The pH of pure water will be less than 7 at
As you know, the pH of pure water is based on water's self-ionization reaction which results in the production of hydronium and hydroxide ions.
The equilibrium constant for this reaction,
Since water's concentration is assumed to be constant at any given temperature, you can write this expression as
The product between
That happens because the formation of the hydronium and hydroxide ions is actually an endothermic process. That means that the equilibrium will counteract any increase in temperature by favoring the forward reaction, since producing hydronium and hydroxide ions consumes energy (think Le Chatelier's Principle.
At any temperature, the pH of pure water is given by the equation
SInce you're dealing with the negative log, any increase in the concentration of the hydronium ions will result in a smaller value for the pH.
However, you need to remember that the water will still be neutral, even if the pH is less than 7!
That happens because the concentrations of hydronium and hydroxide ions are equal, so you can't say that the pH is acidic.