# IMAt 2015 question 43

Pure water self-ionizes. This endothermic reaction is represented by the equation:
H2O(l) + H⁺(aq) ------- H⁺(aq) + OH⁻(aq)

Which is true for pure water between O°C and 100°C?

1. The concentrations of H⁺ and OH⁻ 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

Hi everyone!
The answer is 1) and 2), could someone explain why?

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Hey! so here’s my point of view:

1. This one is quite tricky . The concentrations of H+ and OH- will equally increase; so between this two ions, the concentration will always be “the same”. So this one is correct
2. It’s an endothermic reaction: if you increase the temperature, equilibrium will shift to the right and therefore we will have more H+ ions; hence decreasing the pH (pH is just the measurement of the amounts of ions, so even if the solution is not acid, the pH can decrease)
3. this wouldn’t be true because the electrical conductivity increase when we have more free ions (and that’s what exactly happens here when you increase the temperature, since the equilibrium shifts to the right)
I really hope this is helpful!  4 Likes