IMAT 2015 Q43 [Pure Water]

Pure water self­-ionises. This endothermic reaction is represented by the equation:


Which of the following statements is true for pure water between the temperatures of 0 °C and 100 °C?

  1. The concentrations of H^+ (aq) and OH^– (aq) are equal between 0 °C and 100 °C.
  2. An increase in temperature causes the pH to fall.
  3. An increase in temperature causes the electrical conductivity to decrease.

A. 1 and 3 only
B. 3 only
C. 1 only
D. 1 and 2 only
E. 2 only

Statement 1 is correct. This is because all the H^+ and OH^- ions are produced from the same singular reactant. Therefore, the concentration should stay equal.

Statement 2 is also correct. Since this is an endothermic reaction, an increase in temperature will favour the forwards’ reaction which in turn will produce more H^+ ions. This will make the pH fall.

Statement 3 is incorrect. The amount of free moving electrons available to conduct electricity will not be changed or affected, therefore, we cannot say the increase in temperate can affect the electrical conductivity.

Hence, our answer must be statements 1 and 2, option D.