Why will the pH of pure water at 25 degree celcius be equal to 7.0 ?
This is almost like asking why we call a car a car or a house a house or a mule a mule. The best answer is that "that's the nature of the beast." However, you can go into the math of the thing. Since water dissociates H2O ==> H^+ + OH^- and
(H^+)(OH^-) = 1 x 10^-14, then (H^+) = (OH^-) = 1 x 10^-7 and since
pH = -log (H^+), then pH = -log (1 x 10^-7) = 7