A solution has a hydrogen ion concentration of 10^-4 mol per litre. Its pH and nature are

pH = -log[H+] = -log(10^-4) = 4. Since pH is below 7 the solution is acidic. Basic solutions have pH above 7, and neutral water has pH 7 at 25 degrees Celsius. Taking the exponent -4 as the pH is a common sign slip. A change of one pH unit corresponds to a tenfold change in hydrogen ion concentration, so pH 4 is ten times more acidic than pH 5.