The pH of pure water can vary depending on the temperature it is held at. Does that mean pure water can be acidic or alkaline?

Interestingly, pure water will always be neutral, regardless of pH. To understand why, we have to properly look at what pH means. pH is a measure of the number of free H+ ions in a solution or liquid. The amount of free H+ ions typically determines the acidity of a solution and as such pH can be used as a measure of acidity.However, pure water is the exception to the rule. When water (H2O) dissociates, it forms one hydrogen ion (H+) and one hydroxide ion (OH-). Where H+ ions make a solution acidic, OH- ions make a solution alkaline. As an equal number of H+ and OH- ions are produced when water dissociates, and as we know there are no contaminants (the question specified pure water), the acidic effect of the H+ and the alkaline effect of the OH- cancel each other out, making the water neutral.So why does the pH change? Well as we discussed earlier, pH is only a measure of free H+ ions. It does not account for the free OH- ions. As the water has dissociated, there may be a lot of H+ ions, resulting in a low pH (seemingly acidic), but there is also a lot of OH- ions, resulting in a neutral liquid.This distinction between pH and acidity can be very confusing at first, but it is an important distinction to make, and the process of re-evaluating what you already 'know' when a new situation arises is vital for better learning.