After your freakish friend (from the other pH problem) found out that it was carbon dioxide in the air that made his water acidic, he made sure that all his jars are air-tight and vacuumed out all the air in his jars immediately after the purification. How big of a freak is that?!

During one hot summer day, you and he just finished playing basketball. You went over to his house and again, he offers you a glass of his purified water. And to ensure that it's pure, he measures the pH of it right after he pours it out (so that it doesn't get in contact with the air too long).

And again, the pH of the water is not 7! What happened this time?