 40 , C has a measured pH of about 6.8 but is still classified as neutral
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Pure water at 40\,∘C has a measured pH of about 6.8 but is still classified as neutral. Which of the following best explains why the solution is neutral even though its pH is less than 7?
(a) Because the pH scale compresses at higher temperature; 6.8 is the midpoint of the shortened scale, just as 7 is at 25\,∘C.
(b) Because Kw increases with temperature, equal concentrations of [H3O+] and [OH−] correspond to a pH lower than 7.
(c) Because pH+pOH is always 14, so neutrality must remain at pH 7 regardless of temperature.
(d) Because the endothermic autoionisation of water makes Kw decrease with temperature, shifting the neutral point to pH values below 7.