Neutral solutions have pH = 7 at 298 K. A sample of pure water is found to have pH < 7. Does it mean that it is acidic ?
pH <7 for pure $H _{ 2 }O$ shows that water is at a temperature higher than 298 K. It is neutral at all temperatures. At higher temperature, $H _{ 2 }O$ dissociates more to give large concentrations of ${{H}^{+}}$ ions and ${{OH}^{-}}$ ions. Hence, pH <7.