How can you explain even though you continue to add pure water into a substance, even though the resultant pH might differ a little, the acidity or alkalinity of the substance still remains the same to that of substance?

If the water vapor content stays the same and the temperature drops, then the relative humidity increases. its vise versa.if the water vapor content stats the same and the temperature rises, the relative humidity decreases. so, yes, i'd say that temperature and humidity have a strong relationship.